Re: enum/generics typesafe getter for generic types

Piotr Kobzda <>
Sun, 01 Apr 2007 11:54:00 +0200
Piotr Kobzda wrote:

visionset wrote:

Yes, but how do I now use this class to form a generic getter outside
of Key class?
For this kind of call:

String s = foo.get(Key.A_KEY);

because I'm not instantiating Key, I can't pass in a <Type> so I can't
form a generic method.

You can, declare it e.g. like that:

    public <T> T get(Key<T> key) { ...

Well, honestly, I'm not fully sure /you can/. It depends mostly on what
you are trying to achieve. My understanding of your goal is expressed
in attached example (SSCCE). If that not match your expectations,
explain them deeper.


import java.util.HashMap;
import java.util.Map;

public class MultivalueProperty {

     public static class Key<T> {
         Class<T> type;
         Key(Class<T> type) {
             this.type = type;

         static <T> Key<T> newKey(Class<T> type) {
             return new Key<T>(type);

         public static final Key<String> A_KEY = newKey(String.class);
         public static final Key<String> B_KEY = newKey(String.class);
         public static final Key<Integer> C_KEY = newKey(Integer.class);
         public static final Key<Integer> D_KEY = newKey(Integer.class);

         // possibly better hashCode() is needed here...


     private Map<Key<?>, Object> values = new HashMap<Key<?>, Object>();

     public <T> T get(Key<T> key) {
         return key.type.cast(values.get(key));

     public <T> void put(Key<T> key, T value) {
         values.put(key, value);

     // sample usage...
     public static void main(String[] args) {
         MultivalueProperty foo = new MultivalueProperty();

         foo.put(Key.A_KEY, "A value");
         foo.put(Key.C_KEY, 0);

         String a = foo.get(Key.A_KEY);
         String b = foo.get(Key.B_KEY);
         Integer c = foo.get(Key.C_KEY);
         Integer d = foo.get(Key.D_KEY);


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