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*[如何处理 java.lang.outOfMemoryError PermGen space error](https://github.com/giantray/stackoverflow-java-top-qa/blob/master/contents/dealing-with-java-lang-outofmemoryerror-permgen-space-error.md)
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>An enum type has no instances other than those defined by its enum constants.
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>It is a compile-time error to attempt to explicitly instantiate an enum type. The final clone method in Enum >ensures that enum constants can never be cloned, and the special treatment by the serialization mechanism ensures >that duplicate instances are never created as a result of deserialization. Reflective instantiation of enum types >is prohibited. Together, these four things ensure that no instances of an enum type exist beyond those defined by >the enum constants.
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>Because there is only one instance of each enum constant, it is permissible to use the == operator in place of the >equals method when comparing two object references if it is known that at least one of them refers to an enum ?>constant. (The equals method in Enum is a final method that merely invokes super.equals on its argument and ?>returns the result, thus performing an identity comparison.)
As a reminder, it needs to be said that generally, == is NOT a viable alternative to equals. When it is, however (such as with enum), there are two important differences to consider:
Bloch specifically mentions that immutable classes that have proper control over their instances can guarantee to their clients that == is usable. enum is specifically mentioned to exemplify.
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具体来说，那些提供恰当实例控制的不可变类能够保证 == 是可用的，枚举刚好符合这个条件。
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> Item 1: Consider static factory methods instead of constructors
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[...] it allows an immutable class to make the guarantee that no two equal instances exist: a.equals(b) if and only if a==b. If a class makes this guarantee, then its clients can use the == operator instead of the equals(Object) method, which may result in improved performance. Enum types provide this guarantee.
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