Using Integer as a key with HashMap in Java

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逝去的感伤
逝去的感伤 2021-01-20 10:49

Recently I was looking for good implementation of hashCode() method in Java API and looked through Integer source code. Didn\'t expect that, but th

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  •  -上瘾入骨i
    2021-01-20 11:25

    From the docs:

    The general contract of hashCode is:

    Whenever it is invoked on the same object more than once during an execution of a Java application, the hashCode method must consistently return the same integer, provided no information used in equals comparisons on the object is modified. This integer need not remain consistent from one execution of an application to another execution of the same application.

    --> Integer#hashCode fulfills this.

    If two objects are equal according to the equals(Object) method, then calling the hashCode method on each of the two objects must produce the same integer result.

    --> Integer#hashCode fulfills this too.

    It is not required that if two objects are unequal according to the equals(java.lang.Object) method, then calling the hashCode method on each of the two objects must produce distinct integer results. However, the programmer should be aware that producing distinct integer results for unequal objects may improve the performance of hash tables.

    --> Integer#hashCode fulfills this to the maximum extent, i.e. two unequal Integers will never have the same hash code.

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