題目：設計一個類，我們只能生成該類的一個實例

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public?class?Test02?{??

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????/**?

?????*?單例模式，懶漢式，線程安全?

?????*/??

????public?static?class?Singleton?{??

????????private?final?static?Singleton?INSTANCE?=?new?Singleton();??

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????????private?Singleton()?{??

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????????}??

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????????public?static?Singleton?getInstance()?{??

????????????return?INSTANCE;??

????????}??

????}??

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????/**?

?????*?單例模式，餓漢式，線程不安全?

?????*/??

????public?static?class?Singleton2?{??

????????private?static?Singleton2?instance?=?null;??

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????????private?Singleton2()?{??

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????????}??

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????????public?static?Singleton2?getInstance()?{??

????????????if?(instance?==?null)?{??

????????????????instance?=?new?Singleton2();??

????????????}??

??

????????????return?instance;??

????????}??

????}??

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????/**?

?????*?單例模式，餓漢式，線程安全，多線程環境下效率不高?

?????*/??

????public?static?class?Singleton3?{??

????????private?static?Singleton3?instance?=?null;??

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????????private?Singleton3()?{??

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????????}??

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????????public?static?synchronized?Singleton3?getInstance()?{??

????????????if?(instance?==?null)?{??

????????????????instance?=?new?Singleton3();??

????????????}??

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????????????return?instance;??

????????}??

????}??

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????/**?

?????*?單例模式，懶漢式，變種，線程安全?

?????*/??

????public?static?class?Singleton4?{??

????????private?static?Singleton4?instance?=?null;??

??

????????static?{??

????????????instance?=?new?Singleton4();??

????????}??

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????????private?Singleton4()?{??

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????????}??

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????????public?static?Singleton4?getInstance()?{??

????????????return?instance;??

????????}??

????}??

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????/**?

?????*?單例模式，使用靜態內部類，線程安全【推薦】?

?????*/??

????public?static?class?Singleton5?{??

????????private?final?static?class?SingletonHolder?{??

????????????private?static?final?Singleton5?INSTANCE?=?new?Singleton5();??

????????}??

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????????private?Singleton5()?{??

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????????}??

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????????public?static?Singleton5?getInstance()?{??

????????????return?SingletonHolder.INSTANCE;??

????????}??

????}??

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????/**?

?????*?靜態內部類，使用枚舉方式，線程安全【推薦】?

?????*/??

????public?enum?Singleton6?{??

????????INSTANCE;??

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????????public?void?whateverMethod()?{??

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????????}??

????}??

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????/**?

?????*?靜態內部類，使用雙重校驗鎖，線程安全【推薦】?

?????*/??

????public?static?class?Singleton7?{??

????????private?volatile?static?Singleton7?instance?=?null;??

??

????????private?Singleton7()?{??

??

????????}??

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????????public?static?Singleton7?getInstance()?{??

????????????if?(instance?==?null)?{??

????????????????synchronized?(Singleton7.class)?{??

????????????????????if?(instance?==?null)?{??

????????????????????????instance?=?new?Singleton7();??

????????????????????}??

????????????????}??

????????????}??

??

????????????return?instance;??

????????}??

????}??

??

????public?static?void?main(String[]?args)?{??

????????System.out.println(Singleton.getInstance()?==?Singleton.getInstance());??

????????System.out.println(Singleton2.getInstance()?==?Singleton2.getInstance());??

????????System.out.println(Singleton3.getInstance()?==?Singleton3.getInstance());??

????????System.out.println(Singleton4.getInstance()?==?Singleton4.getInstance());??

????????System.out.println(Singleton5.getInstance()?==?Singleton5.getInstance());??

????????System.out.println(Singleton6.INSTANCE?==?Singleton6.INSTANCE);??

????????System.out.println(Singleton7.getInstance()?==?Singleton7.getInstance());??

????}??

??

}??

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