Java Inheritance - setters only in subtypes

Question

We have two Classes (one parent and a child). both uses private variables to store values, but the parent should NOT provide setters (x and y are given with the constructor and they are some sort of immutable). B should extend A with setters for x and y. Is there a common way to do so?

class A{
    private int x;
    private int y;

    A(int x, int y){
        this.x = x;
        this.y = y;
    }

}

class B extends A{

    public void setx(int x){
        this.x = x;
    }

    //same for y
}

Some thoughts

variables should be private
x and y of parent have to be immutable
B has to provide a public setter

If A's contract is that it's immutable, and B extends A, then B must be immutable. Otherwise, you're breaking the "is a" promise. Code can have a reference with type A to a B object, and despite A's being immutable, find that the object mutates. Not good. :-) — T.J. Crowder
– T.J. Crowder, Commented Nov 13, 2012 at 10:44
I think there is a design issue here. What you are saying is that x and y are final (immutable) so what is the point in changing them? This is even worst if you want to change them in a child class: if a child class is in need to change immutable values of the parent class, that is not a proper child class. — Christian Achilli
– Christian Achilli, Commented Nov 13, 2012 at 10:48
@NaN: I think you've just expressed it in a way that makes it seem more complicated than it actually is. Looking at the comments, you don't actually need x and y to be immutable in A, you just need A not to have any setters. There's a distinction there (in terms of the contract A provides). If it's okay for B to modify the x and y that it inherits from A, then MeNoMore's answer is the correct one. — T.J. Crowder
– T.J. Crowder, Commented Nov 13, 2012 at 10:56

RNJ · Accepted Answer · 2012-11-13 10:44:38Z

3

If you want the variables to be immutable then it should be

class B extends A{

    public B(int x, int y){
          super(x, y);
    }
}

At the moment your x and y variables in A are not immutable. To make them immutable then precede them with final

This is the only way you can assign x and y as they are private. If you want setters then you will have to make the variables protected.

Personally I am a big fan of immutability so would do this rather than setters - creating objects is usually quite cheap.

answered Nov 13, 2012 at 10:44

RNJ

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Comments

CloudyMarble · Accepted Answer · 2012-11-13 10:51:13Z

3

Declare the variables as protected in the base class and write getter and setter in the child classes.

edited Nov 13, 2012 at 10:51

answered Nov 13, 2012 at 10:43

CloudyMarble

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4 Comments

T.J. Crowder Over a year ago

Read the question again: "x and y of parent have to be immutable" Providing setters in the derived class makes them mutable.

CloudyMarble Over a year ago

@T.J.Crowder: Po wrote: "some sort of immutable", PO wrote too: "with setters for x and y", i guess he meant they should be immutable from class A but should be changable in the derived class.

NaN Over a year ago

@MeNoMore yes! in A they are immutable and in B they are mutable!

T.J. Crowder Over a year ago

@NaN: See further comment on the question. If I'm understanding you correctly from the comments (the question itself is unclear), MeNoMore's answer is right.

JohnB · Accepted Answer · 2012-11-13 15:08:57Z

1

You can't have a private member variable and no method (here with method I mean also the constructor) setting it (well, technically you can, but it does not make sense). If you want your variables to be settable from a derived class, they have to be protected.

EDIT: You can, however, define a protected "helper" setter in the base class and call this protected helper setter from a public setter in the derived class.

SECOND EDIT: Another possibility is defining an abstract getter in the base class and implement getter, setter, as well as the private field, in the derived class (template pattern).

edited Nov 13, 2012 at 15:08

answered Nov 13, 2012 at 10:42

JohnB

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4 Comments

T.J. Crowder Over a year ago

There is a way of setting them: The constructor. Entirely normal to have fields which are only set by the constructor. In any case, this is a comment, not an answer.

JohnB Over a year ago

He asked how he can set private variables in a derived class. My answer to it is: That's not possible. So it is an answer.

T.J. Crowder Over a year ago

@ John: No, all you've said is "make them protected." The question is rather more complex than that.

JohnB Over a year ago

Then I suggest you give a more in-depth answer. Feel free to do so.

aaronps · Accepted Answer · 2012-11-13 11:16:09Z

0

This is a weird question, immutable but mutable, private but public... The correct way should be to make them protected, as everyone said.

Anyway, in java you can use dirty tricks if the security manager doesn't complain, check this out:

import java.lang.reflect.Field;
import java.util.logging.Level;
import java.util.logging.Logger;

public class Priv
{
    public static class A
    {
        private final int x;
        public A(int x)
        {
            this.x = x;
        }
    }

    public static class B extends A
    {
        public B(int x)
        {
            super(x);
        }

        public void setX(int x)
        {
            Class c = A.class;
            try
            {
                 Field f = c.getDeclaredField("x");
                f.setAccessible(true);
                f.set(this, x);
            } catch (IllegalArgumentException ex) {
                Logger.getLogger(Priv.class.getName()).log(Level.SEVERE, null, ex);
            } catch (IllegalAccessException ex) {
                Logger.getLogger(Priv.class.getName()).log(Level.SEVERE, null, ex);
            } catch (NoSuchFieldException ex) {
                Logger.getLogger(Priv.class.getName()).log(Level.SEVERE, null, ex);
            } catch (SecurityException ex) {
                Logger.getLogger(Priv.class.getName()).log(Level.SEVERE, null, ex);
            }
        }

        public int getX()
        {
            int v = 0;

            try {
                Class c = A.class;
                Field f = c.getDeclaredField("x");
                f.setAccessible(true);
                v = f.getInt(this);
            } catch (IllegalArgumentException ex) {
                Logger.getLogger(Priv.class.getName()).log(Level.SEVERE, null, ex);
            } catch (IllegalAccessException ex) {
                Logger.getLogger(Priv.class.getName()).log(Level.SEVERE, null, ex);
            } catch (NoSuchFieldException ex) {
                Logger.getLogger(Priv.class.getName()).log(Level.SEVERE, null, ex);
            } catch (SecurityException ex) {
                Logger.getLogger(Priv.class.getName()).log(Level.SEVERE, null, ex);
            }

            return v;
        }
    }

    public static void main(String[] args)
    {
         B b = new B(5);

         System.out.println("b.x is " + b.getX());

         b.setX(42);

        System.out.println("b.x now is " + b.getX());
    }

}

edited Nov 13, 2012 at 11:16

answered Nov 13, 2012 at 11:07

aaronps

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4 Comments

JohnB Over a year ago

You're creating instances from a static class?

aaronps Over a year ago

No, I just copied the code from the IDE, it was classes inside the main class, hence the static.

aaronps Over a year ago

if you don't know what static does in this context, you should check your books again.

JohnB Over a year ago

I know what static does for inner classes, don't worry.

Ravi · Accepted Answer · 2012-11-13 13:29:22Z

0

There is no concept of immutable is you have any setter method or constructor of child class calling super to re-initialize the variable of super class which are private.

immutable is inherently thread-safe.

answered Nov 13, 2012 at 13:29

Ravi

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