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