单例设计模式

class SingleTon{
}

class SingleTon1{
    private SingleTon1(){
    }
    public static SingleTon instance = new SingleTon1();
}

class SingleTon2{
    private SingleTon2(){
    }
    private static SingleTon2 instance;
    
    public static SingleTon2 getInstance() {
        if (instance == null) {
            instance = new SingleTon2();
        }
        return instance;
    }
}