正確的單例寫法

方案一：（無懶加載）

public class SingleInstance{

	private SingleInstance(){}

	private static SingleInstance instance=new SingleInstance();

	public static SingleInstance getInstance(){

		return instance;

	}

}

public class SingleInstance{

private SingleInstance(){}

private static SingleInstance instance=new SingleInstance();

public static SingleInstance getInstance(){

return instance;

}

}

方案二：（靜態內部類）

public class SingleInstance{

	private SIngleInstance(){}

	private static class SingleInner{

	private static SingleInstance instance=new SingleInstance();

	}

	public static SingleInstance getInstance(){

		return SingleInner.instance;

	}

}

方案三（雙重鎖）：

public class SingleInstance{
	private static SingleInstance instance;
	private SingleInstance(){}
	public static SingleInstance getInstance(){
		if(instance==null){
			synchronized (SingleInstance.class){
				if(instance==null){
					instance=new SingleInstance();
				}
			}
		}
	}
}

public class SingleInstance{

private SingleInstance(){}

private static SingleInstance instance=new SingleInstance();

public static SingleInstance getInstance(){

return instance;

}

}