没错啊, 看看JDK, HashMap的put方法的帮助-->Returns:
previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the HashMap previously associated null with the specified key.如果插入的KEY已经存在, 返回原来KEY对应VALUE, 否则返回NULL.
previous value associated with specified key, or null if there was no mapping for key. A null return can also indicate that the HashMap previously associated null with the specified key.如果插入的KEY已经存在, 返回原来KEY对应VALUE, 否则返回NULL.
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