概述
- 在Java8中，得益于lambda所带来的函数式编程，引入了一个全新的Stream流的概念
- 目的：用于简化集合和数组操作的api
案例
需求：创建一个集合存储多个字符串元素，将集合中所有以“z”开头的元素存储到新的集合中，再把这个新集合中长度为3的元素存储到另一个新集合中，遍历输出最新集合的元素
传统方式：
Stream流：
stream流就像一个流水线，先将所有元素放上去，再借助简化的api操作元素并得到最后想要的元素
Stream流的获取
Stream操作集合或者数组的第一步是先得到Stream流，然后才能使用流的功能
代码参考
//Collection集合获取流
Collection<String> list = new ArrayList<>();
Stream<String> s = list.stream();
//Map集合获取流
Map<String,Integer> maps = new HashMap<>();
//先获取键流
Stream<String> keyStream = maps.keySet().stream();
//在获取值流
Stream<Integer> valueStream = maps.values().stream();
//键值对流
Stream<Map.Entry<String,Integer>> keyAndValueStream = maps.entrySet().stream();
//数组获取流
String[] arr = {"gst","zcy","xhk"};
Stream<String> arrStream = Arrays.stream(arr);
Stream<String> arrStream2 = Stream.of(arr);Stream流的常用API
代码参考
List<String> list = new ArrayList<>();
list.add("gst");
list.add("zcy");
list.add("txw");
list.add("xhk");
list.add("zja");
list.add("hb");
list.add("zz");
list.stream().filter(s -> s.startsWith("z")).forEach(s -> System.out.println(s));
System.out.println("------------");
long size = list.stream().filter(s -> s.length() == 3).count();
System.out.println(size);
System.out.println("------------");
list.stream().filter(s -> s.startsWith("z")).limit(2).forEach(s -> System.out.println(s));//个数限制为2个
// list.stream().filter(s -> s.startsWith("z")).limit(2).forEach(System.out::println);//简化
System.out.println("------------");
list.stream().filter(s -> s.startsWith("z")).skip(2).forEach(System.out::println);//跳过前两个
System.out.println("------------");
//Map加工方法
//给集合元素的前面都加上gst
list.stream().map(s -> "gst的"+s).forEach(System.out::println);
System.out.println("------------");
//把所有的名称都加工成学生对象
list.stream().map(s -> new Student(s)).forEach(System.out::println);
// list.stream().map(Student::new).forEach(System.out::println);//简化
System.out.println("------------");
//合并流
Stream<String> s1 = list.stream().filter(s -> s.startsWith("z"));
Stream<String> s2 = Stream.of("Java1","Java2");
Stream<String> s3 = Stream.concat(s1,s2);
s3.forEach(System.out::println);
//注意：这里两个流如果是不同类型的合并时要用Object类型接
Stream流的综合应用
代码参考
public class Topperformer {
private String name;
private double money;//月薪
public Topperformer() {
}
public Topperformer(String name, double money) {
this.name = name;
this.money = money;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public double getMoney() {
return money;
}
public void setMoney(double money) {
this.money = money;
}
@Override
public String toString() {
return "Topperformer{" +
"name='" + name + '\'' +
", money=" + money +
'}';
}
}
public class Employee {
private String name;
private char sex;
private double salary;
private double bonus;
private String punish;//处罚信息
public Employee() {
}
public Employee(String name, char sex, double salary, double bonus, String punish) {
this.name = name;
this.sex = sex;
this.salary = salary;
this.bonus = bonus;
this.punish = punish;
}
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
public char getSex() {
return sex;
}
public void setSex(char sex) {
this.sex = sex;
}
public double getSalary() {
return salary;
}
public void setSalary(double salary) {
this.salary = salary;
}
public double getBonus() {
return bonus;
}
public void setBonus(double bonus) {
this.bonus = bonus;
}
public String getPunish() {
return punish;
}
public void setPunish(String punish) {
this.punish = punish;
}
@Override
public String toString() {
return "Employee{" +
"name='" + name + '\'' +
", sex=" + sex +
", salary=" + salary +
", bonus=" + bonus +
", punish='" + punish + '\'' +
'}';
}
}
public class StreamDemo04 {
public static double allMoney = 0;
public static double allMoney2 = 0;
public static double allMoney3 = 0;
public static void main(String[] args) {
List<Employee> one = new ArrayList<>();
one.add(new Employee("gst",'女',30000,20000,null));
one.add(new Employee("zcy",'男',35000,20000,"被打"));
one.add(new Employee("txw",'女',30000,20000,"被打"));
one.add(new Employee("xhk",'男',35000,20000,"被打"));
List<Employee> two = new ArrayList<>();
two.add(new Employee("ggg",'女',35000,15000,null));
two.add(new Employee("zzz",'男',30000,20000,"被打"));
two.add(new Employee("ttt",'女',20000,10000,null));
two.add(new Employee("xxx",'男',30000,20000,"被打"));
two.add(new Employee("qqq",'男',40000,20000,"被打"));
//1.一部门的最高工资
//制定大小规则
// Employee e = one.stream().max((e1,e2) ->Double.compare(e1.getSalary()+e1.getBonus(),e2.getSalary()+e2.getBonus()))
// .get();
// System.out.println(e);
Topperformer t = one.stream().max((e1,e2) ->Double.compare(e1.getSalary()+e1.getBonus(),e2.getSalary()+e2.getBonus()))
.map(e -> new Topperformer(e.getName(),e.getSalary()+e.getBonus())).get();
System.out.println(t);
//二部门的最高工资
Topperformer t2 = two.stream().max((e1,e2) ->Double.compare(e1.getSalary()+e1.getBonus(),e2.getSalary()+e2.getBonus()))
.map(e -> new Topperformer(e.getName(),e.getSalary()+e.getBonus())).get();
System.out.println(t2);
//2.统计平均工资，去掉最低工资和最高工资
//先排序
one.stream().sorted((e1,e2) ->Double.compare(e1.getSalary()+e1.getBonus(),e2.getSalary()+e2.getBonus()))
.skip(1).limit(one.size()-2).forEach(e -> {
//求出总和
allMoney += (e.getSalary()+e.getBonus());
});
System.out.println("开发一部的平均工资："+allMoney/(one.size()-2));
two.stream().sorted((e1,e2) ->Double.compare(e1.getSalary()+e1.getBonus(),e2.getSalary()+e2.getBonus()))
.skip(1).limit(two.size()-2).forEach(e -> {
//求出总和
allMoney2 += (e.getSalary()+e.getBonus());
});
System.out.println("开发一部的平均工资："+allMoney2/(one.size()-2));
//3.合并2个集合，在统计
Stream<Employee> s1 = one.stream();
Stream<Employee> s2 = two.stream();
Stream<Employee> s3 = Stream.concat(s1,s2);
s3.sorted((e1,e2) ->Double.compare(e1.getSalary()+e1.getBonus(),e2.getSalary()+e2.getBonus()))
.skip(1).limit(one.size()+two.size()-2).forEach(e -> {
//求出总和
allMoney3 += (e.getSalary()+e.getBonus());
});
//用BigDecimal来解决精度问题
BigDecimal a = BigDecimal.valueOf(allMoney2);
BigDecimal b = BigDecimal.valueOf(one.size()+two.size()-2);
System.out.println("开发部门的平均工资是："+a.divide(b,2, RoundingMode.HALF_UP));
}
}收集Stream流
代码参考
List<String> list = new ArrayList<>();
list.add("gst");
list.add("zcy");
list.add("txw");
list.add("xhk");
list.add("zja");
list.add("hb");
list.add("zz");
Stream<String> s1 = list.stream().filter(s -> s.startsWith("z"));
List<String> zlist = s1.collect(Collectors.toList());
System.out.println(zlist);
// //注意：流只能使用一次，上述以及将流里的数据收集走了就等于没有了
// Set<String> zset = s1.collect(Collectors.toSet());
// System.out.println(zset);
//只能再重新获取
// Stream<String> s2 = list.stream().filter(s -> s.startsWith("z"));
// Set<String> zset = s2.collect(Collectors.toSet());
// System.out.println(zset);
Stream<String> s3 = list.stream().filter(s -> s.startsWith("z"));
Object[] arrs = s3.toArray();
System.out.println(Arrays.toString(arrs));
