实现singleton的三种方式

/**
Singleton with public final field
**/

public class Elvis{
    public static final Elvis INSTANCE = new Elvis();
    private Elvis(){ ... }
    public void leaveTheBuilding(){ ... }
}

//Singleton with static factory
public class Elvis{
　　private static final Elvis INSTANCE = new Elvis();
　　private Elvis(){ ... }
　　public static Elvis getInstance(){ return INSTANCE;}
　　public void leaveTheBuilding(){ ... }
}
//Enum singleton - the preferred approach
public enum Elvis{
　　INSTANCE;
　　　　public void leaveTheBuilding(){ ... }
}