Java 8 两个代码段之间的差异

我已经编写了一个示例lambda代码，现在我想了解一些关于语句的知识

我的示例代码：-

public static void main( String[] args )
{
    List<ItemObject> record = new ArrayList();
    ItemObject object1 = new ItemObject();
    ItemObject object2 = new ItemObject();
    ItemObject object3 = new ItemObject();
    ItemObject object4 = new ItemObject();
    record.add(object1);
    record.add(object2);
    record.add(object3);
    record.add(object4);

    Map<String, ItemObject> stbProcessingMap = new HashMap();
    stbProcessingMap = record.stream().collect(Collectors.toMap(recordingItem -> "key", recordingItem -> recordingItem, (r1,r2) -> r2));
    System.out.println(stbProcessingMap);
}

它的工作很好，但当我使用

stbProcessingMap = record.stream().collect(Collectors.toMap(recordingItem -> "key", recordingItem -> recordingItem, (r1,r2) -> r2));

stbProcessingMap = record.stream().collect(Collectors.toMap(recordingItem -> "key", recordingItem -> recordingItem));

我得到以下错误

Exception in thread "main" java.lang.IllegalStateException: Duplicate key com.modle.ItemObject@7cca494b
at java.util.stream.Collectors.lambda$throwingMerger$0(Collectors.java:133)
at java.util.HashMap.merge(HashMap.java:1245)
at java.util.stream.Collectors.lambda$toMap$58(Collectors.java:1320)
at java.util.stream.ReduceOps$3ReducingSink.accept(ReduceOps.java:169)
at java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1374)
at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:481)
at java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:471)
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)
at com.sample.App.main(App.java:30)

---

这两条语句之间的区别是什么？第一个示例代码段使用收集器的此重载：

toMap(Function<? super T,? extends K> keyMapper,
               Function<? super T,? extends U> valueMapper,
               BinaryOperator<U> mergeFunction)

toMap​(Function<? super T,? extends K> keyMapper,
              Function<? super T,? extends U> valueMapper)

---

如果存在键冲突，则会引发异常，因为映射不能有重复的键。

您是否阅读了两个toMap方法的javadoc？很明显！第一种方法使用merge函数放置新值，但第二种方法不使用它，因此它引发异常。事实上，第一种方法使用相同键的新值替换旧值。