Java array cloning strange behavior

Question

I'm trying to clone an instance of a custom class I made called CSP. I have one instance called csp and I want to make a clone of csp called cspclone. Here is what I'm using to do that:

CSP cspclone = new CSP((csp.x).clone(), (csp.d).clone(), (csp.c).clone());

For some reason though when I pass cspclone to a method that modifies it csp gets modified also as if I forgot the .clone() functions but I didn't! Why is this happening?!

`.clone()` only performs a shallow copy of your object, so references to the fields in csp are being copied into cspclone. — Hunter McMillen, Nov 16 '11 at 22:05

Bhesh Gurung · Accepted Answer · 2011-11-16T23:04:15.327

3

Override the clone method in CSP:

public class CSP {
    private String aField;
    private int[] array;
    private int[][] twoDArr;
    private List<ALContent> list; //here ALContent also needs to override clone properly

    @Override
    public Object clone() {
        CSP clone = new CSP();
        clone.aField = this.aField;
        clone.array = new int[this.array.length];
        System.arraycopy(this.array, 0, clone.array, 0, this.array.length);

        clone.list = new ArrayList<ALContent>();
        for(ALContent content : this.list) {
            clone.list.add(content.clone()); //make sure you add the clone of the content
        }

        clone.twoDArr = new int[this.twoDArr.length][];
        for(int i=0; i<this.twoDArr.length; i++) {
            clone.twoDArr[i] = new int[this.twoDArr[i].length];
            System.arraycopy(this.twoDArr[i], 0, clone.twoDArr[i], 0, this.twoDArr[i].length);
        }

        return clone;
    }
}

Then you can do:

CSP csp = new CSP();
CSP cspClone = (CSP) csp.clone();

edited Nov 16 '11 at 23:04

answered Nov 16 '11 at 22:12

Bhesh Gurung

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So what goes where the `//...` is? – Kyle V. Nov 16 '11 at 22:17
Ok, so how would I do it for non-arrays? Here are the fields of a CSP object: `public int[] x; public ArrayList d[]; public int[][] c;` – Kyle V. Nov 16 '11 at 22:28
The default implementation of the `clone` in `ArrayList` does shallow copy: http://download.oracle.com/javase/6/docs/api/java/util/ArrayList.html#clone%28%29. So, you have to do it manully: http://stackoverflow.com/questions/715650/how-to-clone-arraylist-and-also-clone-its-contents. – Bhesh Gurung Nov 16 '11 at 22:35
@StickFigs: Updated the answer again. – Bhesh Gurung Nov 16 '11 at 22:40
@StickFigs: For int[][], check this out: http://stackoverflow.com/questions/5563157/how-to-deep-copy-2-dimensional-array-different-row-sizes – Bhesh Gurung Nov 16 '11 at 22:55
@StickFigs: See the update for the 2 dimensional array int[][]. – Bhesh Gurung Nov 16 '11 at 23:04

score 0 · Answer 2 · answered Nov 16 '11 at 22:08

0

If your properties of array type use System.arraycopy

answered Nov 16 '11 at 22:08

mishadoff

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score 0 · Answer 3 · answered Nov 16 '11 at 22:15

According to http://download.oracle.com/javase/1.3/docs/api/java/lang/Object.html#clone%28%29

this method creates a new instance of the class of this object and initializes all its fields with exactly the contents of the corresponding fields of this object, as if by assignment; the contents of the fields are not themselves cloned. Thus, this method performs a "shallow copy" of this object, not a "deep copy" operation.

You might have to override the clone method and clone() a reference type attribute within the object (i.e., perform deep copy operation).

score 0 · Answer 4 · edited May 23 '17 at 12:21

To solve your problem you need deep cloning. The default clone method does a shallow copy. See Object.clone().

Here are some approaches. All have advantages and drawbacks.

Override: Override the clone() method.
Serialization: Write the object to a stream and a read it back from the stream.
Reflection: Here is a tutorial on reflection.
Use a deep close library, such as Java Deep-Cloning library.

Here are several other stackoverflow discussions of cloning.

Java array cloning strange behavior

4 Answers4