Question

I want a Set which only holds one data type and doesn’t permit any of its superclasses or subclasses from being added to the Set.

Answer 1

you can use Object.getClass() do determine the type of your objects.

if you want a guarantuee that only classes of a type MyClass are added you could just extend HashSet and override the add(Object o) method and only add the element to the collection if (o.getClass() == MyClass.class):

Update: added a working example that you can run as PoC.

public class MySet extends HashSet<MySet.MyClass> {
    public static class MyClass {}

    public static class MySubClass extends MyClass {}

    @Override
    public boolean add(MyClass c) {
        if (c.getClass() != MyClass.class) {
            throw new IllegalArgumentException("illegal class to add " + c.getClass().getName());
        }
        return super.add(c);
    }

    public static void main(String[] args) {
        MySet set = new MySet();
        set.add(new MyClass());   // works
        set.add(new MySubClass()); // throws Exception
    }

}

running this example yields:

Exception in thread “main” java.lang.IllegalArgumentException:
illegal class to add MySet$MySubClass