Java generics (template) specialization possible (overriding template types with specific types)

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 "";     }   }