java的关键字final

final class First{
    int num;
}

class Second extends First{  // 报错
    
}

class First{
    final public void sleep(){
        System.out.println("我爱睡觉");
    }
}

class Second extends First{
    public void sleep(){        // 报错
        System.out.println("啦啦啦");
    }
}

public class FinalDemo {
    public static void main(String[] args){
        First f = new First();
        f.age = 88;    // 报错
    }
}

class First{
    final int age = 18;
}