Java之创建对象>3.Enforce the singleton property with a private constructor or an enum type

 1. 通过一个公开的字段来获取单例

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

The main advantage of the public field approach is that the declarations make it clear that the class is a singleton: the public static field is final, so it will always contain the same object reference. There is no longer any performance advantage to the public field approach (没有任何性能优势)

2. 通过一个公开的方法getInstance()来获取单例

// 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() { ... }
}

One advantage of the factory-method approach is that it gives you the flexibility to change your mind about whether the class should be a singleton without changing its API. The factory method returns the sole instance but could easily be modified to return, say, a unique instance for each thread that invokes it. A second advantage, concerning generic types. Often neither of these advantages is relevant, and the final-field approach is simpler.

3.通过枚举来实现

// Enum singleton - the preferred approach
public enum Elvis {
    INSTANCE;
    public void leaveTheBuilding() { ... }
}

While this approach has yet to be widely adopted, a single-element enum type is the best way to implement a singleton.