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<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1System.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！