Adding functions to Java class libraries

Question

I'm using a Java class library that is in many ways incomplete: there are many classes that I feel ought to have additional member functions built in. However, I am unsure of the best practice of adding these member functions.

Lets call the insufficient base class A.

class A
{
    public A(/*long arbitrary arguments*/)
    {
        //...
    }

    public A(/*long even more arbitrary arguments*/)
    {
        //...
    }

    public int func()
    {
        return 1;
    }
}

Ideally, I would like to add a function to A. However, I can't do that. My choice is between:

class B extends A
{
    //Implement ALL of A's constructors here

    public int reallyUsefulFunction()
    {
        return func()+1;
    }
}

and

class AddedFuncs
{
    public static int reallyUsefulFunction(A a)
    {
        return a.func()+1;
    }
}

The way I see it, they both have advantages and disadvantages. The first choice gives a cleaner syntax than the second, and is more logical, but has problems: Let's say I have a third class, C, within the class library.

class C
{
    public A func()
    {
        return new A(/*...*/);
    }
}

As I see it, there is no easy way of doing this:

C c;
int useful = c.func().reallyUsefulFunction();

as the type returned by C.func() is an A, not a B, and you can't down-cast.

So what is the best way of adding a member function to a read-only library class?

Answer 1

Natural and frequent dilemma. Read about the composition vs inheritance alternative. Your second alternative is basically a composition, if we think that the object A is passed in the constructor instead of passing it in each method - that is, we would be using composition to implement a wrapper or decorator pattern.

The issue for class C returning a new instance of class A has no trivial solution, as you guessed, as long as class C decides to take responsability of creating the new instance. This is why one should pause and think before typing a "new" statement inside a class, if there is the possibility that this class will be subclassed. In yout example, it would be nice if you could tell class C what concrete class to return ... but how would it know to create it? Well we could pass him an object who knows how to instantiate an object of class A (or a subclass)... I guess you are enough motivated to read about Factories now, and design patterns in general.

There is no unique best answer, but if want a quick one: I'd make a wrapper, B class does not extend A but has a constructor with A as parameter, it delegates its methods (except the own) to the inside object. When you need to call the method in class C (I'm assuming you cant touch class C), you could write: B b = new B(c.func())