Map on Closures in Java

import org.junit.Assert;

public class FunctionalMap {

    public interface Map<K, V> {
        V map(K key);
    }

    public static Map empty() {
        return new Map() {
            @Override
            public Object map(Object key) {
                return null;
            }
        };
    }

    public static <K, V> Map<K, V> add(final Map<K, V> map, final K newKey, final V newValue) {
        return new Map<K, V>() {
            @Override
            public V map(K key) {
                return key.equals(newKey) ? newValue : map.map(key);
            }
        };
    }

    public static <K, V> V find(Map<K, V> map, K key) {
        return map.map(key);
    }

    public static void main(String[] args) {
        Map<String, Integer> emptyMap = empty();

        Map<String, Integer> m1 = add(emptyMap, "x", 1);
        Map<String, Integer> m2 = add(m1, "y", 2);
        Map<String, Integer> m3 = add(m2, "x", 3);

        Assert.assertEquals(new Integer(1), find(m1, "x"));

        Assert.assertEquals(new Integer(1), find(m2, "x"));
        Assert.assertEquals(new Integer(2), find(m2, "y"));
        Assert.assertNull(find(m2, "z"));

        Assert.assertEquals(new Integer(3), find(m3, "x"));
        Assert.assertEquals(new Integer(2), find(m3, "y"));
    }
}