Question

Apologies if my terminology is incorrect, I have only started Java (and OO programming in general) 6 weeks ago.

A homework assignment has given me an interface class:

public interface Example {
    void Function();
    //etc
}

And then I have a couple of classes that “implement” that interface class, eg:

public class myExample1 implements Example {
    void Function(){ stuff;}
    public void myExclusiveFunction() { stuff;}
        ...
}

Inside the myExample1 class, I define the functions inside Example, but also add some specific functions to the myExample1 class.

In my main program, I have created a LinkedList<Example> eList = new LinkedList<Example>. Inside that Linked List, I am storing multiple types of Examples (myExample1, myExample2, etc). I want to do something like:

eList.get(i).myExclusiveFunction();

However this will not work. The compiler tells me:

The method myExclusiveFunction() is undefined for the type Example.

How can I use this function? I really want to have the LinkedList be able to hold objects of any Example subclass, and I am under an assignment restriction to NOT edit the Example interface.

Answer 1

Well you have 2 options:

1 – Change your LinkedList to store myExample1 instead of Example.

LinkedList<myExample1> eList

2 – Use instanceof and cast the class to the type myExample1 (messy).

 Example exObj = eList.get(i);
 if (exObj instance of myExample1) {
     ((myExample1)exObj).myExclusiveFunction();
 }