Java generics enforce same type for keys and values of map

Question

What I am looking for:

I'm looking to construct something that enforces type on both the keys and values of a map: kind of like Map<Key<X>, Value<X>>. However, I would additionally like to enforce that types match within each key/value entry, but between entries, no type must should be enforced.

For example, within the same map, these key/value pairs should be considered valid:

Key<Integer> maps to Value<Integer>
Key<String> maps to Value<String>
Key<Double> maps to Value<Double>

However, something like this would be invalid:

Key<Integer> mapping to Value<String>
Key<Double> mapping to Value<Boolean>

How can I accomplish this using Java generics?

What I'm not looking for:

I understand that I can implement something like Set<Pair>, where pair accepts Key/Value of the same type. However, looking this up by key would no longer be a constant time operation.
I understand that I could do something like Map<Key<?>, Value<?>> and just assert that the Key and Value are the same type at runtime. However, I was wondering if this is possible strictly using generics.

Not necessarily. Just looking to create a Map anywhere that just enforces "The keys must be of the same type as the values (or have the same generic type within them, in the case of Key<X>/Value<X>), but each key/value pair can be of a different type." — Jon Newmuis
– Jon Newmuis, Commented Aug 9, 2013 at 22:54
This is exactly what you searched: stackoverflow.com/questions/416540/… — Adir Dayan
– Adir Dayan, Commented Nov 5, 2020 at 9:58

Iulian Popescu · Accepted Answer · 2018-02-12 10:15:55Z

28

You can do this, but you have to roll your own wrapper on top of a Map:

class MyTypeSafeMap {
  private Map<Key<?>, Value<?>> map;
  public <T> void put(Key<T> key, Value<T> value) {
    map.put(key, value); 
  }

  public <T> Value<T> get(Key<T> key) {
    return (Value) map.get(key); 
    // we know it's safe, but the compiler can't prove it
  }
}

Compare e.g. Guava's ClassToInstanceMap.

edited Feb 12, 2018 at 10:15

Iulian Popescu

2,6434 gold badges25 silver badges31 bronze badges

answered Aug 9, 2013 at 22:50

Louis Wasserman

200k26 gold badges367 silver badges437 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

Lucas Kuhlemann Over a year ago

Does it make any difference whether I cast the return value to (Value) or (Value<T>) ?

rzwitserloot Over a year ago

@LucasKuhlemann None whatsoever. Runtime checking of generics bounds isn't possible. Either version doesn't check it (therefore generates a warning), and just assumes you know what you are doing. One via the route of 'oh hey you are using raw types so no type safety', the other via 'oh hey you are casting with generics in the type, which does not do anything, so no type safety'.

Adir Dayan · Accepted Answer · 2020-11-05 12:34:33Z

0

Use GenericMap custom class and another generic class GenericKey for the key, you can see complete example in this answer

Java map with values limited by key's type parameter

answered Nov 5, 2020 at 12:34

Adir Dayan

1,6571 gold badge17 silver badges27 bronze badges

Collectives™ on Stack Overflow

Java generics enforce same type for keys and values of map

2 Answers 2

2 Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related