Java note design_pattern

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public class ClassicSingleton {
	private static ClassicSingleton instance = null;
	protected ClassicSingleton() {
		// Exists only to defeat instantiation.
	}
	public static ClassicSingleton getInstance() {
		if(instance == null) {
			instance = new ClassicSingleton();
		}
		return instance;
	}
}

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public class Singleton {
	private static Singleton instance = null;
	protected Singleton() {
		// Exists only to defeat instantiation.
	}
	public static Singleton getInstance() {
		if(singleton == null) {
			synchronized(Singleton.class) {
				if(singleton == null) {
					singleton = new Singleton();
				}
			}
		}
		return singleton;
	}
}

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public class SimpleSingleton {
	public final static SimpleSingleton INSTANCE = new SimpleSingleton();
	private SimpleSingleton() {
		// Exists only to defeat instantiation.
	}
}

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public enum EnumSingleton {
    INSTANCE;
    public void someOtherMethod() {
        //
	}
}