Rafael Rafael - 1 month ago 6
Java Question

Generic class that accepts either of two types

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

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.

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