Here you can find the source of reverseMap(Map
public static Map<Object, Object> reverseMap(Map<Object, Object> origMap)
//package com.java2s; /*//from w w w .j a va 2 s.co m * Copyright 2012 The Solmix Project * * This is free software; you can redistribute it and/or modify it * under the terms of the GNU Lesser General Public License as * published by the Free Software Foundation; either version 2.1 of * the License, or (at your option) any later version. * * This software is distributed in the hope that it will be useful, * but WITHOUT ANY WARRANTY; without even the implied warranty of * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU * Lesser General Public License for more details. * * you may not use this file except in compliance with the License. * You may obtain a copy of the License at * http://www.gnu.org/licenses/ * or see the FSF site: http://www.fsf.org. */ import java.util.ArrayList; import java.util.HashMap; import java.util.List; import java.util.Map; public class Main { public static Map<Object, Object> reverseMap(Map<Object, Object> origMap) { if (origMap == null) return null; Map<Object, Object> reverseMap = new HashMap<Object, Object>(); for (Object key : origMap.keySet()) { putMultiple(reverseMap, origMap.get(key), key); } return reverseMap; } public static Map<Object, Object> putMultiple(Map<Object, Object> map, Object key, Object value) { Object existingValue = map.get(key); if (existingValue == null) map.put(key, value); else if (existingValue instanceof List) ((List) existingValue).add(value); else map.put(key, buildList(existingValue, value)); return map; } /** * @param lists * @return */ public static List<Object> buildList(List<Object>... lists) { List<Object> result = new ArrayList<Object>(); for (List<Object> list : lists) { result.add(list); } return result; } /** * Build array object into List * * @param lists * @return */ public static List<Object> buildList(Object... lists) { List<Object> result = new ArrayList<Object>(); for (Object list : lists) { result.add(list); } return result; } }