Java 函数式编程(1 万字）

news2024/11/24 6:26:47

此笔记来自于B站黑马程序员 good
Java 历史版本及其优势

在这里插入图片描述

函数式编程, Stream API

一.函数伊始函数、函数对象

函数对象

行为参数法

image-20241007123851139

image-20241007124037094

image-20241007124243417

延迟执行

image-20241007124718046

image-20241007124849860

a-lambda

image-20241007135452414

b-方法引用

image-20241007140002122

复习小测

image-20241007140721800

  1. Math::random
    () -> Math.random()
  2. Math::sqrt
    (double number) -> Math.sqrt(number)
  3. Student::getName
    (student stu) -> stu.getName()
  4. Student::setName
    (Student stu,String newName) -> stu.setName(newName)
  5. Student::hashcode
    (student stu) -> stu.hashcode()
  6. Student::equals

​ (Student stu, object o) -> stu.equals(o)

假设已有对象 Student stu = new Student（“张三”);
  1. stu::getName

​ () -> stu.getName()

  1. stu::setName

​ (String newName) -> stu.setName(newName)

  1. Student::new

​ (String name) -> new Student(name)

二.函数编程语法 Lambda、方法引用、闭包、柯里化、高阶函数

函数对象类型

image-20241007141519019

import java.util.List;
import java.util.function.Function;
import java.math.BigInteger;
import java.util.ArrayList;

public class CategoryTest {
    static class Student{
        private String name;
        public Student(String name) {
        	this.name = name;
        }
        public String getName() { return name; }
        public void setName(String name) {this.name=name;}
        @Override
        public boolean equals(object o){...}
        @0verride
        public int hashCode() { return Objects.hash(name);}
    }
   
   public static void main(String[] args) {
       IntPredicate(int a) -> (a & 1) == 0;
       IntPredicate(int a) -> BigInteger.valueOf(a).isProbablePrime(100);
       (int a, int b, int c) -> a + b + c;
       IntBinaryOperator(int a, int b) -> a - b;
       IntBinaryOperator(int a, int b) -> a * b;
       Supplier<Student>() -> new Student();
       Supplier<List<Student>>() -> new ArrayList<Student>();
       Function<Studetn, String> obj8 <String, Student> obj8 = (Student s) -> s.getName();
       Function<Student, Integer> obj9 (Type7)<Integer, Student> obj9 = (Student s) -> s.getAge();
   }
    
    @FunctionalInterface
    interface Type1 {
        boolean op(int a); // 只能有一个抽象方法
    }
    
    @FunctionalInterface
    interface Type7<O, I> {
        O op(I input);
    }
}

常见的函数接口

image-20241007152935426

image-20241007153356440

使用函数接口来解决问题

public class test04 {
    public static void main(String[] args) {
        
        List<Integer> result = supply(5, () -> ThreadLocalRandom.current().nextInt());
        System.out.println(result);
    }
    
    static List<Integer> filter(List<Integer> list, Predicate<Integer> predicate) { ... };
    static List<String> map(List<Integer> list, Function<Integer, String> func) { ... };
    static void consume(List<Integer> list, Consumer<Integer> consumer) { ... }
    
    static List<Integer> supply(int count, Supplier<Integer> supplier) {
        List<Integer> result = new ArrayList<>();
        
        for (int i = 0; i < count; i++) {
            // 生成：随机数，但以后可能改变生成规则
            result.add(supplier.get());
        }
        
        return result;
        /**
        	() -> ThreadLocalRandom.current().nextInt();
        */
    }
}

方法引用

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类名：静态方法

image-20241007160202182

public class MethodRef1 {
    public static void main(String[] args) {
        /*
        	需求：挑选出所有男性学生
        */
        Stream.of (
        		new Student("张无忌", "男"),
            	new Student("周芷若", "女"),
            	new Student("宋青书"，"男")
        )
            // .filter(stu -> stu.sex().equals("男")); lambda 表达式方式
            .filter(MethodRef1::isMale);
        	// .forEach(sut -> System.out.println(sut)) lambda 表达式方式
        	.forEach(MethodRef1::abc); // 静态方法引用方式
        /*
        	(Student stu) -> stu.sex().equals("男")
        	(Student stu) -> MethodRef1.isMale(stu)
        */
        
        public static boolean isMale(Student stu) {
        	return stu.sex().equals("男");    
        }
        
        public static void abc(Student stu) {
            System.out.println(stu);
        }
        
    }
}
类名：非静态方法

image-20241007162258177

public class MethodRef1 {
    public static void main(String[] args) {
        /*
        	需求：挑选出所有男性学生
        */
        Stream.of (
        		new Student("张无忌", "男"),
            	new Student("周芷若", "女"),
            	new Student("宋青书"，"男")
        )
            // .filter(stu -> stu.sex().equals("男")); lambda 表达式方式
            .filter(MethodRef1::isMale);
        	// .forEach(sut -> System.out.println(sut)) lambda 表达式方式
        //	.forEach(MethodRef1::abc); // 静态方法引用方式
        	.forEach(Student::print);
        /*
        	(Student stu) -> stu.sex().equals("男")
        	(Student stu) -> MethodRef1.isMale(stu)
        */
        
        public static boolean isMale(Student stu) {
        	return stu.sex().equals("男");    
        }
        
        public static void abc(Student stu) {
            System.out.println(stu);
        }
        
        record Student(String name, String sex) {
            public void print() {
                System.out.println(this);
            }
            /**
            * Student::print
            * (stu) -> stu.print()
            */
        }
    }
}

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public class MethodRef3 {
    static class Util {
        public boolean isMale(Student stu) {
            return stu.sex().equals("男");
        }
    }
    
    public static void main(String[] args) {
        Util uitl = new Util();
        Stream.of (
        	new Student("张无忌", "男"),
            new Student("周芷若", "女"),
            new Student("宋青书"，"男")
        )
         .filter(util::isMale)
         .map(Student::getName)
         .forEach(System.out::println);
    }
    
    /**
    * (stu) -> util.isMale(stu)
    */
    record Student(String name, String sex) {
        public String getName() {
            return this.name;
        }
        
        /**
        * Student::name
        * stu -> stu.name()
        */
    }
}
构造方法

image-20241007174908470

import java.util.function.Supplier;

@toString
public class MethodRef4 {
    static class Student {
		private final String name;
        private final Integer age;
        
        public Student() { ... }
        
        public Student(String name) { this.name = name }
        
        public Student(String name, Integer age) {this.name = name; this.age = age}
        
        @Override
        public String toString() {
            
        }
    }
    
    public static void main(String[] args) {
        Supplier<Student> s1 = Student::new;
        Function<String, Student> s2 = Student::new;
        BiFunction<String, Integer, Student> s3 = Student::new;
        
        System.out.println(s1.get());
        System.out.println(s2.apply("张三"));
        System.out.println(s3.apply("李四", 25));
    }
}

image-20241007175645967

public class MethodRef5 {
    public static void main(String[] args) {
        Util util = new UtilExt();
        util.hiOrder(Stream.of (
        	new Student("张无忌", "男"),
            new Student("周芷若", "女"),
            new Student("宋青书"，"男")
        ));
    }
    
    record Student(String name, String sex) { }
    
    static class Util {
        // 过滤男性学生并打印
        private bollean isMale(Student stu) {
            return stu.sex().equals("男");
        }
        
         private boolean isFemale(Student stu) {
            return stu.sex().equals("女");
        }
        
        void hiOrder(Stream<Student> stream) {
            stream
         //       .filter(stu->this.isMale(stu))
                .filter(this.isMale)
                .forEach(System.out::println);
        }
    }
    
    static class UtilExt extends Util {
        // 过滤女性学生并打印
       
        void hiOrder(Stream<Student> stream) {
            stream
                .filter(super::isFemale)
                .forEach(System.out::println);
        }
        
    }
}

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对于无需返回值的函数接口，例如 Consumer 和 Runnable 它们可以配合有返回值的函数对象使用

import java.util.function.Consumer;

public class MethodRef7 {
    public static void main(String[] args) {
        Consumer<Object> x = MethodRef7::print1;
        Function<Object, Integer> y = MethodRef7::print2;
        Consumer<Object> z = MethodRef7::print2;
        
        static void print1(Object obj) {
            System.out.println(obj);
        }
        
        static int print2(Object obj) {
            System.out.println(obj);
            
            return 1;
        }
    }
}

public class Exercise4 {
    record Student(String name) { }
    
    // 写出与下列 lamdba表达式 等价的方法引用
    public static void main(String[] args) {
        Function<String, Integer> lambda1 = Integer::parseInt;
        // Function<String, Integer> lambda = (String s) -> Integer.parseInt(S);
        
        // BiPredicate<List<String>, String> lambda2 = (list, element) -> list.contains(element);
        BiPredicate<List<String>, String> lambda2 = List::contains;
        
        // BiPredicate<Student, Object> lambda3 =  (stu, obj) -> stu.equals(obj);
        BiPredicate<Student, Object> lambda3 = Student::equals;
        
        // Predicate<File> lambda4 = (file) -> file.exists();
        Predicate<File> lambda4 = File::exists;
        
        // Runtime runtime = Runtime.getRuntime();
        
        // Supplier<Long> lambda5 = () -> runtime.freeMemory();
        Supplier<Long> lambda5 = Runtime.getRuntime()::freeMemory;
    }
}
例二
public class Exercise5 {
	record Color(Integer red, Integer green, Integer blue) { }
    
    // 如果想用 'Color::new' 来构造 Color 对象，还应当补充哪些代码
    
    public static void main(Stirng[] args) {
        TrenaryFunction lambda = Color::new; // (Integer, Integer, Integer) -> Color
        
        Color white = lambda.create(255, 255, 255);
        System.out.println(white);
        
    }
    
    @FunctionalInterface
    interface TernaryFunction {
        Color create(Integer red, Integer green, Integer blue);
    }
}

public class Exercise6 {
    /**
    * 传入参数时，分别用
    * 类名::静态方法
    * 类名::非静态方法
    * 来表示 [学生年龄大于等于18] 的条件
    */
    static void highOrder(Predicate<Student> predicate) {
        List<Student> list = List.of(
        	new Student("张三", 18),
            new Student("张三", 17),
            new Student("张三", 20)
        );
        
        for (Student stu : list) {
            if (predicate.test(stu)) {
                System.out.println(stu + "通过测试");
            }
        }
        record Student(String name, int age) {
            boolean abc() {
            	return this.age() >= 18;
        	}
        }
       
        static boolean ageGreaterOrEquals18(Student student) {
            return studetn.age() >= 18;
        }
        
        public static void main(String[] args) {
            highOrder(Exercise6::abc);
            
            /**
            * (Exercise6 obj, Student student) -> obj.abc(student)
            */
        }
    }
    
}

闭包

image-20241007191118724

public class ClosureTest1 {
    
    @FunctionalInterface
    interface Lambda {
        int op(int y);
    }
    
    static void highOrder(Lambda lambda) { System.out.println(lambda.op(1)); }
    
    public static void main(String[] args) {
        /**
        *
        * 函数对象 (int y) -> x + y 与它外部的变量 x 形成闭包
        * effective final <===> final
        */
        final int x = 10;
        highOrder((int y) -> x + y);
        
        stu.d = 40;
        highOrder(lambda);
    }
    
    static int a = 1;
    int b = 2;
    
    static class Student {
        int d;
        public Student(int d) {
            this.d = d;
        }
    }
    
    public static void test(int c) {
        highOrder(y -> a + y);
        highOrder(y -> b + y);
        highOrder(y -> c + y);
    }
}

public class ClosureTest2 {
    
    public static void main(String[] args) throws IOException {
        // 闭包作用：给函数对象提供参数以外的数据
        List<Runnable> list = new ArrayList<>();
        for (int i = 0; i < 10; i++) {
            int k = i + 1;
            Runnable task = () -> System.out.println(Thread.currentThread() + "执行任务" + k);
            list.add(task);
        }
        
        ExecutorService service = Executors.newVirtualThreadPerTaskExcutor();
        for (Runnable task : list) {
            service.submit(task);
        }
        
        System.in.read();
    } 
    
}

public class CarryingOTest {
    @FunctionalInterface
    interface F2 {
        int op(int a, int b);
    }
    
    @FunctionalInterface
    interface Fa {
        Fb op(int a);
    }
    
    @FunctionalInterface
    interface Fb {
        int op(int b);
    }
    
    public static void main(String[] args) {
        // 两个参数的函数对象
        F2 f2 = (a, b) -> a + b;
        System.out.println(f2.op(10, 20));
        
        /**
        * 改造
        * (a) -> 返回另一个参数
        * (b) -> a + b
        */ 
        
        Fa fa = (a) -> (b) -> a + b;
        Fb fb = fa.op(10);
        int r = fb.op(20);
        System.out.println(r);
    }
    
    
}

示例二

import java.util.ArrayList;
import java.util.List;

public class Carrying1Test {
    @FunctionalInterface
    interface Fa {
        Fb op(List<Integer> a);
    }
    
    @FunctionalInterface
    interface Fb {
        Fc op(List<Interger> b);
    }
    
    @FunctionalInterface
    interface Fc {
        List<Integer> op(List<Integer> c);
    }
    
    static Fc step2(Fb fb) {
        List<Integer> y = List.of(4, 5, 6);
        return fb.op(y);
    }
    
    static void step3(Fc fc) {
        List<Integer> z = List.of(7, 8, 9);
        List<Integer> result = fc.op(z);
        System.out.println(result);
    }
    
    public static void main(String[] args) {
        step3(step2(step1()));
    }
    
}

高阶函数

所谓高阶，就是指它是其它函数对象的使用者

  • 将通用、复杂的逻辑隐含在高阶函数内
  • 将易变、未定的逻辑放在外部的函数对象中

image-20241007202851624

public class InnerLoop {
	
    public static void main(String[] args) {
        List<Integer> list = List.of(1, 2, 3, 4, 5, 6, 7);
        
        // 需求：逆序遍历集合，只想负责元素处理，不改变集合
        hiOrder(list, (value) -> System.out.println(value));
    }
    
    public static void void hiOrder(List<Integer> list, Consumer<Integer> consumer) {
        ListIterator<Integer> iterator = list.listInterator(list.size());
        while (iterator.hasPrevious()) {
            Integer value = iterator.previous();
            consumer.accept(value);
        }
        
        
    }
}

image-20241007202748128

import java.util.LinkedList;

public class BinaryTree {
    
    public record TreeNode(int value, TreeNode left, TreeNode right) {
        public String toString() { return "%d".formatted(value); }
    }
    
    enum Type {
        PRE, IN, POST
    }
    public static void traversal(TreeNode root, Type type, Consumer<TreeNode> consumer) { 
   		// 用来记住回去的路
        LinkedList<TreeNode> stack = new LinkedList<>();
        // 当前节点
        TreeNode curr = root;
        // 记录最近一次处理完的节点
        TreeNode last = null;
        while (curr != null || !stack.isEmpty()) {
            // 左边未走完
            if (curr != null) {
                // 记住来时的路
                stack.push(curr);
                if (type == Type.PRE) {
                    consumer.accept(curr);
                }
                // 下次向左走
                curr = curr.left;
            } else {
                TreeNode peek = stack.peek();
                
                if (peek.right == null) {
                    if (type == Type.IN || type == Type.POST) {
                        consumer.accept(peek);
                    }
                    last = stack.pop();
                } else if (peek.right == last) {
                    if (type == Type.POST) {
                        consumer.accept(peek);
                    }
                    last = stack.pop();
                } else {
                    
                    if (type == Type.POST) {
                        consumer.accept(peek);
                    }
                    curr = peek.right;
                }
            }
        }
        
        
    }
    
    public static void main(String[] args) {
        /* 
        	1
           / \
          2   3
         /   / \ 
        4   5   6
        *
        */
        
        TreeNode root = new TreeNode(
        	new TreeNode(2, new TreeNode(4, null, null), null),
            new TreeNode(3, new TreeNode(5, null, null), new TreeNode(6, null, null))
        );
        
        traversal(root, Type.PRE, System.out::print);
        System.out.println();
        traversal(root, Type.IN, System.out::print);
        System.out.println();
        traversal(root, Type.POST, System.out::print);
        System.out.println();
    }
}

stream

image-20241007205851325

import java.util.Collection;
import java.util.List;

public class SimpleStream<T> {
    public static void main(String[] args) {
        List<Integer> list = List.of(1, 2, 3, 4, 5);
        
        SimpleStream.of(list)
            		.filter(x -> (x & 1) == 1) 
            		.map(x -> x * x)
            		.forEach(System.out::println);
    }
    
    public static <T> SimpleStream<T> of(Collection<T> collection) {
        return new SimpleStream<>(Collection);
    }
    
    private Collection<T> collection;
    
    private SimpleStream(Collection<T> collection) {
        this.collection = collection;
    }
    
    public SimpleStream filter(Predicate<T> predicate) {
        List<T> result = new ArrayList<>();
        for (T t : collection) {
            if (predicate.test(t)) {
                result.add(t);
            }
        }
        
        return new SimpleStream<>(result);
    }
    
    public <U> SimpleStream<U> map(Function<T, U> function) {
        List<U> result = new ArrayList<>();
        for (T t : collection) {
            U u = function.apply(t);
            result.add(U);
        }
        
        return new SimpleStream<>(result);
    }
    
    public void forEach(Consumer<T> consumer) {
        for (T t : collection) {
			consumer.accept(t);
        }
    }
}

image-20241007212646149

import java.util.Collection;
import java.util.List;

public class SimpleStream<T> {
    public static void main(String[] args) {
        List<Integer> list = List.of(1, 2, 3, 4, 5);
        
        HashSet<Integer> collect = SimpleStream.of(list)
            						.collect(HashSet::new, HashSet::add); // HashSet::add (set, t) -> set.add(t)
        System.out.println(collect);
        
        StringBuilder collect2 = SimpleStream.of(list).collect(StringBuilder::new, StringBuilder::append);
        System.out.println(collect2);
        
        SimpleStream.of(list).collect(() -> new StringJoiner("-"), (joiner, t) -> joiner.add(String.valueOf(t)));
        
        SimpleStream.of(list)
            		.map(t->String.valueOf(t))
            		.collect(()->new StringJoiner("-"), StringJoiner::add);
        System.out.println(collect3);
        
        // (StringJoiner, Integer) -> void
        // (StringJoiner, CharSequence) -> void
        
        
        SimpleStream.of(list)
            		.filter(x -> (x & 1) == 1) 
            		.map(x -> x * x)
            		.forEach(System.out::println);
        
        System.out.println(SimpleStream.of(list).reduce(0, Integer::sum));
        System.out.println(SimpleStream.of(list).reduce(Integer.MAX_VALUE, Math::min));
        System.out.println(SimpleStream.of(list).reduce(Integer.MIN_VALUE, Math.max));
        
    }
    
    public static <T> SimpleStream<T> of(Collection<T> collection) {
        return new SimpleStream<>(Collection);
    }
    
    private Collection<T> collection;
    
    private SimpleStream(Collection<T> collection) {
        this.collection = collection;
    }
    
    // C 代表容器类型，supplier 用来创建容器
	public <C> C collect(Supplier<C> supplier, BiConsumer<C, T> consumer) {
        C c = supplier.get(); //创建了容器
		for (T t : collection) {
            consumer.accept(c, t); // 向容器中添加元素
        }
        
        return c;
    }
    
    // 新增
    public T reduce(T o, BinaryOperator<T> operator) {
        T p = o;
        
        for (T t : collection) { // t是本次遍历的元素
            p = operator.apply(p, t); // 累加器
        }
        
        return p;
    }
    
    public SimpleStream filter(Predicate<T> predicate) {
        List<T> result = new ArrayList<>();
        for (T t : collection) {
            if (predicate.test(t)) { // 过滤器
                result.add(t);
            }
        }
        
        return new SimpleStream<>(result);
    }
    
    public <U> SimpleStream<U> map(Function<T, U> function) {
        List<U> result = new ArrayList<>();
        for (T t : collection) {
            U u = function.apply(t);
            result.add(U);
        }
        
        return new SimpleStream<>(result);
    }
    
    public void forEach(Consumer<T> consumer) {
        for (T t : collection) {
			consumer.accept(t); // 遍历
        }
    }
    
}

image-20241007223754622

public class SimpleStream<T> {
    public static void main(String[] args) {
        List<Integer> list = List.of(1, 2, 3, 4, 5, 1, 2, 3);
        /*
        	key			value
        	1             1
        	2             2
        	3             2
        	4             1
        	5             1
        */
        
        HashMap<Integer, Integer> collect = SimpleStream.of(list)
            .collect(HashMap::new, (map, t) -> {
                if (!map.containsKey(t)) {
                    map.put(t, 1);
                } else {
                    Integer v = map.get(t);
                    map.put(t, v + 1);
                }
            });
        System.out.println(collect);
        
        HashMap<Integer, AtomicInteger> collect2 = SimpleStream.of(List)
            .collect(HashMap::new, (map, t) -> map.computeIfAbsent(t, k -> new AtomicInteger()).getAndIncrement());
        System.out.println(collect2);
        
    }
}

三. Stream API

image-20241007231429134

filter过滤 Predicate

public class FilterTest {
    public static void main(String[] args) {
        Stream.of (
                new Fruit(cname: "草莓", name: "Strawberry", category: "浆果", color: "红色"),
                new Fruit(cname: "桑葚", name: "Mulberry", category: "浆果", color: "紫色"),
                new Fruit(cname: "杨梅", name: "Waxberry", category: "浆果", color: "红色"),
                new Fruit(cname: "核桃", name: "Walnut", category: "坚果", color: "棕色"),
                new Fruit(cname: "花生", name: "Peanut", category: "坚果", color: "棕色"),
                new Fruit(cname: "蓝莓", name: "Blueberry", category: "浆果", color: "蓝色")
        )
            .filter(f->f.category().equals("浆果")) // && f.color().equals("蓝色")
            .filter(f->f.color().equals("蓝色"))
            .forEach(System.out::println);
    }
    
    // Java 17新特性
    record Fruit(String cname, String name, String category, String color) {
        
    }
}

image-20241007232348630

public class FilterTest {
    public static void main(String[] args) {
        Stream.of (
                new Fruit(cname: "草莓", name: "Strawberry", category: "浆果", color: "红色"),
                new Fruit(cname: "桑葚", name: "Mulberry", category: "浆果", color: "紫色"),
                new Fruit(cname: "杨梅", name: "Waxberry", category: "浆果", color: "红色"),
                new Fruit(cname: "核桃", name: "Walnut", category: "坚果", color: "棕色"),
                new Fruit(cname: "花生", name: "Peanut", category: "坚果", color: "棕色"),
                new Fruit(cname: "蓝莓", name: "Blueberry", category: "浆果", color: "蓝色")
        )
        .map(f->f.cname()+"酱") // Stream<String>
        .forEach(System.out::println);
    }
    
    // Java 17新特性
    record Fruit(String cname, String name, String category, String color) {
        
    }
}

image-20241007232741938

public class FilterTest {
    public static void main(String[] args) {
        Integer[][] array2D = {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9},
        };
    
        Arrays.stream(array2D)
            	.flatMap(array -> Arrays.stream(array))
            	.forEach(System.out::println);
        
    // Java 17新特性
    record Fruit(String cname, String name, String category, String color) {
        
    }
}

image-20241007233412077

import java.util.List;

public class BuildStreamTest {
    public static void main(String[] args) {
        
        // 1. 从集合构建
        Set.of(1, 2, 3).stream().forEach(System.out::println);
        Map.of("a", 1, "b", 2).entrySet().stream().forEach(System.out::println);
        
        int[] array = {1, 2, 3}
        Arrays.stream(array).forEach(System.out::println);
        
        // 3. 从对象构建
        Stream.of(1, 2, 3, 4, 5).forEach(System.out::println);
        
        
    }
}

image-20241007233959266

public class ConcatSplitTest {
    public static void main(String[] args) {
        // 1. 合并
        Stream<Integer> s1 = Stream.of(1, 2, 3);
        Stream<Integer> s2 = Stream.of(4, 5);
        
        Stream<Integer> concat = Stream.concat(s1, s2);
        concat.forEach(System.out::println);
        /*
        	2. 截取 - 直接给出截取位置
        	skip(long n)  跳过 n 个数据， 保留剩下的
        	limit(long n) 保留 n 个数据，剩下的不要
        */
        
//        concat.skip(2).forEach(System.out::print);
        concat.limit(2).forEach(System.out::print);
        
        /**
        	takeWhile(Predicate p)  条件成立保留，一旦条件不成立，剩下的不要
        	dropWhile(Predicate p)  条件成立舍弃，一旦条件不成立，剩下的保留
        */
        
        concat.takeWhile(x -> x < 3).forEach(System.out::print);
        concat.dropWhile(x -> x < 3).forEach(System.out::print);
    }
}

image-20241007234741879

public class GenerateTest {
    public static void main(String[] args) {
        // 1. IntStream.range
        
        IntStream.rang(1, 10).forEach(System.out::println);
        IntStream.rangeClosed(1, 1).forEach(System.out::println);
        
        // 2. IntStream.iterate 生成 1 3 5 7 9 ... 奇数序列 可以根据上一个元素值来生成当前元素
        InStream.iterator(1, x -> x + 2).forEach(System.out::println);	
        InStream.iterator(1, x -> x <= 9, x -> x + 2).forEach(System.out::println);
        
        // IntStream.generate
        IntStream.generate(() -> ThreadLocalRandom.current().nextInt(100)).limit(5).forEach(System.out::println);
        
        ThreadLocalRandom.current().ints(streamSize: 5, randomNumberOrigin: 0, randomNumber: 100).forEach(System.out::println);
    }
}

image-20241008091700932

import java.util.stream.IntStream;

public class FindTest {
    public static void main(String[] args) {
        IntStream stream = IntStream.of(1, 2, 3, 4, 5, 6);
        
        // 找到第一个偶数
        System.out.println(stream.filter(x -> (x & 1) == 0).findFirst().orElse(-1));
        
        stream.filter(x -> (x & 1) == 0).findFirst().ifPresent((x)->System.out.println(x));
        // 找到任意一个偶数
        stream.filter(x -> (x & 1) == 0).findAny().ifPresent((x) -> System.out.println(x));
        
        // 见名知意
        System.out.println(stream.anyMatch(x -> (x&1) == 0));
        System.out.println(stream.allMatch(x -> (x & 1)) == 0);
        System.out.println(stream.noneMatch(x -> (x & 1)) == 0);
        
        	
    }
}

去重与排序

public class SortTest {
    public static void main(String[] args) {
        // 去重
        IntStream.of(1, 2, 3, 1, 2, 3, 3, 4, 5)
            	  .distince()
            	  .forEach(System.out::println);
        
        // 排序
        Stream.of (
        			// 排序
                new Hero(name: "令狐冲", strength: 90),
                new Hero(name: "风清扬", strength: 98),
                new Hero(name: "独孤求败", strength: 100),
                new Hero(name: "方证", strength: 92),
                new Hero(name: "东方不败", strength: 98),
                new Hero(name: "冲虚", strength: 90),
                new Hero(name: "向问天", strength: 88),
                new Hero(name: "任我行", strength: 92),
                new Hero(name: "不戒", strength: 88)
        )
            // 重要:简洁但可能存在溢出问题。
            .sorted((a, b) -> a.strength() - b.strength()) // a.strenght < b.strength ? -1 : a.strength() == b.strength() ? 0 : 1
            // .sorted((a, b) -> Integer.compare(a.strength(), b.strength()))
            // .sorted((Comparator.comparingInt(h -> h.strength()))
             .sorted(Comparator.comparingInt(Hero::strength).reverse().thenComparingInt(h->h.name.length()))         
            .forEach(System.out::println);
        
        // Hero::strength <==> (h) -> h.strength();
        record Hero(String name, int strength) {
            
        }
    }
}

reduce

import java.util.stream.Stream;

/*
	简化：两两合并，只剩下一个
	适合：最大值，最小值，求和，求个数...
	.reduce((p, x) -> r) p 上次的合并结果，x 当前元素， r 本次合并结果
	.reduce(init, (p, x) -> r)
	.reduce(init, (p, x) -> r, (r1, r2) -> r)
*/ 
public class ReduceTest {
    record Hero(String name, int strength) { ... }

    public static void main(String[] args) {
        Stream<Hero> result = Stream.of (
            new Hero(name: "令狐冲", strength: 90),
            new Hero(name: "风清扬", strength: 98),
            new Hero(name: "独孤求败", strength: 100),
            new Hero(name: "方证", strength: 92),
            new Hero(name: "东方不败", strength: 98),
            new Hero(name: "冲虚", strength: 90),
            new Hero(name: "向问天", strength: 88),
            new Hero(name: "任我行", strength: 92),
            new Hero(name: "不戒", strength: 88)
        );
        
        // 1.求武力最高的 hero
        Optional<Hero> result = stream.reduce((h1, h2) -> h1.strength() > h2.strength() ? h1 : h2)
		
        Hero result = stream.reduce(new Hero("-", -1), (h1, h2) -> h1.strength() > h2.strength() ? h1 : h2);
        System.out.println(result);
        
        // 求高手总数
        System.out.println(stream.map(h -> 1).reduce(0, (a, b) -> a + b));
        
        System.out.println(stream.count());
        stream.max(Comparator.comparingInt(Hero::strength));
        stream.min(Comparator.comparingInt(Hero::strength));
        
        stream.mapToInt(Hero::strength).sum());
        stream.mapToInt(Hero::strength).average());
    }
}

收集器

image-20241008101936930

import java.util.stream.Stream;

public class C11CollectTest {
    record Hero(String name, int strength) { ... }
    /*
    	收集：将元素收集入容器
    	.collect(() -> c, (c, x) -> void, ?)
    	() -> c			创建容器 c
    	(c, x) -> void  将元素 x 加入 容器 c
    */
    
    public static void main(String[] args) {
        Stream<String> stream = Stream.of("令狐冲", "风清扬", "孤独求败", "方证", "东方不败","冲虚","向问天","任我行","不戒");
        // 1.收集到 List
        List<String> result = stream.collect(() -> new ArrayList<>(), (list, x) -> list.add(x), (a, b) -> { });
        /*
            ArrayList::new 			() -> new ArrayList()
            ArrayList::add 			(list, x) -> list.add(x)
            能等价就等价看自己的风格或要求
        */    
        // 缺点：不太方便调试
        List<String> result = stream.collect(ArrayList::new, ArrayList::add, (a, b) -> { });
        
        Set<String> result = stream.collect(LinkeeHashSet::new, Set::add, (a, b) -> { });
        
        Map<String, Integer> result = stream.collect(HashMap::new, (map, x) -> map.put(x, 1), (a, b) -> { });
        
        // 流空，不实现
        stream.collect(StringBuilder::new, StringBuilder::append, (a, b) -> { });
        
        stream.collect(() -> new StringJoiner(","), StringJoiner:add, (a, b) -> { });
    }
}

image-20241008105156898

image-20241008111021453

import java.util.stream.Stream;

// 收集器 Collectors
public class C11CollectTest {
    record Hero(String name, int strength) { ... }
    /*
    	收集：将元素收集入容器
    	.collect(() -> c, (c, x) -> void, ?)
    	() -> c			创建容器 c
    	(c, x) -> void  将元素 x 加入 容器 c
    */
    
    public static void main(String[] args) {
        Stream<String> stream = Stream.of("令狐冲", "风清扬", "孤独求败", "方证", "东方不败","冲虚","向问天","任我行","不戒");
        // 1.收集到 List
        List<String> result = stream.collect(() -> new ArrayList<>(), (list, x) -> list.add(x), (a, b) -> { });
        /*
            ArrayList::new 			() -> new ArrayList()
            ArrayList::add 			(list, x) -> list.add(x)
            能等价就等价看自己的风格或要求
        */    
        // 缺点：不太方便调试
        List<String> result = stream.collect(ArrayList::new, ArrayList::add, (a, b) -> { }); 
        List<String> result = stream.collect(Collectors.toList());
        
        Set<String> result = stream.collect(LinkeeHashSet::new, Set::add, (a, b) -> { });
        Set<String> result = stream.collect(Collectors.toSet());
        
        Map<String, Integer> result = stream.collect(HashMap::new, (map, x) -> map.put(x, 1), (a, b) -> { });
        Map<String, Integer> result = stream.collect(Collectors.toMap(x -> x, x -> 1));
        
        // 流空，不实现
        stream.collect(StringBuilder::new, StringBuilder::append, (a, b) -> { });
        stream.collect(Collectors.joining;
        
        stream.collect(() -> new StringJoiner(","), StringJoiner:add, (a, b) -> { });
        stream.collect(Collectors.joining(","));
                       
        /*
            3：new ArrayList（["令狐冲", “风清扬", “向问天",“任我行"])
            4：new ArrayList（["独孤求败"，"东方不败"])
            2: new ArrayList（["方证"，“冲虚",“不戒"])
        */
        Map<Integer, List<String>> result = stream.collect(Collectors.groupingBy(x -> x.length(), Collectors.toList()));
                       
        for (Map.Entry<Integer, List<String>> e : result.entrySet()) {
                System.out.println(e);
        }
        
       /**
       	1. mapping(x->y, dc) 需求：根据名字长度分组，分组后组内只保留他们的武力值
            new Hero("令狐冲", 90) -> 90
            dc 下游收集器 down collector 
       */
        Map<Integer, List<Integer>> collect = stream.collect (
        	groupingBy(h -> h.name().length(), mapping(h -> h.strength(), toList())));
                       
        for (Map.Entry<Integer, List<Integer>> e : collect.entrySet()) {
            System.out.println(e);
        }
        
        /*
        * 2. filtering(x -> boolean, dc) 需求：根据名字长度分组，分组后组内过滤掉武力小于 90 的
        */
        
        // 在分组收集的过程中，执行过滤
        Map<Integer, List<Hero>> collect1 = stream.collect (
        	groupingBy(h -> h.name().length(), filtering(h -> h.strength() >= 90, toList()));
        )
        
        // 先过滤，再来分组收集
        Map<Integer, List<Hero>> collect1 = stream.filter(h -> h.strength() >= 90)
                       			.collect(groupingBy(h -> h.name().length(), toList()));
                       
        for (Map.Entry<Integer, List<Hero>> e : collect1.entrySet()) {
            System.out.println(e);
        }
           
        /**
        * 3. flatMapping(x -> substream, dc)          需求：根据名字长度分组，分组后组内保留人名，并且人名切分成单个字符
        */
         // Character::toString (x) -> x.toString(); 
        "令狐冲".chars().mapToObj(Character.toString.forEach(System.out::println);
        
        stream.collect(groupingBy(h -> h.name().length(), flatMapping(h->h.name().chars().mapToObj(Character::toString), toList())));
                               
                               
        for (Map.Entry<Integer, List<String>> e : collect.entrySet()) {
            System.out.println(e);
        }
     
        /*
        * 4. counting() 需求：根据名字长度分组，分组后求每组个数
        */
        Map<Integer, Long> collect = stream.collect(groupingBy(h -> h.name().length(), count()));
        for (Map.Entry<Integer, Long> e : collect.entrySet()) {
            System.out.println(e);
        }
                               
        /*
        * minBy((a, b) -> int) 需求：根据名字长度分组，分组后求每组武功最低的人
        * maxBy((a, b) -> int) 需求：根据名字长度分组，分组后求每组武功最高的人
        */
        stream.collect(groupingBy(h -> h.name().length(), maxBy(Comparator.comparingInt(Hero::strength))));
         
        /*
        * 7. summingInt(x -> int)       需求：根据名字长度分组，分组后求每组武力和
        * 8. averagingDouble(x -> double)  需求：根据名字长度分组，分组后求每组武力平均值
        *
        * stream.collect(groupingBy(h -> h.name().length(), averagingDouble(h -> h.strength())));
        */
        for (Map.Entry<Integer, Optional<Hero>> e : collect.entrySet()) {
            System.out.println(e);
        }
                               
        
        /*
        * 9. reducing(init, (p, x) -> r)
        */
         stream.collect(groupingBy(h -> h.name().length(), mapping(h -> h.strength(), reducing(0, (p, x) -> p + x))));
         
    }
}

基本流

image-20241008160903373

image-20241008161139270

import java.util.stream.DoubleStream;
import java.util.stream.IntStream;
import java.uyil.stream.LongStream;
import java.util.stream.Stream;

public class C14Effective {
    /*
    	三种基本流
    */
    public static void main(String[] args) {
        IntStream a = IntStream.of(97, 98, 99);
        LongStream b = LongStream.of(1L, 2L, 3L);
        DoubleStream c = DoubleStream.of(1.0, 2.0, 3.0);
        
        Stream<Integer> d = Stream.of(1, 2, 3);
        
        a.mapToObj(Character::toString).forEach(System.out::println);
        
        IntSummaryStatistics stat = a.summaryStatistics();
        System.out.println(stat.getSum());
        System.out.println(stat.getCount());
        System.out.println(stat.getMax());
        System.out.println(stat.getMin());
        System.out.println(stat.getAverage());
        
        Stream<Hero> stream = Stream.of(
        		new Hero("令狐葱", 90),
            	new Hero("风清扬"， 98)
        );
            
        stream.mapToInt(Hero::strength).forEach(System.out::println);
        
    }
}

流的特性

import java.util.stream.Stream;

public class C15Summary {
    public static void main(String[] args) {
        /**
        * 掌握 Stream 流的特性
        * 1. 一次使用
        * 2. 两类操作(中间操作 lazy 懒惰， 终结操作 eager 迫切)
        */
        
        Stream<Integer> s1 = Stream.of(1, 2, 3, 4, 5);
        
        s1
            .map(x -> x + 1)
            .filter(x -> x <= 5)
            .forEach(x -> System.out::println);
        
    }
}

image-20241008162707310

image-20241008162741576

image-20241008163214599

image-20241008163345241

stream 并行

import static java.util.stream.Collectors.toList;

// 并行流
public class C16Parallel {
    public static void main(String[] args) {
        /* ... */
        
        List<Integer> collect = Stream.of(1, 2, 3, 4)
            					.parallel()
            					.collect(Collector.of (
                                	() -> new ArrayList(), // 如何创建容器
                                    (list, x) -> list.add(x), // 如何向容器添加数据
                                    (list1, list2) -> {
                                    	list1.addAll(list2);
                                        return list1;
                                    },    // 如何合并两个容器的数据
                                   list -> list      // 收尾
                           // 特性：并发，是否需要收尾，是否要保证收集顺序   (默认)容器不支持并发，需要收尾，要保证收集顺序
                                ));
        
        System.out.println(collect);
    }
    
    private static String simple() { }
}

import static java.util.stream.Collectors.toList;

// 并行流
public class C16Parallel {
    public static void main(String[] args) {
        /* ... */
        
        List<Integer> collect = Stream.of(1, 2, 3, 4)
            		.parallel()
            		.collect(Collertor.of)(
        			() -> {
                        System.out.printf("%-12s %s%n", simple(), "create"); // 1.如何创建容器
                        return new ArrayList<Integer>();
                    },
        			(list, x) -> {
                        List<Integer> old = new ArrayList<>(list);
                        list.add(x);
                        System.out.printf("%-12s %s.add(%d)=>%s%n", simple(), old, x, list);
                    },														// 2. 如何向容器添加数据
            		(list1, list2) -> {
                        List<Integer> old = new ArrayList<>(list1);
                        list1.addAll(list2);
                        System.out.println("%-12s %s.add(%s)=>%s%n", simple(), lod, list2, list3);
                        return list1;
                    }														// 3.如何合并两个容器的数据
            		list -> {
                        System.out.printf("%-12s finish: %s=>%s%n", simple(), list, list);
                        return list;
                    }													// 4.收尾
            		// 5. 特性：容器不支持并发，需要收尾，要保证收集顺序
            		
        	)
            System.out.println(e);
    }
    
    private static String simple() { }
}

image-20241008171250518

public class C16Parallel {
    public static void main(String[] args) {
        /*
        * 1. 数据量问题：数据量大时才建议用并行流
        * 2. 线程会无限增加吗：跟 cpu 能处理的线程数相关
        * 3. 收尾的意义: 转不可变集合，StringBuilder 转 String
        * 4. 是否线程安全: 不会有线程安全问题
        * 5. 特性
        *		是否需要收尾(默认收尾),
        		是否需要保证顺序(默认保证)
        		容器是否支持并发(默认不支持)
        		
        		到达选择哪一种?
        			A. Characteristics.CONCURRENT + Characteristics.UNORDERED + 线程安全容器
                    B. 默认 + 线程不安全容器
        */
        
        List<Integer> collect = Stream.of(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12)
            		.parallel()
            		.collect(Collertor.of(
        			() -> {
                        System.out.printf("%-12s %s%n", simple(), "create"); // 1.如何创建容器
                        return new ArrayList<Integer>();
                    },
        			(list, x) -> {
                        List<Integer> old = new ArrayList<>(list);
                        list.add(x);
                        System.out.printf("%-12s %s.add(%d)=>%s%n", simple(), old, x, list);
                    },														// 2. 如何向容器添加数据
            		(list1, list2) -> {
                        List<Integer> old = new ArrayList<>(list1);
                        list1.addAll(list2);
                        System.out.println("%-12s %s.add(%s)=>%s%n", simple(), lod, list2, list3);
                        return list1;
                    }														// 3.如何合并两个容器的数据
            		list -> {
                        System.out.printf("%-12s finish: %s=>%s%n", simple(), list, list);
                        return Collections.unmodifiableList(list);
                    }													// 4.收尾
            		// 5. 特性：容器不支持并发，需要收尾，要保证收集顺序
            		, Collector.Characteristics.IDENTITY_FINISH			// 不需要收尾
                    ，Collector.Characteristics.UNORDERED				// 不需要保证顺序
                    , Collector.Characteristics.CONCURRENT				// 容器需要支持并发
        	)
                             
            System.out.println(e);
            collect.add(100);
    }
    
    private static String simple() { }

Stream 流的性能

// 性能: 求和 JMH
public class T01Sum {
    @State(Scope.Benchmark)
    public static class MyState { ... }
    
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public int primitive(MyState state) {
        int sum = 0;
        for (int number : state.numbers) {
            sum += number;
        }
        return sum;
    }
    
    @Benchmark
    @BenchmarkMode(Mode.AveragetTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public int boxed(MyState state) { 
    	int sum = 0;
        for (Integer i : state.numberList) {
            sum += i;
        }
        return sum;
    }
    
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public int stream(MyState state) {
        return state.numberList.stream().reduce(0, (a, b) -> a + b);
    }
    
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public int intStream(MyState state) {
        return IntStream.of(state.numbers).sum();
    }
    
    public static void main(String[] args) throws RunnerException {
        Options opt = new OptionsBuilder()
            	.include(TO1Sum.class.getSimpleName())
            	.forks(1)
            	.build();
        
        new Runner(opt).run();
        
    }
}

image-20241008174801962

image-20241008174824092

pubilc class T02Parallel {
    static final int n = 1000000;
    
    @State(Scope.Benchmark)
    pubilc static class MyState {
        int[] numbers = new int[n];
        {
            for (int i = 0; i < n; i++) {
                numbers[i] = ThreadLocalRandom.current().nextInt(10000000);
            }
        }
    }
    
    
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public int primitive(MyState state) {
        int max = 0;
        for (int number : state.numbers) {
            if (number > max) {
                max = number;
            }
        }
        
        return max;
    }
    
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public int sequence(MyState state) {
        return IntStream.of(state.numbers).max().orElse(0);
    }
    
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public int parallel(MyState state) {
        return IntStram.of(state.number).parallel().max().orElse(0);
    }
    
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public int custom(Mystate state) throws ExecutionException, InterruptedException {
        int[] numbers = state.numbers;
        int step = n / 10;
        ArrayList<Future<Integer>> result = new ArrayList<>();
        try (ExecutorService service = Executors.newVirtualThreadPerTaskExecutor()) {
            for (int j = 0; j < n; j += step) {
                int k = j;
                result.add(service.submit(() -> {
                    int max = 0;
                    for (int i = k; i < k + step; i++) {
                        if (numbers[i] > max) {
                            max = numbers[i];
                        }
                    }
                    
                    return max;
                }));
            }
            
            System.out.println(result.size());
			int max = 0;
            for (Future<Integer> future : result) {
                if (future.get() > max) {
                    max = future.get();
                }
            }
            
            return max;
        }
    }
}

image-20241008191153029

image-20241008191332644

import static java.util.stream.Collectors.*;

public class T03Concurrent {
    
    static final int n = 1000000;
    
    @State(Scope.Benchmark)
    public static class MyState {
        int[] numbers = new int[n];
        {
            for (int i = 0; i < n; i++) {
                numbers[i] = ThreadLocalRandom().current().nextInt(n / 10);
            }
        }
    }
    
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public Map<Integer, Integer> loop1(MyState state) {
        Map<Integer, Integer> map = new HashMap<>();
        
        for (int number : state.numbers) {
            map.merge(number, 1, Integer::sum);
        }
        return map;
    }
    
  	@Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public Map<Integer, Integer> loop1(MyState state) {
        Map<Integer, Integer> map = new HashMap<>();
        
        for (int number : state.numbers) {
            map.computeIfAbsent(number, k -> new AtomicInteger()).getAndIncrement();
        }
        return map;
    }  
    
    
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public Map<Integer, Long> sequence(MyState state) {
        return Arrays.stream(state.numbers).boxed()
            	.collect(groupingBy(Function.identity(), counting()));
    }
    
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public Map<Integer, Long> parallelNoConcurrent(MyState state) {
        return Arrays.stream(state.numbers).boxed()
            	.parallel()
            	.collect(groupingBy(Function.identity(), counting()));
    }
    
    @Benchmark
    @BenchmarkMode(Mode.AverageTime)
    @OutputTimeUnit(TimeUnit.NANOSECONDS)
    public ConcurrentMap<Integer, Long> parallelConcurrent(MyState state) {
        return Arrays.stream(state.numbers).boxed()
            	.parallel()
            	.collect(groupingByConcurrent(Function.identity(), counting()));
    }
    
    public static void main(String[] args) throws RunnerException, ExecutionException, InterruptedException {
        
    }
}

image-20241008192804304

image-20241008192959684

四.实际应用统计、异步、框架、并行、事件

统计

static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

public static void main(String[] args) {
    try (Stream<String> lines = Files.lines(Path.of("./data.txt"))) {
        lines.skip(1).limit(5).forEach(line->System.out.println(line));
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
每月的销售量
static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

public static void main(String[] args) {
    try (Stream<String> lines = Files.lines(Path.of("./data.txt"))) {
       long start = System.currentTimeMillis();
        Map<YearMonth, Long> collect = lines.skip(1)
            						.map(line -> line.split(","))
            						.collect(groupingBy(array -> YearMonth).from(formatter.parse(array[TIME]), TreeMap::new, counting()));
        
        System.out.println("花费" + (System.currentTimeMillis() - start));
        
        for (Map.Entry<YearMoth, Long> e : collect.entrySet()) {
            System.out.println(e);
        }
        
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
统计销量最高的月份
static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

public static void main(String[] args) {
   try (Stream<String> lines = Files.lines(Path.of("./data.txt"))) {
     	lines.skip(1)
            	.map(line -> line.split(","))
            	.collect(groupingBy(array -> YearMonth.from(formatter.parse(array[TIME])), counting()))
            	.entrySet().stream()
            	// .max(Comparator.comparingLong(e -> e.getValue()));
            	.max(Map.Entry.comparingByValue())
            	.ifPresent(x -> System.out.println(x));
   } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
销量最高的商品
static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

public static void main(String[] args) {
	case3();
}
private static void case3() {
    try (Stream<String> lines = Files.lines(Paths.get("./data.txt")) {
        lines.skip(1)
            	.map(line->line.split(","))
            	.collect(groupingBy(array->array[PRODUCT_ID], counting()))
            	.entrySet().stream()
            	.max(Map.Entry.comparingByValue())
            	.ifPresent(System.out::println);
        
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
下单最多的前10用户
static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

public static void main(String[] args) {
	case4();
}
private static void case4() {
    try (Stream<String> lines = Files.lines(Paths.get("./data.txt")) {
        lines.skip(1)
            	.map(line -> line.split(",")
                .collect(groupingBy(array -> array[USER_ID], counting));
         
        for (Map.Entry<String, Long> e : collect.entrySet()) {
            System.out.println(e);
        }
         
         collect.entrySet().stream()
                .sorted(Map.Entry.<String, Long>comparingByValue().reversed())
                .limit(10)
                .forEach(System.out::println);
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
最小堆
static class MyQueue<E> extends PriorityQueue<E> {
    private int max;
    public MyQueue(Comparator<? super E> comparator, int max) {
        super(comparator);
        this.max = max;
    }
    
    @Override
    public boolean offer(E e) {
        boolean r = supper.offer(e);
        if (this.size()  > max) {
            this.poll();
        }
        
        return r;
    }
  
}
private static void case4() {
    try (Stream<String> lines = Files.lines(Path.of("./data.txt"))) {
        Map<String, Long> collect = lines.skip(1)
        .map(line -> line.split(","))
        .collect(groupingBy(array -> array[USER_ID], counting()));
        
        // 小顶推
        MyQueue<Map.Entry<String, Long>> map = collect.entrySet().stream()
            .collect (
                () -> new MyQueue<Map.Entry<String, Long>(Map.Entry.comparingByValue())
                (queue, x) -> queue.offer(x),
                (q1, q2) -> q1.addAll(q2)
        );
        
        while(!queue.isEmpty()) {
            System.out.println(queue.poll());
        }
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
每个地区下单最多的用户
static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

public static void main(String[] args) {
	case5();
}
private static void case5() {
    try (Stream<String> lines = Files.lines(Paths.get("./data.txt")) {
       Map<String, Map<String, Long>> collect = lines.skip(1)
           			.map(line -> line.split(","))
           			.collect(groupingBy(array -> array[USER_GEGION], groupingBy(array -> array[USER_ID], counting())));
        
        collect.entrySet().stream.map(e -> Map.entry(
        			e.getKey(),
            		e.getValue().entrySet().stream()
            			.max(Map.Entry.comparingByValue())
        ))
            .forEach(System.out::println);
        
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
每个地区下单最多的前3用户
static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

public static void main(String[] args) {
	case5();
}
private static void case5() {
    try (Stream<String> lines = Files.lines(Paths.get("./data.txt")) {
       Map<String, Map<String, Long>> collect = lines.skip(1)
           			.map(line -> line.split(","))
           			.collect(groupingBy(array -> array[USER_GEGION], groupingBy(array -> array[USER_ID], counting())));
        
        collect.entrySet().stream.map(e -> Map.entry(
        			e.getKey(),
            //		e.getValue().entrySet().stream()
            //			.sorted(Map.Entry.comparingByValue().reverse())
            //			.limit(3)
            //			.collect(toList())
            		e.getValue().entrySet().stream()
            			.collect (
                        		() -> new MyQueue<Map.Entry<String, Long>> (Map.Entry.comparingByValue(), 3)
                            	MyQueue::offer
                            	MyQueue::addAll
                        )
        ))
            .forEach(System.out::println);
        
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
按一级类别统计销量
static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

public static void main(String[] args) {
	case6();
}
static String firstCategory(String[] array) {
    String c = array[CATEGORY_CODE];
    int idx = c.indexOf(".");
    return c.substring(0, idx);
}
private static void case6() {
    try (Stream<String> lines = Files.lines(Paths.get("./data.txt")) {
       Map<String, Long> collect = lines.skip(1)
           				.map(line -> line.split(","))
           				.filter(array -> !array[CATEGORY_CODE].isEmpty())
           				.collect(groupingBy(TestData::firstCategory, TreeMap::new, counting()));
        
        for (Map.Entry<String, Long> e : collect.entrySet()) {
            System.out.println(e);
        }
        
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
按区间统计销量
static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

public static void main(String[] args) {
	case7();
}
static String priceRange(Double price) {
    if (price < 100) {
        return "[0, 100)";
    } else if (price >= 100 && price < 500) {
        return "[100, 500)"
    } else if (price >= 500 && price < 1000) {
        return "[500, 1000)";
    } else {
        return "[1000, 无穷)";
    }
}
private static void case6() {
    try (Stream<String> lines = Files.lines(Paths.get("./data.txt")) {
       lines.skip(1)
           	.map(line -> line.split(","))
            .map(array->Double.valueOf(array[PRICE]))
            .collect(groupingBy(TestData::priceRange, counting()));
        
        // 打印集合 略
           
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
不同年龄段女性下不同类别订单
static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

public static void main(String[] args) {
	case8();
}
static String ageRange(Double price) {
    int age = Double.valueOf(array[USER_AGE]).intValue();
    if (age < 18) {
        return "[0, 18)";
    } else if (age < 30) {
        return "[18, 30)";
    } else if (age < 50) {
        return "[30, 50)";
    } else {
        return "[50, 无穷)"
    }
}
private static void case8() {
    try (Stream<String> lines = Files.lines(Paths.get("./data.txt")) {
       lines.skip(1)
           	.map(line -> line.split(","))
            .filter(array->array[USER_SEX].equals("女"))
        	.filter(array-> array[CATEGORY_CODE].isEmpty())
            .collect(groupingBy(TestData::ageRange, 
                                groupingBy(TestData::firstCategory, TreeMap::new, counting())));
        
        for (Map.Entry<String, Map<String, Long>> e1 : map.entrySet()) {
            for (Map.Entry<String, Long> e2 : e1.getValue().entrySet()) {
                System.out.println("%-12s%-15s%d%n", e1.getKey(), e2.getKey(), e2.getValue());
            }
        }
        
        
        // 打印集合 略
           
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}

异步处理

static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

static Logger logger = LoggerFactory.getLogger("Test");

// 1. 显式使用了线程池
// 2. 函数对象嵌套使用，可读行差
public static void main(String[] args) {
	logger.info("开始统");
    // monthlySalesReport();
    new Thread()->monthlySalesReport().start(); // 异步代码调用
    logger.info("执行其他操作");
    /*
    	目标：将处理结果的逻辑放在 monthlySalesReport 之外
    	做法1：将结果作为方法的返回值返回
    	做法2：将处理结果的逻辑作为函数对象传递给方法
    */
    try (ExcutorService service = Executors.newFixedThreadPool(3)) {
        logger.info("开始统计");
        service.submit(()-> {
            monthlySalesReport((map) -> {
                logger.info(e.toString());
            });
        });
        logger.info("执行其他操作");
    }
    
}

private static void monthlySalesReport(Counsumer<Map<YearMonth, Long>> consumer) {
    try (Stream<String> lines = Files.lines(Paths.get("./data.txt")) {
       		Map<YearMonth, Long> map = lines.skip(1)
                			.map(line -> line.split(","))
                			.collect(groupingBy(array -> YearMonth.from(formatter.parse(array[TIME], TreeMap::new, counting())));
                  
          return collect;                           
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}
异步优化
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import java.util.concurrent.CompletebleFuture;

public class C03CompletableFutureTest {
    static Logger logger = loggerFactory.getLogger("Test");
    
    public static void main(String[] args) {
        // 1. 异步执行
        CompletableFuture.runAsync()     // 在任务不需要返回结果时
        CompletableFuture.supplyAsync() // 在任务需要处理结果时
            
            
         CompletableFuture.runAsync(() -> logger.info("异步操作1"));   
         CompletableFuture.supplyAsync(() -> {
             logger.info("异步操作2");
             return "结果";
         }).thenApply(r -> r + "转换后")
             .thenAccept(r -> logger.info(r));
        
        System.in.read(); // 不让主线程立刻结束
        
        
         // 2.处理异步任务的结果
         /*
          thenApply(Function)
          thenApplyAsync
          thenAccept(Consumer)
          thenAcceptAsunc(Consumer)
         */
        
        
    }
}

static final int INDEX = 0;
static final int TIME = 1;
static final int ORDER_ID = 2;
static final int PRODUCT_ID = 3;
static final int CATEGORY_ID = 4;
static final int CATEGORY_CODE = 5;
static final int BRAND = 6;
static final PRICE = 7;
static final int USER_ID = 8;
static final int USER_AGE = 9;
static final int USER_SEX = 10;
static final int USER_REGION = 11;
static final int DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy-MM--dd HH:mm:ss z");

static Logger logger = LoggerFactory.getLogger("Test");

// 1. 显式使用了线程池
// 2. 函数对象嵌套使用，可读行差
public static void main(String[] args) {
	
   CompletableFuture
       	.supplyAsync(() -> monthlySalesReport())
    	.thenAccept(map -> {
            for (Map.Entry<YearMonth, Long> e : map.entrySet()) {
                logger.info(e.toString());
            }
        });
    
    System.in.read();
    
    try (ExcutorService service = Executors.newFixedThreadPool(3)) {
        logger.info("开始统计");
        CompletableFuture
            	.supplyAsync(() -> monthlySalesReport)
            	.thenAccept(map -> {
                    for (Map.Entry<YearMonth, Long> e : map.entrySet()) {
                        logger.info(e. toString());
                    }
                });
        
        logger.info("执行其他操作");
        System.in.read();
    }
    
}

private static void monthlySalesReport(Counsumer<Map<YearMonth, Long>> consumer) {
    try (Stream<String> lines = Files.lines(Paths.get("./data.txt")) {
       		Map<YearMonth, Long> map = lines.skip(1)
                			.map(line -> line.split(","))
                			.collect(groupingBy(array -> YearMonth.from(formatter.parse(array[TIME], TreeMap::new, counting())));
                  
          return collect;                           
    } catch (IOException e) {
        throw new RuntimeException(e);
    }
}

框架 跳过 博主 还没学呢 😂（预计11月份开Spring全家桶)

  • 什么是框架?
    • 半成品软件，帮助开发者快速构建应用程序
    • 框架提供的都是固定不变的、已知的、可以重用的代码
    • 而那些每个应用不同的业务逻辑，变化的、未知的部分，则在框架外由开发者自己实现
将未知交给子类
  • Spring 延迟创建 bean

image-20241008223538442

  1. public class C01JdbcTemlate {
    public static void main(String[] args) {
        HikariDataSoure dataSouce = new HikariDataSource();
        // 链接 mysql 服务器
        dataSource.setJdbcUrl("jdbc:mysql://localhost:3306/test")
        dataSource.setUsername("root")
        dataSource.setPassword("root");
        
     //   try (Connection conn = dataSource.getConnection()) {
    //        PreparedStatement stat = conn.prepareStatement("select * from student");
     //       ResultSet rs = stat.executeQuery();
       //     while (rs.next()) {
      //          int id = rs.getInt("id");
    //            String name = rs.getString("name");
    //            String sex = rs.getString("sex");
         //       list.add(new Student(id, name, sex));
         //   }
      //  } catch (SQLException e) {
      //      throw new RuntimeException(e);
      //  }
        
        JdbcTemplate template = new JdbcTemplate(dataSource);
        List<Student> list = template.query("select * from student", (rs, row) -> {
            int id = rs.getInt("id");
            String name = rs.getString("name");
            String sex = rs.getString("sex");
            return new Student(id, name, sex);
        });
        
        for (Student student : list) {
            System.out.println(student);
        }
    }
    
    record Student(int id, String name, String sex) {
        
    }
}
  • Spring
  • SpringBoot
序列化
public class TestSerializable { 
    public static void main(String[] args) throws Excepton {
        // 可序列化的函数对象
        Type1 lambda = (Type1 & Serializable) Student::getName;
        
        // 函数对象 <==> 字节码 会额外存储类和方法的信息，运行时就可以根据这些信息找到属性，从而进一步确定(列名)
        /*
        	for (Method method : lambda.getClass().getDeclaredMethods()) {
        		System.out.println(method);
        	}
        */
        
        SerializaedLambda invoke = (SerializedLambda) lambda.getClass().getDeclaredMethod("writeReplace").invoke(lambda);
        
        System.out.println(invoke.getClass()); // 哪个类使用了这个函数对象
        System.out.println(invoke.getImpClass()); // 哪个类实现了函数对象的逻辑
        System.out.println(invoke.getImplMethodName()); // 哪个方法实现了函数对象的逻辑
    }
    
    interface Type1 {
        String abc(Student student);
    }
}

并行

统计 Web 页面的访问次数
import static java.util.stream.Collectors.*;

public class ParalletTest {
    static Pattern reg = Pattern.compile("(\\S+) - \\[(.+)] (.+) (.+)");
    
    private static final int FILES = 100;
    
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        Map<String, Long> map = one(0);
        for (Map.Entry<String, Long> e : map.entrySet()) {
            System.out.println(e);
        }
        
    }
    
    private static Map<String, Long> one(int i) {
        try(Stream<String> lines = Files.lines(Path.of(String.format("web_server_access_%d.log", i)))) {
        	    return lines.map(line->reg.matcher(line)) // reg::matcher
                  //  	.limit(10)
                    	.filter(mather -> mather.find()) // Matcher::find
                    	.map(matcher -> new String[]{matcher.group(1), matcher.group(2), matcher.group(3), matcher.group(4)})
                    	.collect(groupingBy(array -> array[2], counting()));
        } catch {
			throw new RuntimeException(e);
        }
        
        
    }
    
    private static void sequence() {
        long start == System.currentTimeMillis();
        Map<String, Long> m0 = new HashMap<>();
        for (int i = 0; i < FILES; i++) {
            Map<String, Long> mi = one(i);
            m0 = merge(m0, mi);
        }
        
        for (Map.Entry<String, Long> e : m0.entrySet()) {
            System.out.println(e);
        }
        
        System.our.println("cost: " + (System.currentTimeMillis() - start));
    }
    
    static Map<String, Long> merge(Map<String, Long> m1, Map<String, Long> m2) {
        
        return Stream.of(m1, m2)
            	.flatMap(m -> m.entrySet().stream())
            	.collect(toMap(Map.Entry::getKey, Map.Entry::getValue, (v1, v2) -> v1 + v2)); 
    }
    
    private static void parallel() throws InterruptedException, ExecutionException {
        long start = System.currentTimeMillsi();
        for (int i = 0; i < FiLES; i++) {
            int k = i;
            CompletableFuture.supplyAsync(() -> one(k));
        }
        
        Completablefuture<Map<String, Long>> f0 = futures.getFirst();
        for (int i = 1; i < futures.size(); i++) {
            CompletableFuture<Map<String, Long>> fi = futures.get(i);
            f0.thenCombine(fi, (m0, mi) -> merge(m0, mi));
        }
        
        Map<String, Long> map = f0.get();
        for (Map.Entry<String, Long> e : map.entrySet()) {
            System.out.println(e);
        }
        
        System.out.println("cost: " + (System.currentTimeMillis() - start));
    }
}

UI 事件

import javax.swing.*;

// 在 UI 设计时的应用
public class UITest {
    public static void main(String[] args) {
        JFrame frame = new JFrame();
        
        JButton button = new JButton("点我");
        
        button.addActionListener(e -> System.out.println("已点击"));
        frame.add(button);
        
        
        frame.setSize(300, 200);
        frame.setVisible(true);
    }
}

五.实现原理 Lambda、方法引用、闭包、可切分迭代器

原理 Lambda

import java.util.function.BinaryOperator;

public class C01Lambda1 {
    
    public static void main(String[] args) throws Throwable {
        
        BinaryOperator<Integer> lambda = (a, b) -> a + b;
        
        /*
        	lambda 表达式是一种语法糖， 它仍然会被翻译成 类，对象，方法
        1. 方法从哪来：编译器发现代码中出现了 lambda, 就会在当前类中生成 private static 方法，方法内包含的就是 lambda 的逻辑实验代码
            for (Method method : C01Lambda1.class.getDeclaredMethods()) {
                System.out.println(method);
            }
          
        2. 类和对象从哪来: 运行期间动态生成
        		MethodHandle
       		MethodHandle 的执行权限与上下文相关
            	原本有权限调用的方法，正常能运行，通过 MethodHandle 反射也能调用
            	原本没权限调用的方法，正常不能调用， MethodHandle 反射也调用不了
        3. 
       
        1. 反射调用静态方法
        MethodHandle mh = MethodHandles.lookup().findStatic(C01Lambda1.class, "lambda$main$2"),
        	MethodType.methodType(Integer.class, Integer.class, Integer.class);
        
        System.out.println(mh.invoke(1, 2));
        // 反射调用非静态方法
        MethodHandle mh2 = MethodHandles.lookup().findVirtual(MyLambda.class, "apply", MethodType.methodType(Integer.class, Integer.class, Integer.class));
        
        // 反射调用构造器方法
        MethodHandle mh3 = MethodHandles.lookup().findConstructor(MyLambda.class, MethodType.methodType(void.class));
        System.out.println(mh3.invoke()); //new MyLambda()
        
        System.out.println(new MyLambda());
        
        */
        
        MethodHandles.Lookup lookup = MethodHandles.lookup();
        MethodHandle imp1 = lookup.findStatic(CO1Lambda1.class, "lambda$main$2",
                   MethodType.methodType(Integer.class, Integer.class, Integer.class));
        
        // 内部: 生成函数对象所需的类
        LambdaMetafactory.metafactory (
        	, // 1. lookup
            , // 2. 接口方法名
            , // 3. 创建函数对象工厂方法长相   BinaryOperator factory()
            , // 4. 接口方法长相
            , // 5. 实现方法(本例就是下面的静态方法 lambda$main$2)
            , // 6. 函数对象实际长相
            
            lookup,
            "apply",
            MethodType.methodType(BinaryOperator.class),
            MetodType.methodType(Object.class, Object.class, Object.class)
            impl,
            MethodType.methodType(Integer.class, Integer.class, Integer.class)
        );
        
        // BinaryOperator factory() { return new MyLambda() }
        MethodHandle mh = cs.getTarget(); // 就是函数对象工厂方法
		BinaryOperator<Integer> invoke = (BinaryOperator<Integer>) mh.invoke();
        System.out.println(invoke.apply(5, 6));
        
    }
        
        static final class MyLambda implements BinaryOperator<Integer> {
            private MyLambda() { }
            @Override
            public Integer apply(Integer e, Integer b) {
                returen lambda$main$0(a, b);
            }
        }
        
        
        private static Integer lambda$main$2(Integer a, Integer b) {
            return a + b;
        }
}

image-20241009091026289

image-20241009091117261

函数入口
import java.util.function.BinaryOperator;

public class C01Lambda1 {
    
    public static void main(String[] args) throws Throwable {
        
        BinaryOperator<Integer> lambda = (a, b) -> a + b;
       
    }
        
        static final class MyLambda implements BinaryOperator<Integer> {
            private MyLambda() { }
            @Override
            public Integer apply(Integer e, Integer b) {
                returen lambda$main$0(a, b);
            }
        }
        
        
        private static Integer lambda$main$2(Integer a, Integer b) {
            return a + b;
        }
}

重定向

image-20241009094105166

方法引用

public class C02MethodReference {
    public static void main(String[] args) {
        // 方法引用是一种语法糖，它仍然会被翻译成 类，对象，方法
        // 1.方法从哪来
        // 2.类，对象从哪来
        
        Function<Student, String> func = Student::getName(); // stu-> stu.getName()
        
        MethodHandles.Lookup lookup = MethodHandles.lookup();
        
        // 反射调用非静态方法
        MethodHandle impl = lookup.findVirtual(Student.class, "getName", MethodType.methodType(String.class));
        
        CallSite cs = LambdaMetafactory.metafactory (
        		lookup,
            	"apply",
            	MethodType.methodType(Function.class),
            	MethodType.methodType(Object.class, Object.class), // 实现接口：两个泛型参数
            	impl,	
            	MethodType.methodType(String.class, Student.class) // 返回类型 + 参数类型
        );
        
        Function<Student, String> invoke = (Function<Student, String>) cs.getTarget().invoke();
        Student stu = new Student();
        stu.name = "张三";
        System.out.println(invoke.apply(stu));
        
    }
    
    static final class MyMethodReference implements Function<Student, String> {
        
        @Override
        public String apply(Student student) {
            return student.getName();
        }
    }
    
    
    static class Student {
        
        private String name;
        
        public String getName() {
            return name;
        }
    }
}

image-20241009100513631

闭包

import java.util.function.BinaryOperator;

// -Djdk.invoke.LambdaMetafactory.dumpProxyClassFiles
public class C03Closure {
    public static void main(String[] args) {
        int c = 10;
        BinaryOperator<Integer> lambda = (a, b) -> a + b + c; // invoke dynamic
        test(lambda);
        
        for (Method method : C03Closure.class.getDeclaremethods()) {
            System.out.println(method);
        }
        
        // 1.方法
        // 2.类和对象
        static void test(BinaryOperator<Integer> lambda) {
            
        }
        
        
        final static class C03Closure$$Lambda implements BinaryOperator {
            private final int c;
            
            private C03Closure$$Lambda(int c) { this.c = c;}
            
            public Object apply(Object a, Object b) {
                return C03Clsure.lambda$main$1(this.c, (Integer)a, (Integer)b);
            }
        }
        // 反编译
        static private Integer lambda$main$1(int c, Integer a, Integer b) {
            reutrn a + b + c;
        }
    }
}
静态变量
public class C04Closure {
    static int c = 10;
    public static void main(String[] args) {
        BinaryOperator<Integer> lambda = (a, b) - a + b + c;
        /*
        	System.out.println(lambda.apply(1, 2));
        	c = 20;
        	System.out.println(lambda.apply(1, 2));
        */
        
        for (Method method : C04Closure.class.getDeclaredMethods()) {
            System.out.println(method);
        }
        
        static final class C04Closure$$Lambda implements BinaryOperator {
            private c04Closure$$Lambda() {
                
            }
            
            public Object apply(Object a, Object b) {
                return C04Closure.lambda$main$1((Integer)a, (Integer)b );
            }
        }
    }
    
    private static Integer lambda$1(Integer a, Integer b) {
        return a + b + C04Closure.c;
    }
}

可切分迭代器

image-20241009103201318

public class C06Spliterator {
    public static void main(String[] args) {
        List<Integer> list = List.of(1, 2, 3, 4, 5, 6, 7, 8, 9);
        Integer[] array = {1, 2, 3, 4, 5, 6, 7, 8, 9};
        
        // Stream<Integer> s1 = list.stream();
        // Stream<Integer> s2 = Arrays.stream(array);
        
        Spliterator<Integer> sp1 = list.spliterator();
        
        Spliterator<Integer> sp2 = sp1.trySplit();
        Spliterator<Integer> sp3 = sp2.trySplit();
        
        System.out.println("sp1:");
        sp1.tryAdvance(System.out::println);
        System.out.println("sp2:");
        sp2.tryAdvance(System.out::println);
        System.out.println("sp3:")
        sp3.tryAdvance(System.out::println);
        System.out.println("==========================");
        
        
        CompletableFuture.supplyAsync(() -> StreamSupport.stream(sp2, false).reduce(0, Integer::sum))
            	.thenAccept(x -> logger.info("{}", x));
        
        CompletableFuture.supplyAsync(() -> StreamSupport.stream(sp3, false).reduce(0, Integer::sum))
            	.thenAccept(x -> logger.info("{}", x));
        
        System.in.read();
    }
}

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