Question

What is a quick and easy way to fill a Java array with clones of a single object?

e.g. after:

Rectangle[] rectangles = new Rectangle[N];
fillWithClones(rectangles, new Rectangle(1, 2, 3, 4));

the rectangles array would contain N distinct Rectangle instances, initialised with the same coordinates.

I am aware of the flaws of Object.clone() in Java, but in this case I know that the objects to be copied have non-throwing, public clone() methods but may or may not have a public copy constructor.

I’m guessing there is a library method somewhere that does this, but I don’t think it’s in the JDK, Commons-collections or Guava.

Answer 1

If you don’t have the specific type you want to work with at compile time, you will have to invoke the clone method by reflection.

private static <T> T cloneByReflection(T object) {
    try {
        return (T) object.getClass().getMethod("clone").invoke(object);
    } catch (Exception e) {
        return null;    // or whatever you want to do
    }
}

public static <T> void fillWithClones(T[] array, T template) {
    for (int i = 0; i < array.length; ++i)
        array[i] = cloneByReflection(template);
}