Map with generic entries-CodePudding

I have two generic classes A<T> and B<T> and a list map entries of the type (A<T>, B<T>). However, the generic type T can be different between entries, but is the same in key-value pairs. I want a way ensure that a key of type A<T> can only insert a value of type B<T> and not some other generic class, and similarly, ensure that any value retrieved from key A<T> is of type B<T>.

CodePudding user response：

It's not possible to declare the Map with those constraints, but you can use unsafe casting privately and expose generic methods for public access:

private Map<A<?>, B<?>> map = new HashMap<>();

public <T> void put(A<T> key, B<T> value) {
    map.put(key, value);
}

public <T> B<T> get(A<T> key) {
    return (A<T>)map.get(key);
}

CodePudding user response：

Same solution as shmodel, but complete example:

class A<T> {
}

class B<T> {
}

class ABMap {
    private final Map<A, B> map = new HashMap<>();

    public <T> void put(A<T> key, B<T> value) {
        map.put(key, value);
    }
    
    public <T> B<T> get(A<T> key) {
        return map.get(key);
    }
}

public class Test {

    public static void main(String[] args) {

        A<String> aString = new A<>();
        B<String> bString = new B<>();
        A<Integer> aInt = new A<>();
        B<Integer> bInt = new B<>();

        ABMap map = new ABMap();
        map.put(aString, bInt); // compile error
        map.put(aString, bString); // OK
        B<String> b = map.get(aInt); // compile error
        b = map.get(aString); // must be the correct type because we only allow correct puts
    }
}

(And it is coincidence that the names are identical! This was written entirely in parallel)