How to swap keys and values in a Map

How to swap keys and values in a Map 

package com.javanotes2all.java.collectionfw;

import java.util.HashMap;
import java.util.Map;

public class SwapKeysAndValuesinMap {

 public static void main(String[] args) {
  HashMap map=new HashMap();
  map.put("1", "a");
  map.put("2", "b");
  map.put("3", "c");
  map.put("4", "d");
  map.put("5", "e");
  map.put("6", "f");
  map.put("7", "g");
  System.out.println("before swap"+map);
  map=reverse(map);
  System.out.println("after swap"+map);
 }
 public static  HashMap reverse(Map map) {
     HashMap rev = new HashMap();
     for(Map.Entry entry : map.entrySet())
         rev.put(entry.getValue(), entry.getKey());
     return rev;
 }
}