单利模式实现有饿汉式，懒汉式，双重检测等待，接下来介绍的是一个枚举类的单利，线程安全
public class SingletonDemo {
// private final static class SingletonHolder {
// private static final Singleton INSTANCE = new Singleton();
// }
private SingletonDemo() {
}
public static SingletonDemo getInstance() {
return Singleton.INSTANCE.getInstance();
}
private enum Singleton {
INSTANCE;
private SingletonDemo singleton;
Singleton() {
singleton = new SingletonDemo();
}
public SingletonDemo getInstance() {
return singleton;
}
}
}