Question

Is there a way in Java to have a map where the type parameter of a value is tied to the type parameter of a key? What I want to write is something like the following:

public class Foo {     // This declaration won't compile - what should it be?     private static Map<Class<T>, T> defaultValues;      // These two methods are just fine     public static <T> void setDefaultValue(Class<T> clazz, T value) {         defaultValues.put(clazz, value);     }      public static <T> T getDefaultValue(Class<T> clazz) {         return defaultValues.get(clazz);     } } 

That is, I can store any default value against a Class object, provided the value’s type matches that of the Class object. I don’t see why this shouldn’t be allowed since I can ensure when setting/getting values that the types are correct.

EDIT: Thanks to cletus for his answer. I don’t actually need the type parameters on the map itself since I can ensure consistency in the methods which get/set values, even if it means using some slightly ugly casts.

Answer 1

You’re not trying to implement Joshua Bloch’s typesafe hetereogeneous container pattern are you? Basically:

public class Favorites {   private Map<Class<?>, Object> favorites =     new HashMap<Class<?>, Object>();    public <T> void setFavorite(Class<T> klass, T thing) {     favorites.put(klass, thing);   }    public <T> T getFavorite(Class<T> klass) {     return klass.cast(favorites.get(klass));   }    public static void main(String[] args) {     Favorites f = new Favorites();     f.setFavorite(String.class, 'Java');     f.setFavorite(Integer.class, 0xcafebabe);     String s = f.getFavorite(String.class);     int i = f.getFavorite(Integer.class);   } } 

From Effective Java (2nd edition) and this presentation.