Java inheritance from superclass fields

Question

I have made these two classes in Java, and I need to define my structure like this:

class UnitType {
    Position myPos;
    public Position getPosition() {
        return myPos;
    }
}
class Unit extends UnitType {
    Position myPos;
    public Unit(Position myPos) {
        this.myPos = myPos;
    }
}

My question, can I do this following:

class UnitTest {
    private Unit testUnit;
    @Before
    public void setUp() {
        testUnit = new Unit(new Position(1,0));
    }
    @Test
    public void testPositionUnit() {
        assertEquals("Unit should be placed at (1,0)", new Position(1,0), testUnit.getPosition());
    }
}

And if I can do this, what would happen if I updated the position in my Unit class. Will I be looking at a wrong pointer when calling getPosition()?

Many is wondering about my .equals method for my Position class so here it is: https://pastebin.com/Tfer1kqR

Have you tried to do "this following"?

Simon Forsberg
– Simon Forsberg

2012-09-15 16:01:48 +00:00
Commented Sep 15, 2012 at 16:01 — Simon Forsberg
– Simon Forsberg, Commented Sep 15, 2012 at 16:01

João Silva · Accepted Answer · 2012-09-15 16:21:54Z

4

If you do so, you'll be hiding the myPos field of the superclass UnitType. For example:

Unit u = new Unit(new Position(1, 0));
System.out.println(u.myPos);         // (1, 0) assuming toString() was overriden
System.out.println(u.getPosition()); // null
UnitType ut = u;
System.out.println(ut.myPos);        // null

Then, even though you may update the myPos field of your Unit class, getPosition() will always return null, since it will return the myPos field from the superclass, which was never set.

Also, if you want to determine if two Position objects are the same, make sure you implement .equals() (and .hashCode()).

edited Sep 15, 2012 at 16:21

answered Sep 15, 2012 at 16:02

João Silva

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1 Comment

miniwolf Over a year ago

Could I pass on the internal pointer? Like in the super constructor passing on the myPos and also updating in the Unit class? Then in super class getPosition would point to the position insert in Unit.

Mike Samuel · Accepted Answer · 2012-09-15 18:25:46Z

1

To make sure you're looking at the right position, set the position via the super constructor

class UnitType {
    Position myPos;  // This could be private final.

    // Only needed for backwards compatibility if you do new UnitType().
    UnitType() { this(null); }  // this(...) delegates to another constructor.
    // sets the position field.
    UnitType(Position myPos) { this.myPos = myPos; }

    public void getPosition() {
        return myPos;
    }
}
class Unit extends UnitType {
    // No field definition here that masks the super-class one.
    public Unit(Position myPos) {
        super(myPos);  // Invoke UnitType(Position) to set the field
    }
}

then to get your unit-tests to work, make sure the Position class overrides equals(Object) so that assertEquals will test for equivalence instead of checking that they are the same object.

answered Sep 15, 2012 at 18:25

Mike Samuel

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4 Comments

miniwolf Over a year ago

If I do this trick wouldn't I run into an update in the Unit instance would cause the pointer to the element would be different. So UnitType and Unit would have two different myPos internal pointers.

Mike Samuel Over a year ago

@miniwolf, no. There is only one myPos field.

miniwolf Over a year ago

Alright but say I wanted to update this position inside Unit and not inside UnitType.

Mike Samuel Over a year ago

@miniwolf, make the field protected, and then do this.myPos = ...

Peter Ilfrich · Accepted Answer · 2012-09-15 16:19:56Z

0

First of all, you method getPosition() has to return something. In your code it returns nothing, cause you've specified the method with a return type void

Secondly, depending on the Position class you're using (own class?, geometric position?, ...) you probably need to override/write the equals(Object o) method according to what you want to achieve.

In general testUnit.getPosition() and new Position(..) won't return the same object (although they might contain the same values), thus making the assertion false.

answered Sep 15, 2012 at 16:19

Peter Ilfrich

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3 Comments

miniwolf Over a year ago

Exactly my issue, because parsing the value from Unit constructor on to Super constructor would originally give me the same pointer, but afterwards this would break if an update were to appear on Unit.myPos.

Peter Ilfrich Over a year ago

Not exactly. new Position(..) is not the same object instance in the heap as testUnit.getPosition(). Thus the equals() method of Object (which will most likely be used) will return false.

miniwolf Over a year ago

This is my Position class, the .equals() method should fix this. pastebin.com/Tfer1kqR

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