Creating a copy constructor and an overridden clone() method

Question

I'm having trouble understanding how to create and implement a copy constructor, as well as an overridden clone() method (not within the same class).

I know that for an "actual copy" you have to deep clone instead of simple clone (that took me a long time to figure out...). One of the more obvious solutions (to me) I found on here was to make a copy constructor.

For the sake of brevity, let's say I have a Dog object that holds two integers. In another class, Cat, I have a Cat object that consists of an array of 100 Dog objects (e.g., Dog[] cat = new Dog[100]). I've tried the following thus far:

// for a copy constructor public Cat(Cat cat) { Dog[] copy = new Dog[100]; for (int i = 0; i < 100; i++) { copy[i] = cat[i]; } }

Something like that will always give me a type mismatch ("The type of expression must be array type but it resolved to Cat" and "Cannot convert from Cat to Dog"). It may be because it's 5AM where I am, but I just can't see why this doesn't work. Aren't copy and cat of the same type?

I get the same errors if I try to override with a clone() method of my own. I've never used clone() before (or seen it for that matter), so I'd like to see what I'm not understanding before trying to come up with my own. For what it's worth, though, I had the following commented out earlier and the compiler threw up on the return statement:

protected Cat clone() { Dog[] copy = new Dog[100]; int count = 0; (while (count < cat.length) { copy[count] = (Dog) cat[count]; count++; } return copy; }

Same type mismatch error. Changing the header to "protected Dog[] clone()" made the compiler error go away, but clearly that didn't get me anywhere.

Thanks for any help!

Your example is somehow misleading (cat and dogs don't mix up that well!); but the sure thing is that cat is an instance of Cat and not Dog[]. — Julien Royer
– Julien Royer, Commented Dec 4, 2012 at 10:52

Thirler · Accepted Answer · 2012-12-04 10:58:16Z

In your case you are trying to copy an array of Dogs that is in the Cat object. However you are trying to use the Cat object as an array (by using [] and length)

What you probably mean is to copy the contents of the cat into the new cat. Then your constructor would become something like:

private Dogs[] myCatsDogs; // for a copy constructor public Cat(Cat otherCat) { this.myCatsDogs = new Dog[100]; for (int i = 0; i < 100; i++) { this.myCatsDogs[i] = otherCat.myCatsDogs[i]; } }

For the clone() method the same applies. Also do note that clone is not used very often (because it is hard to implement correctly, see also Effective Java Item 10 by Bloch). Always use the copy constructor if you can.

Java Developer · Accepted Answer · 2012-12-04 11:10:25Z

This is my Simple Example to Under Stand copy Constructor....

public class Dog { public final List<String> names; public int age; public int weight; public Dog() { names = new ArrayList<String>(); } protected Dog(Dog original) { names = new ArrayList<String>(original.names); age = original.age; weight = original.weight; } public Dog copy() { return new Dog(this); }

}

public class Dalmatian extends Dog { public int spots; public Dalmatian() { } protected Dalmatian(Dalmatian original) { super(original); spots = original.spots; } public Dalmatian copy() { return new Dalmation(this); }

}

hope helpful....

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