访问子类中的私有实例

Question

I've seen answers for questions related to mine on Stack Overflow, but I am still left with some ambiguity. A parent class method has access to its own private instance variables. If a child class inherits the class, what happens when the getA() method is called on an instance of the Child class? Does it return the a from the Parent class or the a from the Child class?

class Parent {
    private int a = 10;  
    public int getA() {
       return a;
    }
 }

class Child extends Parent {
    private int a = 22;              
 }

public class Test { 
   public static void main(String []args) throws Exception {        
       Child c = new Child(); 
       System.out.println(c.getA());
   }    
}

Answer 1

As the Method getA() is inherited, if you call this Method, you'll always invoke the Parent 's Method.

The current Object will be treated as a Parent and not as a Child , and the Parent 'sa will be returned.

Even though you have your own variable a , this variable wont override the Parent 's a . They are different from each other, have different addresses and different values.

If you want getA() to return Child 's a , you need to override the Method to return your new variable.

class Child extends Parent {
    private int a = 22;

    @Override
    public int getA(){
        return a;
    }
 }

You could also "go crazy" and do stuff like the following:

class Child extends Parent {
    private int a = 22;       

    @Override
    public int getA(){
        int superA = super.getA();

        return a+superA;    
    }
 }

That way you could return the sum of Parent 's and Child 's a .

(Just an example)

Answer 2

This is a duplicate of this SO post .

The variable a in subclass Child hides the a in the parent class Parent .

Answer 3

Fields cannot be overridden.

In your code, an instance of child (like the one referred to by c ) has two different fields, which are both called a .

You cannot access Parent 's private variables inside Child , full stop. That's the entire point of private . There is nothing* you can write inside Child to make the parent's a field equal 22.

* Technically, you could with reflection. Reflection doesn't count though, since its purpose is essentially to allow you to break things, and do things that are otherwise impossible.

Answer 4

Private variables are local to the class and in your code you are inheriting the properties of the parent class so you can access getA() and it will return the parent's attribute. And you cannot access child's variable unless you have public getter method for child attribute.

Answer 5

坚持基础“私有成员变量只能通过自己的成员函数在类中访问，并且字段不能被继承”所以基本上当你访问父类中的私有变量A时，该方法应该访问它自己的私有成员而不是然后是任何子类的私有领域。