Question

I have to define a List and it has two types of possible values

1. String
2. Some user defined Class

How can I make a List that is type safe in that it only accepts these two types?

I want to avoid the use of raw List.

Answer 1

I want to define a List which can accept objects of only two types(above defined) any attempt of addition other than those two types should prompt me “COMPILE TIME ERROR”

This does what you are asking for (with a runtime error):

public class MyList extends ArrayList<Object> {

    public MyList() {
        super();
    }

    public MyList(int initialSize) {
        super(initialSize);
    }

    @Override
    public void add(Object obj) {
        if ((obj instanceof String) || (obj instanceof SomeType)) {
            add(obj);
        } else {
            throw new IllegalArgumentException("not a String or SomeType");
        }
    }

    public void add(String s) {
        super.add(s);
    }

    public void add(SomeType s) {
        super.add(s);
    }
}

There is no way to implement this in Java that will give you a compile time error you add an element of the wrong type (in your sense) to a List. However, if this was not a List, you could define the class to have overloaded add methods, etcetera. Creating new “adder” methods won’t help you here because the existing add(T) method will still exist in the interface. No matter what you do (in Java), it won’t be a compile time error to call it.