Change superclass instance variables from subclass

Question

I got this task and I can't quite figure out how to solve it: "Change all three of the x-variables related to the C-class."

class A {
    public int x;
}

class B extends A {
    public int x;
}

class C extends B {
    public int x;

    public void test() {
        //There are two ways to put x in C from the method test():
        x = 10;
        this.x = 20;

        //There are to ways to put x in B from the method test():
        ---- //Let's call this Bx1 for good measure.
        ---- //Bx2

        //There is one way to put x in A from the method test();
        ---- //Ax1
    }
}

To test, I set up this:

public class test {
    public static void main(String[] args)
    {
        C c1=new C();
        c1.test();
        System.out.println(c1.x);

        B b1=new B();
        System.out.println(b1.x);

        A a1=new A();
        System.out.println(a1.x);
    }
}

Which gives 20, 0, 0.

Now, I figured out I could write Bx1 like this:

super.x=10;

That would change the x in B, but I could not figure out how to call it in my test.java.

How do you get Bx1, Bx2, Ax1, and how do you call them for a test?

Related: http://stackoverflow.com/questions/427756/overriding-a-super-class-instance-variables — Rahul Tripathi, Oct 15 '15 at 14:07
@RahulTripathi i guess he knows that he doesn´t override them, since he is speaking about changing all three `x` members. — SomeJavaGuy, Oct 15 '15 at 14:09
"Bx1, Bx2" - there are no two x's in B. But most importantly, unless you ran into an extreme edge-case (which I can't think about) - you shouldn't do things like that. In most cases, the need to interfere with the parent's class variable implies of a poor design. — Nir Alfasi, Oct 15 '15 at 14:10

T.J. Crowder · Accepted Answer · 2015-10-15T14:16:55.997

You can access the superclass's version of x by using a superclass type reference:

System.out.println("A's x is " + ((A)this).x);

That will get A#x.

But in general, it's a very bad idea to shadow a superclass's public instance members.

Example: (live copy on IDEOne)

class Example
{
    public static void main (String[] args) throws java.lang.Exception
    {
        new C().test();
    }
}

class A {
    public int x = 1;
}

class B extends A {
    public int x = 2;
}

class C extends B {
    public int x = 3;

    public void test() {
        //There are two ways to put x in C from the method test():
        System.out.println("(Before) A.x = " + ((A)this).x);
        System.out.println("(Before) B.x = " + ((B)this).x);
        System.out.println("(Before) C.x = " + this.x);
        ((A)this).x = 4;
        System.out.println("(After) A.x = " + ((A)this).x);
        System.out.println("(After) B.x = " + ((B)this).x);
        System.out.println("(After) C.x = " + this.x);
    }
}

Output:

(Before) A.x = 1
(Before) B.x = 2
(Before) C.x = 3
(After) A.x = 4
(After) B.x = 2
(After) C.x = 3

Are there more than 2 ways to change B.x? Besides: super.x = 5; ((B)this).x = 5; — CookieJarJar, Oct 15 '15 at 15:16
@CookieJarJar: Those are the only two I can think of, other than (obviously) if `B` has some mutator or setter method. — T.J. Crowder, Oct 15 '15 at 15:19
Ah, ok. The task text just said that the last one, `A`, was hard, so I assumed it wasn't the same as one of the `B`s. — CookieJarJar, Oct 15 '15 at 15:23

score 0 · Answer 2 · answered Oct 15 '15 at 14:12

this is how your test method could look like

void test() {
    this.x = 30;
    A a = this;
    a.x = 10;
    B b = this;
    b.x = 20;
}

It´s important to note, that you are accessing the variable of the Type of the class that you defined, so in that case you would Access, x from A, and x from B by defining a variable due to the this keyword.

score 0 · Answer 3 · answered Oct 15 '15 at 14:21

Using getters and setters

class A {

public int x;

}

class B extends A {

public int x;

public void setAx(int x) {
    super.x = x;
}
public int getAx() {
    return super.x;
}

}

class C extends B {

public int x;

public void test() {

    x = 10;
    this.x = 20;

}
public void setBx(int x){
    super.x = x;
}
public int getBx() {
    return super.x;
}

public static void main(String[] args)
{
    C c1= new C();
    c1.x = 1;
    c1.setAx(2);
    c1.setBx(3);

    System.out.println(c1.getAx()+"/"+c1.getBx()+"/"+c1.x);
}

}

Change superclass instance variables from subclass

3 Answers3