Question

I try to use a simple example for better undersanding: I’ve got a class Tool and child classes which are extending class Tool: Hammer, Saw. Both have defined some fields like weight and both are overriding method getCost with own implementation.

    Tool first_tool = new Hammer();
    Tool second_tool = new Saw();

I need a method in Tool class, that will to do a copy of any tool, such way, that first_tool_copy is from the same subclass as first_tool. How can I make this possible? I need something like:

    /* Copy tool, change parameters of copy, the original won't change */
    /* first_tool_copy will be instance of Hammer class */
    first_tool_copy = first_tool.copy
    first_tool_copy.weight = 100

Conclusions: I would like to have some simple copy constructor common for all subclasses.

Answer 1

I would make Tool abstract, and add an abstract copy method to Tool. Then each subclass is forced to provide its own implementation. This is a fairly OO approach, and takes advantage of dynamic dispatch.

abstract class Tool
{
    // snip...
    public abstract Tool copy();
    // snip...
}

class Hammer
{
    // snip...
    public Tool copy()
    {
        Hammer h = new Hammer();
        // copy fields from this to h
        return h;
    }
    // snip...
}

Otherwise, you’d provide a concrete implementation in Tool which you’d have to update every time to want to handle a new subclass of Tool. This method would have to use instanceof, getClass(), or similar non-OO techniques to create the right class. Ew.

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Remember what Effective Java Item 10 tells us: Override clone judiciously.

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I should mention, that copy is made just by copying the weight attribute.

Assuming the Tool implementation looks something like the below class, you could do this with reflection:

class Tool
{
    // ...

    public Tool () {}

    public int getWeight ()
    {
        // don't care about implementation
    }

    public void setWeight()
    {
        // don't care about implementation
    }

    // ignores all exceptions - not production code!
    public Tool copy() throws Exception
    {
        Tool copy = this.getClass().getConstructor().newInstance();
        copy.setWeight(this.getWeight());
        return copy;
    }

    // ...
}

I wouldn’t recommend this, though. It’s just smelly and ugly to me.