jdk8

新特性stream深受喜爱，平时使用比较多，其中有：

Map<String, String> collect2 = 
list.stream().collect(Collectors.toMap(Book::getName, Book::getIdNO,(pre, after) -> pre));

 现象如下：

package MainTest.stream;

import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

public class StreamListToMap {

    public static void main(String[] args) {
        List<Book> list = new ArrayList<>();
        list.add(new Book("master amber", 1987, "ASHJBYGSFDFG15"));
        list.add(new Book("slaver amber", 1987, "IJFGNMKETRE58"));
        list.add(new Book("master amber", 1999, "MYHGORPJFJ64"));
        list.add(new Book("ssdas", 12414, null));
        list.add(new Book("fkajh", 32423, null));


        // 问题1：jdk8这种情况下，value为空会报错，jdk9解决了这个问题(key为null没有问题)
        System.out.println("==============");
        Map<String, String> collect2 = list.stream().collect(Collectors.toMap(Book::getName, Book::getIdNo,(pre, after) -> pre));
        for (String key : collect2.keySet()) {
            System.out.println("key:"+key + "=======value:"+collect2.get(key));
        }

    }


    public static class Book {
        private String name;

        private int year;

        private String idNo;

        Book(String name, int year, String idNo) {
            this.name = name;
            this.year = year;
            this.idNo = idNo;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getYear() {
            return year;
        }

        public void setYear(int year) {
            this.year = year;
        }

        public String getIdNo() {
            return idNo;
        }

        public void setIdNo(String idNo) {
            this.idNo = idNo;
        }
    }
}

问题：JDK8环境下，如果Book::getIdNo 为null的话会报空指针,即JDK8环境下的，该接口不支持value为null（key为null没这个问题）

<R, A> R collect(Collector<? super T, A, R> collector);

该接口不支持valueMapper为null，JDK9解决了这个问题 

解决方案：

1.升级JDK9

2.按需求可以过滤掉null的value 

3.使用另一个collect接口

<R> R collect(Supplier<R> supplier,BiConsumer<R, ? super T> accumulator,
              BiConsumer<R, R> combiner);

Supplier<R> supplier ：收集器返回的容器

BiConsumer<R, ? super T> accumulator ： 消费者，R参数是返回结果类型，? super T 一般值list内元素

BiConsumer<R, R> combiner：联合器，主要是并行流下使用（parallel），非并行流不使用改参数，但不能为null

package MainTest.stream;

import java.util.*;
import java.util.function.Function;
import java.util.stream.Collectors;

public class StreamListToMap {

    public static void main(String[] args) {
        List<Book> list = new ArrayList<>();
        list.add(new Book("master amber", 1987, "ASHJBYGSFDFG15"));
        list.add(new Book("slaver amber", 1987, "IJFGNMKETRE58"));
        list.add(new Book("master amber", 1999, "MYHGORPJFJ64"));
        list.add(new Book("ssdas", 12414, null));
        list.add(new Book("fkajh", 32423, null));


        /*// 问题1：jdk8这种情况下，value为空会报错，jdk9解决了这个问题(key为null没有问题)
        System.out.println("==============");
        Map<String, String> collect2 = list.stream().collect(Collectors.toMap(Book::getName, Book::getIdNo,(pre, after) -> pre));
        for (String key : collect2.keySet()) {
            System.out.println("key:"+key + "=======value:"+collect2.get(key));
        }*/

        // 问题1解决方案1
        System.out.println("==============");
        HashMap<String, String> collect3 = list.stream().parallel().collect(HashMap::new, (n, v) -> n.put(v.getName(), v.getIdNo()), HashMap::putAll);
        for (String key : collect3.keySet()) {
            System.out.println("key:"+key + "=======value:"+collect3.get(key));
        }
        // 问题1解决方案2 过滤掉null的，看需求
        System.out.println("==============");
        HashMap<String, String> collect4 = list.stream().filter(l-> Objects.nonNull(l) && Objects.nonNull(l.idNo)).collect(HashMap::new, (n, v) -> n.put(v.getName(), v.getIdNo()), HashMap::putAll);
        for (String key : collect4.keySet()) {
            System.out.println("key:"+key + "=======value:"+collect4.get(key));
        }


    }


    public static class Book {
        private String name;

        private int year;

        private String idNo;

        Book(String name, int year, String idNo) {
            this.name = name;
            this.year = year;
            this.idNo = idNo;
        }

        public String getName() {
            return name;
        }

        public void setName(String name) {
            this.name = name;
        }

        public int getYear() {
            return year;
        }

        public void setYear(int year) {
            this.year = year;
        }

        public String getIdNo() {
            return idNo;
        }

        public void setIdNo(String idNo) {
            this.idNo = idNo;
        }
    }
}

 

JDK8的bug ，还是新特性功能的bug，并且在下个版本做了修复，特此记录！！希望能帮到你奥！