请教在map集合中通过value如何得到key

        HashMap map = new HashMap();        map.put("1", "11");        map.put("2", "22");        map.put("3", "33");        map.put("4", "44");        map.put("5", "55");        map.put("6", "66");        map.put("7", "77");        int num = 0 ;        String key;        String value;        Iterator it1 = map.keySet().iterator();        Iterator it2 = map.entrySet().iterator();                while(it2.hasNext()){            num++;            value = ((Object) it2.next()).toString();             if("55".equals(value))                break;        }        System.out.println("num = " + num);        while(it1.hasNext()){            num--;            value = (String) it1.next();             if(num ==0){                System.out.println("value = " + value);                break;            }        }
------解决方案--------------------
继承 HashMap，覆盖一些方法：

Java code
import java.util.Collections;import java.util.HashMap;import java.util.HashSet;import java.util.Map;import java.util.Set;public class ValueHashMap<K, V> extends HashMap<K, V> {    private static final long serialVersionUID = 4136683943755968213L;        protected Map<V, Set<K>> valueMap;        protected boolean isDebug = false;    public ValueHashMap() {        super();        initValueMap();    }    public ValueHashMap(int initialCapacity, float loadFactor) {        super(initialCapacity, loadFactor);        initValueMap();    }    public ValueHashMap(int initialCapacity) {        super(initialCapacity);        initValueMap();    }    public ValueHashMap(Map<? extends K, ? extends V> m) {        super(m);        initValueMap();    }    private void initValueMap() {        valueMap = new HashMap<V, Set<K>>();    }    public void clear() {        super.clear();        valueMap.clear();    }    public V put(K key, V value) {        V v = super.put(key, value);        putValue(key, value);        debugValueMap(key, value);        return v;    }    public void putAll(Map<? extends K, ? extends V> m) {        super.putAll(m);        for(Map.Entry<? extends K, ? extends V> entry : m.entrySet()) {            putValue(entry.getKey(), entry.getValue());            debugValueMap(entry.getKey(), entry.getValue());        }            }    public V remove(Object key) {        if(!containsKey(key)) {            return null;        }        V v = super.remove(key);        removeValueMapKey(key, v);        debugValueMap(key, null);        return v;    }        public Set<K> getKeySet(V value) {        return Collections.unmodifiableSet(valueMap.get(value));    }        protected void putValue(K key, V value) {        Set<K> keys = valueMap.get(value);        if(keys == null) {            keys = new HashSet<K>();            valueMap.put(value, keys);        }        keys.add(key);    }        protected void removeValueMapKey(Object key, V value) {        valueMap.get(value).remove(key);    }        protected void debugValueMap(Object key, V value) {        if(!isDebug) {            return;        }        StackTraceElement se = new Throwable().getStackTrace()[1];        System.out.print(se.getMethodName() + ": ");        System.out.print("key = [" + key + "]");        if(!"remove".equals(se.getMethodName())) {            System.out.print(", value = [" + value + "]");        }        System.out.println();        for(Map.Entry<V, Set<K>> en : valueMap.entrySet()) {            System.out.print("  [" + en.getKey() + "] --> ");            int i = 0;            for(K k : en.getValue()) {                if(i++ > 0) {                    System.out.print(", ");                }                System.out.print("[" + k + "]");            }            System.out.println();        }        System.out.println();    }}