最简单的单例

不啰嗦，因为太简单，直接上代码

//枚举式单例模式
public enum SingleMode {

    INSTANCE;

}

使用时：

 //枚举式单例模式
 SingleMode singleMode = SingleMode.INSTANCE;

原来的饿汉：

public class SingleMode {

    private static SingleMode INSTANCE = new SingleMode();

    private SingleMode() {}

    public static SingleMode getInstance() {
        return INSTANCE;
    }

}

原来的懒汉：

public class SingleMode {

    //private static SingleMode INSTANCE;
    //加上 volatile 关键字，更安全
    private static volatile SingleMode INSTANCE;

    private SingleMode() {
    }

    public static SingleMode getInstance() {
        if (INSTANCE == null) {
            synchronized (SingleMode.class) {
                if (INSTANCE == null) 
                    INSTANCE = new SingleMode();
            }
        }
        return INSTANCE;
    }

}