Cloning subclasses in Java

Question

I need to clone a subclass in Java, but at the point in the code where this happens, I won't know the subclass type, only the super class. What's the best design pattern for doing this?

Example:

class Foo {
    String myFoo;
    public Foo(){}
    public Foo(Foo old) {
        this.myFoo = old.myFoo;
    }
}

class Bar extends Foo {
    String myBar;
    public Bar(){}
    public Bar(Bar old) {
        super(old); // copies myFoo
        this.myBar = old.myBar;
    }
}

class Copier {
    Foo foo;

    public Foo makeCopy(Foo oldFoo) {

        // this doesn't work if oldFoo is actually an
        // instance of Bar, because myBar is not copied
        Foo newFoo = new Foo(oldFoo);
        return newFoo;

        // unfortunately, I can't predict what oldFoo's the actual class
        // is, so I can't go:
        // if (oldFoo instanceof Bar) { // copy Bar here }
    }
}

Answer 1

If you have control of the classes you are trying to copy, then a virtual method is the way forward:

class Foo {
    ...
    public Foo copy() {
        return new Foo(this);
    }
}
class Bar extends Foo {
    ...
    @Override public Bar copy() {
        return new Bar(this);
    }
}

(Ideally make classes either abstract or effectively final.)

Answer 2

The most standard way in Java would be to make each class that may be cloned implement Cloneable, and override Object.clone() with a public version that does the cloning appropiately for that class (by default Object.clone() makes a shallow copy of the object).

Note that many people think that Cloneable/Object.clone() is bad design.