Re: Retrieve the key from a map (2)
On 07.09.2006 15:17, ralf@rw7.de wrote:
Sorry to everyone,
probably I simplified my example a bit too much. So here is what I
really want;
I want to implement a cache, where the key class is implemented by
myself, but the value class is java.util.List. I want to count the hits
of this cache, separately for each cache entry. Therefore I want to
store the hits-counter at the key class.
I'd rather not do that. The reason is: the key must be know outside of
your class Cache and thus the key should not carry information which is
internal to your cache. Also it may make usage of the Cache class more
complicated; for example: if you use String as key then the client must
create a StringKeyWithCounts and hand that off to your class (at least
the way you do it right now). If you do it internally in class Cache
you still pay the overhead of object creation which you need to do the
lookup.
Rather do this: create a value class that is internal to your Cache
(private static) with a field for the value and a field for the hit
counter. See the sample below.
Kind regards
robert
import java.util.HashMap;
import java.util.Map;
public class Cache<K, V> {
private Map<K, Val<V>> values = new HashMap<K, Val<V>>();
public void set( K key, V val ) {
Val<V> tmp = values.get( key );
if ( tmp == null ) {
tmp = new Val<V>();
values.put( key, tmp );
}
tmp.setValue( val );
}
public V get( K key ) {
Val<V> tmp = values.get( key );
if ( tmp == null ) {
return null;
}
else {
tmp.increment();
return tmp.getValue();
}
}
public int getHits( K key ) {
Val<V> tmp = values.get( key );
return tmp == null ? 0 : tmp.getCount();
}
}
class Val<V> {
private V value;
private int count;
public void setValue( V val ) {
this.value = val;
}
public V getValue() {
return value;
}
public void increment() {
++count;
}
public int getCount() {
return count;
}
}