Generic class that accepts either of two types

Question

I want to make a generic class of this form:

class MyGenericClass<T extends Number> {} 

Problem is, I want to be acceptable for T to be either Integer or Long, but not Double. So the only two acceptable declarations will be:

MyGenericClass<Integer> instance; MyGenericClass<Long> instance; 

Is there any way to do that?

Answer 1

The answer is no. At least there is no way to do it using generic types. I would recommend a combination of generics and factory methods to do what you want.

class MyGenericClass<T extends Number> {   public static MyGenericClass<Long> newInstance(Long value) {     return new MyGenericClass<Long>(value);   }    public static MyGenericClass<Integer> newInstance(Integer value) {     return new MyGenericClass<Integer>(value);   }    // hide constructor so you have to use factory methods   private MyGenericClass(T value) {     // implement the constructor   }   // ... implement the class   public void frob(T number) {     // do something with T   } } 

This ensures that only MyGenericClass<Integer> and MyGenericClass<Long> instances can be created. Though you can still declare an variable of type MyGenericClass<Double> it will just have to be null.

Answer 2

No, there's nothing in Java generics to allow this. You might want to consider having a non-generic interface, implemented by FooIntegerImpl and FooLongImpl. It's hard to say without knowing more about what you're trying to achieve.