If the key was a mutable object, its value could be changed. 如果鍵是可變對象，它的值可改變。
Classes in the Java class library are examples of immutable objects - they represent a single value that cannot change over the lifetime of the object. 類(lèi)就是不變對象的示例它們表示在對象的生命期內無(wú)法更改的單個(gè)值。
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