Restricting types in a generic class

Question

This is my first time working with generics, so I'm a little unfamiliar with the terminology. Anyway, supposing I'm creating my own implementation of AbstractList:

class MyList<T> extends AbstractList<T>

And I've got the following three classes that I cannot change:

 class TestNumber class One extends TestNumber class Two extends TestNumber class Three extends TestNumber // etc...

Is it possible to change the type parameter for MyList to allow only types One and Two, but not other classes that inherit from TestNumber?

Create a dummy interface, have only One and Two implement it, and use it as generic type. See docs.oracle.com/javase/tutorial/java/generics/bounded.html — PM 77-1
– PM 77-1, Commented Jan 7, 2015 at 4:59
@PM77-1 what if i wasn't allowed to change those classes? i forgot to mention this crucial bit earlier, sorry. — user928112
– user928112, Commented Jan 7, 2015 at 5:07

Paul Boddington · Accepted Answer · 2015-01-07 06:09:06Z

It is not possible to restrict T to being one of One or Two. However you can do something very similar.

Make a class NumberType with exactly two instances, like this:

public final class NumberType<T extends TestNumber> { public static final NumberType<One> ONE = new NumberType<One>(); public static final NumberType<Two> TWO = new NumberType<Two>(); private NumberType() {} }

Then make your class MyList look like this:

public class MyList<T extends TestNumber> extends AbstractList<T> { public static <T extends TestNumber> MyList<T> newInstance(NumberType<T> type) { return new MyList<T>(); } private MyList() {} // Class body omitted }

Because the constructor of MyList is private, the only way to instantiate a new MyList is to use the static newInstance() method. The code to instantiate a MyList looks like this:

List<One> list1 = MyList.newInstance(NumberType.ONE); List<Two> list2 = MyList.newInstance(NumberType.TWO);

It is not possible to instantiate a MyList<Three>.

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