Java implementing clone, generic class.

Question

I have a class that implements a 2Dtable. The elements in the table is generic. And the table is stored like this:

public Object[][] list;

The problem is that calling clone on this apparently doesn't work. Note that my testcase initializes the table to store normal Integers.

Tabell2D<Integer> en = new Tabell2D<Integer>(5,5);
        en.sett(0, 0, 55);
        Tabell2D<Integer> to = en.clone();
        to.sett(0, 0, 11);
        assertTrue(en.equals(to));

Here I make a table. Change it, clone it. Change the clone, and compare them. Obviously the clone is changed. Even so, this assertTrue is well true.

The equal method is generated by eclipse:

public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        Tabell2D other = (Tabell2D) obj;
        if (bredde == null) {
            if (other.bredde != null)
                return false;
        } else if (!bredde.equals(other.bredde))
            return false;
        if (høyde == null) {
            if (other.høyde != null)
                return false;
        } else if (!høyde.equals(other.høyde))
            return false;
        if (!Arrays.equals(liste, other.liste))
            return false;
        return true;
    }

I assume that the problem is either in the compare of the list variable in equal, or the problem is in the clone method. Clone method:

public Tabell2D<E> clone(){
        Tabell2D<E> nyTab = new Tabell2D<E>(this.bredde,this.bredde);
        nyTab.liste = liste.clone(); 
        return nyTab; 
    }

This question might have some useful answers, they also talk about, and propose methods, for deep array copies: http://stackoverflow.com/questions/419858/how-to-deep-copy-an-irregular-2d-array — Frederik Wordenskjold, Feb 14 '10 at 01:57

score 2 · Accepted Answer · answered Feb 13 '10 at 23:24

2

I think that the root of the problem is that cloning a 2d array doesn't go deep. If you want a deep copy of 'liste' you need to write your own code to copy each row (or column depending on how you look at it).

answered Feb 13 '10 at 23:24

bmargulies

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This worked! Manually transferring each element in the table. And I also needed a custom equal method, the one eclipse generates did not work out. – Algific Feb 13 '10 at 23:30

Paul Tomblin · Answer 2 · 2010-02-13T23:25:35.150

-2

Clone is supposed to return Object. Your "clone" isn't overriding the normal one.

Also, I don't know that cloning an array does a deep copy. You'd probably be better off to create a new array and assign the elements.

edited Feb 13 '10 at 23:25

answered Feb 13 '10 at 23:18

Paul Tomblin

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So my professor would be an ... for including this: public abstract ITabell2D clone(); in the Tabell2D interface? – Algific Feb 13 '10 at 23:21
He should call it something else. – Paul Tomblin Feb 13 '10 at 23:22
2

Since Java 1.5 an overriding method may specialize the return type, i.e. `Foo clone()` can override `Object clone()`. This is called a *covariant return type*. That is, the answer's first sentence is incorrect for recent versions of Java. – meriton Feb 13 '10 at 23:39
@misterfixit: Making a shallow copy of an array isn't necessarily wrong. It just depends what you're trying to accomplish. So I'd hesitate before ridiculing your professor. – Jay Feb 14 '10 at 01:07

Java implementing clone, generic class.

2 Answers2