Re: Delegation and generics craziness

Roedy Green <>
Tue, 12 Aug 2008 21:02:08 GMT
On Mon, 11 Aug 2008 10:00:23 -0700 (PDT), Sideswipe
<> wrote, quoted or indirectly quoted
someone who said :

public class ExceptionOnDuplicateKeyMap<K, V> implements Map<K,V> {
   private final Map<? extends K, ? extends V> delegate;

   public ExceptionOnDuplicateKeyMap(Map<? extends K, ? extends V>
delegate) {
       this.delegate = delegate;

   public void clear() {

   public boolean containsKey(Object key) {
       return delegate.containsKey(key);

   public boolean containsValue(Object value) {
       return delegate.containsValue(value);

   public Set<Entry<K, V>> entrySet() {
       return delegate.entrySet(); // error here

   public V get(Object key) {
       return delegate.get(key);

   public boolean isEmpty() {
       return delegate.isEmpty();

   public Set<K> keySet() {
       return delegate.keySet(); // error here

   public V put(K key, V value) {
           throw new IllegalArgumentException();
       return delegate.put(key,value); // error here

   public void putAll(Map<? extends K, ? extends V> m) {
       for (Entry<? extends K, ? extends V> entry : m.entrySet())

   public V remove(Object key) {
       return delegate.remove(key);

   public int size() {
       return delegate.size();

   public Collection<V> values() {
       return delegate.values(); // error here


your first problem is you are missing your imports:
import java.util.Collection;
import java.util.Map;
import java.util.Set;

After you fix that you are left with: incompatible types
found : java.util.Set<java.util.Map.Entry<capture#674 of ? extends
K,capture#787 of ? extends V>>
required: java.util.Set<java.util.Map.Entry<K,V>>
      return delegate.entrySet(); // error here
                              ^ incompatible types
found : java.util.Set<capture#774 of ? extends K>
required: java.util.Set<K>
      return delegate.keySet(); // error here
                            ^ put(capture#371 of ? extends
K,capture#681 of ? extends V) in ja
va.util.Map<capture#371 of ? extends K,capture#681 of ? extends V>
cannot be applied to (K,V)
      return delegate.put(key,value); // error here
                     ^ incompatible types
found : java.util.Collection<capture#972 of ? extends V>
required: java.util.Collection<V>
      return delegate.values(); // error here
The mismatches are fairly obvious. The problem what do you do to fix
them. Have a look at source code for

     * Constructs a new <tt>HashMap</tt> with the same mappings as the
     * specified <tt>Map</tt>. The <tt>HashMap</tt> is created with
     * default load factor (0.75) and an initial capacity sufficient
     * hold the mappings in the specified <tt>Map</tt>.
     * @param m the map whose mappings are to be placed in this map
     * @throws NullPointerException if the specified map is null
    public HashMap(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1,

It is similar to what you want to do.

Roedy Green Canadian Mind Products
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