How to deduce type variable

Question

First, sorry for my poor English.
This piece of code compiled with no problem nor errors.
interface Singularity<T> {
    public T unique();
class UniqueArrayList<K, V extends Singularity<K> > {
    Map<K, V> map = new HashMap<K, V>();
    public void add(V value) {
        map.put(value.unique(), value);
    public boolean contains(K key) {
        return map.containsKey(key);
class Dimension implements Singularity<String> {
    public String unique() {
        return null;
class Test {
    UniqueArrayList<String, Dimension> list = new UniqueArrayList<String, Dimension>();
}My problem is simple, in this line:
UniqueArrayList<String, Dimension> list = new UniqueArrayList<String, Dimension>(); I need to specify both types K and V, but this is a burden: from definition of Dimension it is clear that K is String
How can i define UniqueArrayList in such way that K will be automatically deduced from V.
Thanks
Boaz

Answer 1

How can i define UniqueArrayList in such way that K will be automatically deduced from >V.I don't know if this is possible. But I have played around a little bit with your code and I think there is bug. Look at the following:
import java.util.HashMap;
import java.util.Map;
interface Singularity<T> {
    public T unique();
class UniqueArrayList<K, V extends Singularity<K> > {
    Map<K, V> map = new HashMap<K, V>();
    public void add(V value) {
        map.put(value.unique(), value);
    public boolean contains(K key) {
        return map.containsKey(key);
class Dimension<T> implements Singularity<T> {
    private T unique;
    public Dimension(T unique){
        this.unique=unique;
    public T unique() {
        return (T)null;
public class Test {
    public static void main(String[] args) {
        Dimension dim=new Dimension<Integer>(new Integer(0));    
        Map<String, Dimension> map = new HashMap<String, Dimension>();
        map.put(dim.unique(), dim);
        UniqueArrayList<String, Dimension> list = new UniqueArrayList<String, Dimension>(); 
        list.add(dim);
}When using the map the compiler complains about a not compatible method call. When using the array List he doesn't although I think he should.
I am not sure if this really is bug or if I missunderstand something about the genreics in Java.