Public attributes and inheritance Java

Question

I'm checking some Java exercises but I'm confused with this one:

We have a Foo class with this structure:

public class Foo {
    public int a = 3;

    public void addFive() {
        a += 5;
    }
}

A Bar class who inherits from Foo:

public class Bar extends Foo {
    public int a = 8;

    public void addFive() {
        a += 5;
    }
}

And a Test class :

public class Test {

    public static void main(String[] args) {
        Foo f = new Bar();
        f.addFive();
        System.out.println(f.a);
    }
}

I would think the output is 13, but it's 3, my question is Why?..

The line int a += 5; won't compile. I suspect you meant a += 5; to match the code in the superclass method, which needs a semicolon. — rgettman
– rgettman, Commented Dec 2, 2013 at 21:07

kiruwka · Accepted Answer · 2013-12-02 21:25:50Z

7

Adding to Peter's answer : keep in mind that member variables are not polymorphic, so they are not overriden.
I.e. f.a still gives you reference to super class's a (since f's declared type is Foo) whereas f.addFive() calls method on Bar, (since f's runtime type is Bar);

So, for example

Bar b = new Bar();
Foo f = b;
f.addFive();
System.out.println(f.a); // prints 3
System.out.println(b.a); // prints 13 as you have expected

Hope it is clear.

edited Dec 2, 2013 at 21:25

answered Dec 2, 2013 at 21:11

kiruwka

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Comments

Peter Lawrey · Accepted Answer · 2013-12-02 21:06:14Z

5

You have two fields, one which hides, not replaces the other. You have Foo.a and Bar.a You overridden method alters Bar.a but you are looking up Foo.a

answered Dec 2, 2013 at 21:06

Peter Lawrey

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1 Comment

Sotirios Delimanolis Over a year ago

Polymorphism doesn't apply to fields.

Pshemo · Accepted Answer · 2013-12-02 21:34:54Z

5

You need to know that in Java late-binding (mechanism of polymorphism that finds body of method based on actual object at runtime) works only for methods, not for fields.

So if you have reference of base type Foo that holds object of type derived from Foo like in our case Bar

Foo f = new Bar();

then with f.field you will use field from Foo class (no polymorphic behaviour here).
But if you use f.someMethod() then thanks to late-binding at runtime Java will find and execute code of this method from Bar class (since f holds insteance of Bar object).

So in your code

Foo f = new Bar();//you are creating reference of type Foo to instance of Bar
f.addFive();//body of this method will increment `a` declared in `Bar`
System.out.println(f.a);//here you are using `a` declared in Foo.

You can get value a from Bar class if you use reference of type Bar.

System.out.println(((Bar)f).a);//prints 13.

edited Dec 2, 2013 at 21:34

answered Dec 2, 2013 at 21:24

Pshemo

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1 Comment

kiruwka Over a year ago

+1 for System.out.println(((Bar)f).a); Makes a good point too.

dm78 · Accepted Answer · 2013-12-02 21:09:45Z

0

In your Bar#addFive() method, you declare a local int a. Changes to this local int a do not affect the class member int a. Since f is a Foo, the value of a will be 3 in this case.

Also, you do not need to redeclare your addFive() method in your Bar class since there is no difference from the Foo#addFive() method.

EDIT: Oddly, the code you originally posted has changed and the int a is now no longer local. What is the actual code you are using?

answered Dec 2, 2013 at 21:09

dm78

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3 Comments

Joe Over a year ago

I think that was a typo as that wouldn't compile if it was written that way.

dm78 Over a year ago

It will compile even if it is a typo.

Joe Over a year ago

It really won't, try it.

Joe · Accepted Answer · 2013-12-02 21:19:21Z

0

Because you are initializing a Bar object as a Foo. You have two choices here, either initialize it as a straight Bar object or add a getter to the Bar object that connects directly to that property.

Bar bar = new Bar();
bar.a;

or

public class Bar extends Foo{
    public int a = 8;

    public int getA() {
        return a;
    }

    public void addFive(){
        int a += 5;
    }
}

bar.getA();

edited Dec 2, 2013 at 21:19

answered Dec 2, 2013 at 21:09

Joe

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1 Comment

Joe Over a year ago

Why did I get all those down votes for this? At least write something in response.

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