JAVA8新特性stream流收集为Map,value为null导致空指针的问题

jdk8

新特性stream深受喜爱,平时使用比较多,其中有:

Map 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 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 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;
        }
    }
}

JAVA8新特性stream流收集为Map,value为null导致空指针的问题_第1张图片

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 collect(Collector 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 collect(Supplier supplier,BiConsumer accumulator,
              BiConsumer combiner);

Supplier supplier :收集器返回的容器

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

BiConsumer 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 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 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 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 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;
        }
    }
}

 JAVA8新特性stream流收集为Map,value为null导致空指针的问题_第2张图片

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!

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