[Java基础] stream流中Collectors.toMap报空指针异常情况

代码示例：

        List<BookEntity> bookEntityList = new ArrayList<>();BookEntity bookEntity = BookEntity.builder().bookName("钢铁是怎样炼成的").bookOwner("xinliushijian").build();BookEntity bookEntity1 = BookEntity.builder().bookName("悲惨世界").bookOwner("xinliu").build();bookEntityList.add(bookEntity);bookEntityList.add(bookEntity1);BookEntity bookEntity2 = BookEntity.builder().bookName(null).bookOwner("shijian1").build();BookEntity bookEntity3 = BookEntity.builder().bookName(null).bookOwner("shijian2").build();bookEntityList.add(bookEntity2);bookEntityList.add(bookEntity3);// key中有null，不报错Map<String, String> map = bookEntityList.stream().collect(Collectors.toMap(BookEntity::getBookName, BookEntity::getBookOwner, (v1, v2)  -> v2));System.out.println(map); // 打印1BookEntity bookEntity4 = BookEntity.builder().bookName("笑面人").bookOwner(null).build();bookEntityList.add(bookEntity4);// value中有null，报空指针异常Map<String, String> map1 = bookEntityList.stream().collect(Collectors.toMap(BookEntity::getBookName, BookEntity::getBookOwner, (v1, v2)  -> v2));System.out.println(map1); // 打印2

打印1：

{null=shijian2, 悲惨世界=xinliu, 钢铁是怎样炼成的=xinliushijian}

打印2：

Exception in thread “main” java.lang.NullPointerException
at java.util.HashMap.merge(HashMap.java:1225)
at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)atjava.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1384)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:472)
at java.util.stream.ReduceOps$ReduceOp.evaluateSequential(ReduceOps.java:708)
at java.util.stream.AbstractPipeline.evaluate(AbstractPipeline.java:234)
at java.util.stream.ReferencePipeline.collect(ReferencePipeline.java:499)

报错原因：
并不是因为map不允许value值为null，我们都知道map中key和value都可以为null，而是merge方法中value不允许为null

解决办法：
所以在进行map转换之前，若无法保证数据非空，还是需要过滤下数据，只保留非空的部分数据（key为null一般无意义，也不要了）

	// 正确写法Map<String, String> map2 = bookEntityList.stream().filter(v -> !StringUtils.isAnyBlank(v.getBookName(), v.getBookOwner())).collect(Collectors.toMap(BookEntity::getBookName, BookEntity::getBookOwner, (v1, v2)  -> v2));System.out.println(map2); // 打印3

打印3：

{悲惨世界=xinliu, 钢铁是怎样炼成的=xinliushijian}