Question

I’m a bit of a Java N00B, but I am majorly confused about clone.

Consider the following incorrect class

class blah {
   public blah(Collection<Integer> c){
       member_collection = c.clone(); //the clone here is defensive
   }
   private final Collection<Integer> member_collection;
}

I think what I want to do is clear.

I understand why the clone() symbol is not found: clone is not a public method in any possibly Collection. However, in any concrete class I would want to use it is, and I want to just tell the compiler that.

I’m sure I’m not the first person to want to clone a generic collection, but the tutorials I have looked at only explain why you cannot clone a Collection, not how to get around the problem. I have tried hacky stuff with getClass() and casting, but those usually require me to make ugly try-catch blocks and it’s difficult to ensure exactly one initialization (the pointer member_collection) is final!

What I’d really like to do is somehow put that information into the type somehow. Is there a nice way to do this?

Thanks!

Answer 1

You’re better off copying like so:

public class Blah {
    private final Collection<Integer> collection;

    public Blah(Collection<Integer> collection) {
        this.collection = new ArrayList<Integer>(collection);
    }
}

Or like so:

public class Blah {
    private final Collection<Integer> collection = new ArrayList<Integer>();

    public Blah(Collection<Integer> collection) {
        this.collection.addAll(collection);
    }
}

This is the same as what you’re trying and guarantees that any order of the given Collection is preserved.

Edit: If you absolutely need it to be the same implementation as the one given to you and you don’t want to rethink your design, then you can do some of the hacky reflection stuff and put it in a static method.

public class Blah {
    private final Collection<Integer> collection;

    public Blah(Collection<Integer> collection) {
        this.collection = clone(collection);
    }

    @SuppressWarnings("unchecked")
    private static <T> T clone(T t) {
        try {
            Method cloneMethod = t.getClass().getDeclaredMethod("clone");
            return (T) cloneMethod.invoke(t);
        } catch (Exception ex) {
            // TODO Log ex
            return null;
        }
    }
}

Must of the concrete collections implement clone as a public method, so this should work for most of them. It will be null for the ones that don’t. You may want to just rethrow whatever exception you get instead.