Named placeholders in string formatting

Question

Hello!

In Python, when formatting string, I can fill placeholders by name rather than by position, like that:

print "There's an incorrect value '%(value)s' in column # %(column)d" % \
  { 'value': x, 'column': y }

I wonder if that is possible in Java (hopefully, without external libraries)? Thanks in advance.

Answer 1

Try Freemarker, templating library.

Answer 2

not quite, but you can use MessageFormat to reference one value multiple times:

MessageeFormat.format("There's an incorrect value \"{0}\" in column # {1}", x, y);

The above can be done with String.format() as well, but I find messageFormat syntax cleaner if you need to build complex expressions, plus you dont need to care about the type of the object you are putting into the string

Answer 3

You could have something like this on a string helper class

/**
     * An interpreter for strings with named placeholders.
     *
     * For example given the string "hello %(myName)" and the map <code>
     *      <p>Map<String, Object> map = new HashMap<String, Object>();</p>
     *      <p>map.put("myName", "world");</p>
     * </code>
     *
     * the call {@code format("hello %(myName)", map)} returns "hello world"
     *
     * It replaces every occurrence of a named placeholder with its given value
     * in the map. If there is a named place holder which is not found in the
     * map then the string will retain that placeholder. Likewise, if there is
     * an entry in the map that does not have its respective placeholder, it is
     * ignored.
     *
     * @param str
     *            string to format
     * @param values
     *            to replace
     * @return formatted string
     */
    public static String format(String str, Map<String, Object> values) {

        StringBuilder builder = new StringBuilder(str);

        for (Entry<String, Object> entry : values.entrySet()) {

            int start;
            String pattern = "%(" + entry.getKey() + ")";
            String value = entry.getValue().toString();

            // Replace every occurence of %(key) with value
            while ((start = builder.indexOf(pattern)) != -1) {
                builder.replace(start, start + pattern.length(), value);
            }
        }

        return builder.toString();
    }

Answer 4

Thanks for all your help! Using all your clues, I've written routine to do exactly what I want -- python-like string formatting using dictionary. Since I'm Java newbie, any hints are appreciated.

  public static String dictFormat(String format, Hashtable<String, Object> values)
  {
    StringBuilder convFormat = new StringBuilder(format);
    Enumeration<String> keys = values.keys();
    ArrayList valueList = new ArrayList();
    int currentPos = 1;
    while (keys.hasMoreElements())
      {
        String key = keys.nextElement(),
          formatKey = "%(" + key + ")",
          formatPos = "%" + Integer.toString(currentPos) + "$";
        int index = -1;
        while ((index = convFormat.indexOf(formatKey, index)) != -1)
          {
            convFormat.replace(index, index + formatKey.length(), formatPos);
            index += formatPos.length();
          }
        valueList.add(values.get(key));
        ++currentPos;
      }
    return String.format(convFormat.toString(), valueList.toArray());
  }

Answer 5

You can use StringTemplate library, it offers what you want and much more on the top of that: http://www.antlr.org/wiki/display/ST/Introduction