People are often asking Java questions such as 'how do I sort a Map on value', as opposed to key, so the following is a class that reverses the Map, so the keys and values are swapped.
This provides greater abstraction of these problems, so sorting on value becomes trivial. Download HERE
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
/**
* * Reverse a Map, such that the keys become
* * the values and vice versa. The former
* * need to be held in a List owing to the
* * possible presence of duplicates
* *
* * @author Charles Johnson
* * @version 1.0
* *
* * @param <K> The key type of the original map
* * @param <V> The value type of the original map
* */
public class MapReverser<K, V> {
private Map<K, V> map;
public MapReverser(Map<K, V> map) {
this.map = map;
}
public Map<V, List<K>> getMap() {
Map<V, List<K>> reversed = new HashMap<V, List<K>>();
Iterator<Map.Entry<K, V>> i = map.entrySet().iterator();
while (i.hasNext()) {
Map.Entry<K, V> e = i.next();
K key = e.getKey();
V value = e.getValue();
List<K> reversedKeys = null;
if (reversed.containsKey(value)) {
reversedKeys = reversed.get(value);
} else {
reversedKeys = new ArrayList<K>();
reversed.put(value, reversedKeys);
}
reversedKeys.add(key);
}
return reversed;
}
}
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
/**
* * Reverse a Map, such that the keys become
* * the values and vice versa. The former
* * need to be held in a List owing to the
* * possible presence of duplicates
* *
* * @author Charles Johnson
* * @version 1.0
* *
* * @param <K> The key type of the original map
* * @param <V> The value type of the original map
* */
public class MapReverser<K, V> {
private Map<K, V> map;
public MapReverser(Map<K, V> map) {
this.map = map;
}
public Map<V, List<K>> getMap() {
Map<V, List<K>> reversed = new HashMap<V, List<K>>();
Iterator<Map.Entry<K, V>> i = map.entrySet().iterator();
while (i.hasNext()) {
Map.Entry<K, V> e = i.next();
K key = e.getKey();
V value = e.getValue();
List<K> reversedKeys = null;
if (reversed.containsKey(value)) {
reversedKeys = reversed.get(value);
} else {
reversedKeys = new ArrayList<K>();
reversed.put(value, reversedKeys);
}
reversedKeys.add(key);
}
return reversed;
}
}
