java_实现一个类只能声明一个对象

public class StaticDemo {
    public  int a=10;
    private static StaticDemo s= new StaticDemo();
    private StaticDemo(){

    }
    public static StaticDemo getS(){//为使外部能访问 故此用static
        return s;
    }

    public static void main(String[] args) {
        //验证的d1和d2指向同一对象
        StaticDemo d1=StaticDemo.getS();
        StaticDemo d2=StaticDemo.getS();
        d2.a=100;
        System.out.println(d1.a);

    }

}

有知道这个有什么用处的大佬 还请留下您宝贵的见解 谢谢