Polymorphism argument and return type

class A
{
    public void output() { System.out.println("Called A."); }
}

class B extends A
{
    /* Override isn't required. It's just for clarity purposes and depending on
     * your dev software it might give you a warning if you override something
     * without specifying @Override just in case you've done it accidentally.
    */
    @Override
    public void output() { System.out.println("Called B."); }

    public void somethingSpecificToB() { System.out.println("B only."); }
}

class C extends A 
{
    public void somethingSpecificToC() { System.out.println("C only."); } 
}

class Test
{
    public static void main(String[] args)
    {
        polymorphicParam(new A());
        polymorphicParam(new B());
        polymorphicParam(new C());

        A value = polymorphicReturn(true); //Return value is actually type 'A'
        value.output();
        value = polymorphicReturn(false); //Return value is actually type 'B'
        //Can only call output(), since we're treating what was returned as
        //an instance of class A
        value.output();
    }

    private static void polymorphicParam(A param)
    {
        //Even if the actual input class is B or C, you can still only
        //use the members defined in class A since that's what the parameter
        //type is.
        param.output();
    }

    private static A polymorphicReturn(bool createA)
    {
        if (createA)
           return new A();
        return new B();
    }
}

//Output:
// Called A.
// Called B.
// Called A.
// Called A.
// Called B.