java - 未定义的构造函数(java)

Question

所以我有一个名为 User 的 java 类，它包含这样的构造函数:

public class User extends Visitor {
    public User(String UName, String Ufname){
        setName(UName);
        setFname(Ufname);
    }
}

然后他的问题出现在我的另一个名为 Admin 的类中:

public class Admin extends User {  //error "Implicit super constructor User() is undefined for default constructor. Must define an explicit constructor"

//public Admin() { } //tried to add empty constructor but error stays there

    //}


    public void manage_items() {            

        throw new UnsupportedOperationException();
    }

    public void manage_users() {   

        throw new UnsupportedOperationException();
    }
}

我收到错误默认构造函数的隐式 super 构造函数 User() 未定义。必须在代码中标记的地方定义一个显式构造函数。我不知道如何解决这个问题，而且我对java真的很陌生。

Answer 1

如果您没有向类添加构造函数，系统会自动为您添加一个构造函数。对于 Admin 类，它看起来像这样:

public Admin() { 
    super();
}

调用 super() 正在调用父 User 类中的以下构造函数:

public User() {
    super();
}

由于此构造函数不存在，您将收到错误消息。

有关默认构造函数的更多信息，请参见此处: http://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html

You don't have to provide any constructors for your class, but you must be careful when doing this. The compiler automatically provides a no-argument, default constructor for any class without constructors. This default constructor will call the no-argument constructor of the superclass. In this situation, the compiler will complain if the superclass doesn't have a no-argument constructor so you must verify that it does. If your class has no explicit superclass, then it has an implicit superclass of Object, which does have a no-argument constructor.*