Question

I’m wondering what are the options to specialize generic types in Java, i.e. in a templated class to have specific overrides for certain types.

In my case I was a generic class (of type T) to return null usually, but return “” (the empty string), when T is the String type, or 0 (zero) when its the Integer type, etc.

Merely providing a type-specific overload of a method produces a “method is ambiguous” error:

e.g.:

public class Hacking {

  public static void main(String[] args) {
    Bar<Integer> barInt = new Bar<Integer>();
    Bar<String> barString = new Bar<String>();

    // OK, returns null
    System.out.println(barInt.get(new Integer(4)));

    // ERROR: The method get(String) is ambiguous for the type Bar<String>
    System.out.println(barString.get(new String("foo")));
  }

  public static class Bar<T> {

    public T get(T x) {
      return null;
    }

    public String get(String x) {
      return "";
    }  
  }
}

Is the only option to subclass the generic class with a specific type (see StringBar in the following example?

  public static void main(String[] args) {
    Bar<Integer> barInt = new Bar<Integer>();
    StringBar barString2 = new StringBar();

    // OK, returns null
    System.out.println(barInt.get());

    // OK, returns ""
    System.out.println(barString2.get());
  }

  public static class Bar<T> {

    public T get() {
      return null;
    }
  }

  public static class StringBar extends Bar<String> {
    public String get() {
      return "";
    }
  }
}

Is this is the only way, it’s a bit of a pain to have to create a subclass for every type I want to specialize instead of an overload of get() in the Bar class.

I’m guessing I could check the instanceof in the Bar.get() method, e.g.
T get(T t) {
if (t instanceof String) return “”;
if (t instanceof Integer) return 0;
else return null;
}

However I’ve been taught to avoid instanceof and use polymorphism when possible.

Answer 1

All things considered, the concensus appears to be that the StringBar method mentioned in the question is the only way to go.

  public static class StringBar extends Bar<String> {
    public String get() {
      return "";
    }
  }