只可以初始化一次的类实例

class Single{
    private static Single s = new Single();
    private Single() {
    }
    public static Single getSingle() {
        return s;
    }

    public void print() {
        System.out.println("hello world");
    }
};

public class SingleDemo2 {
    public static void main(String args[]) {
        Single s1 = Single.getSingle();
        s1.print();

    }
}