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Taming Tiger: Formatted output
By John Zukowski - 2004-05-03 Page:  1 2 3 4 5

String support

The String class has two new static format() methods that work similarly to their printf() equivalents. Send a format string and arguments (with a possible Locale) and use what is specified in the format string to convert the arguments. In the case of the String version of the method, the results are sent back as a String object, instead of going through the stream. These methods aren't overly spectacular, but they allow you to avoid using the Formatter object directly and creating an intermediate StringBuilder.

Formatting arbitrary objects

Everything you've seen so far describes how to use the new formatting capabilities to format existing objects and primitive types. If you want to provide support for using your own objects with Formatter, that's where the Formattable interface comes into play. By implementing the single formatTo() method shown in Listing 6 in your own class, you can use your own classes with format strings:

Listing 6. Formattable interface

void formatTo(Formatter formatter,
              int flags,
              Integer width,
              Integer precision)

Listing 7 demonstrates the use of the Formattable interface by providing a simple class with a name property. That name is displayed in the output, with support for controlling the width of the output and the justification.

Listing 7. Example formattable usage

import java.util.Locale;
import java.util.Formatter;
import java.util.Formattable;

public class MyObject implements Formattable {
  String name;
  public MyObject(String name) { = name;
  public void formatTo(
         Formatter fmt,
         int f,
         Integer width,
         Integer precision) {

    StringBuilder sb = new StringBuilder();
    if (precision == null) {
      // no max width
    } else if (name.length() < precision) {
    } else {
      sb.append(name.substring(0, precision - 1)).append('*');

    // apply width and justification
    if ((width != null) && (sb.length() < width)) {
      for (int i = 0, n=sb.length(); i < width - n; i++) {
        if ((f & Formattable.LEFT_JUSTIFY) == Formattable.LEFT_JUSTIFY) {
          sb.append(' ');
        } else {
          sb.insert(0, ' ');

  public static void main(String args[]) {
   MyObject my1 = new MyObject("John");
   MyObject my2 = new MyObject("Really Long Name");
   // First / Using toString()
   System.out.println("First Object : " + my1);
   // Second / Using Formatter
   System.out.format("First Object : '%s'\n", my1);
   // Second / Using Formatter
   System.out.format("Second Object: '%s'\n", my2);
   // Second / Using Formatter with width
   System.out.format("Second Object: '%10.5s'\n", my2);
   // Second / Using Formatter with width and left justification
   System.out.format("Second Object: '%-10.5s'\n", my2);

Running this program produces the output in Listing 8. The first two lines demonstrate the difference between using toString and Formatter. The last three show options for width and justification control.

Listing 9. Example formattable output

  First Object : MyObject@10b62c9
  First Object : 'John'
  Second Object: 'Really Long Name'
  Second Object: '     Real*'
  Second Object: 'Real*     '

View Taming Tiger: Formatted output Discussion

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First published by IBM developerWorks

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