Restrict a generic Class parameter to classes that implement Map

Question

I'm trying to write a Map builder. One of the constructors will allow the client to specify the type of Map they wish to build

public class MapBuilder<K, V> {

    private Map<K, V> map;

    /**
     * Create a Map builder
     * @param mapType the type of Map to build. This type must support a default constructor
     * @throws Exception
     */
    public MapBuilder(Class<? extends Map<K, V>> mapType) throws Exception {
        map = mapType.newInstance();
    }

    // remaining implementation omitted
}

The intent is that it should be possible to construct instances of the builder with:

MapBuilder<Integer, String> builder = new MapBuilder<Integer, String>(LinkedHashMap.class);

or

MapBuilder<Integer, String> builder = new MapBuilder<Integer, String>(HashMap.class);

It seems that the type signature of the constructor argument doesn't currently support this, because the line above causes a "Cannot resolve constructor" compilation error.

How can I change my constructor so that it accepts classes that implement Map only?

Answer 1

Use a Supplier instead of a Class:

public MapBuilder(Supplier<? extends Map<K, V>> supplier) {
    map = supplier.get();
}

Which then can be called like this:

MapBuilder<Integer, Integer> builder = new MapBuilder<>(LinkedHashMap::new);

This is also safer, because a Class<Map> could have no default constructor, which would throw an error (which is not very responsive code)

Answer 2

The following will work:

public MapBuilder(Class<? extends Map> mapType) throws Exception {
    map = mapType.newInstance();
}