基本单例模式：

final class Singleton{
    private static Singleton s=new Singleton(47);
    private int i;
    private Singleton(int x){i=x;}

    public static Singleton getReference(){
        return s;
    }
    public int getValue(){return i;}
    public void setValue(int x){i=x;}
}

静态延时初始化：

final class StaticSingleton{
    private static StaticSingleton s;
    private int i;
    private StaticSingleton(int x){i=x;}

    public static StaticSingleton getReference(){
        if(s == null){
            s=new StaticSingleton(47);
        }
        return s;
    }

    public int getValue(){return i;}
    public void setValue(int x){i=x;}
}
类加载延时初始化：

final class InnerSingleton{
    private static InnerSingleton s;
    private int i;
    private InnerSingleton(int x){i=x;}

    private static class SingletonHolder{
        static InnerSingleton instance =new InnerSingleton(47);
    }
    public static InnerSingleton getReference(){
        return SingletonHolder.instance;
    }

    public int getValue(){return i;}
    public void setValue(int x){i=x;}
}

单例模式--延时初始化