/**
   * 对{@link Map}中元素以key排序后，每行以{key}={value}形式输出到{@link Writer}<br>
   * map为空或null时则不向writer写入任何内容
   * @param map
   * @param writer 为null抛出{@link IllegalArgumentException}
   * @throws IOException
   */
  public static void storeSortedMap(Map<String,String> map,Writer writer) throws IOException {
    if(null==writer)
      throw new IllegalArgumentException("the arugment 'writer' must not be null ");
    TreeMap<String, String> sortedMap = new TreeMap<String,String>();
    if(null!=map)
      sortedMap.putAll(map);
    BufferedWriter bw=(writer instanceof BufferedWriter)?(BufferedWriter)writer
        : new BufferedWriter(writer);
    for (Entry<String,String> e:sortedMap.entrySet()) {
      bw.write(e.getKey() + "=" + e.getValue());
      bw.newLine();
    }
    bw.flush();
  }
  /**
   * 对 {@link Collection}中元素排序后(去除重复)，元素分行输出到{@link Writer}<br>
   * collection为空或null时则不向writer写入任何内容
   * @param collection
   * @param writer 为null抛出{@link IllegalArgumentException}
   * @throws IOException
   */
  public static void storeSortedSet(Collection<String> collection,Writer writer) throws IOException {
    if(null==writer)
      throw new IllegalArgumentException("the arugment 'writer' must not be null ");
    TreeSet<String> sortedSet = new TreeSet<String>();
    if(null!=collection)
      sortedSet.addAll(collection);
    BufferedWriter bw=(writer instanceof BufferedWriter)?(BufferedWriter)writer
        : new BufferedWriter(writer);
    for (String e:sortedSet) {     
      bw.write(e);
      bw.newLine();
    }
    bw.flush();
  }

Map<String,String> map;
//....
storeSortedMap(map,new FileWriter(new File("c:\\id.txt")));
Set<String,String> set;
//....
storeSortedSet(set,new FileWriter(new File("c:\\trainval.txt")));