Formatting and Parsing Numbers

Part:   1   2 

Sample programs listed in this chapter have been tested with JDK 1.3.1, 1.4.1 and 1.5.0.

Formatting Numbers to Strings

The Number class, java.lang.Number, is an abstract class representing a numerical value. It has several concrete subclasses: Byte, Short, Integer, Long, Float, and Double.

The NumberFormat class, java.text.NumberFormat, is an abstract class providing a foundation for the derived subclasses to format numbers to strings and parse strings back to numbers.

The DecimalFormat class, java.text.DecimalFormat, is a concrete subclass of NumberFormat. An object of DecimalFormat contains a formatting pattern and locale information. It can be used to format numbers into string representations; or parsing strings for numbers.

There are some factory methods in NumberFormat class that return predefined default DecimalFormat objects with commonly used formatting patterns. For example, NumberFormat.getCurrencyInstance() returns a DecimalFormat object with a pattern of "\u00A4#,##0.00;(\u00A4#,##0.00)" good for formatting a number into a currency representation.

The following programs shows some examples of how to format numbers to strings:

import java.util.*;
import java.text.*;
class NumberFormatTest {
   public static void main(String[] a) {
      testDefault();
      myFormats();
   }
   public static void testDefault() {
      NumberFormat nf;
      double x = -1234.5678;
      nf = NumberFormat.getInstance();
      System.out.println("Default format: "
         + ((DecimalFormat)nf).toPattern() + " -> " + nf.format(x));
      nf = NumberFormat.getNumberInstance();
      System.out.println("Default number format: "
         + ((DecimalFormat)nf).toPattern() + " -> " + nf.format(x));
      nf = NumberFormat.getCurrencyInstance();
      System.out.println("Default currency format: "
         + ((DecimalFormat)nf).toPattern() + " -> " + nf.format(x));
      nf = NumberFormat.getPercentInstance();
      System.out.println("Default percent format: "
         + ((DecimalFormat)nf).toPattern() + " -> " + nf.format(x));
   }   
   public static void myFormats() {
      double myNumber = -1234.5678;
      // getting a formater for default locale
      DecimalFormat df = new DecimalFormat("#,##0.00");
      System.out.println("Pattern: " + df.toPattern()
         + " -> " + df.format(myNumber));

      // locale sensitive currency symbole
      df = new DecimalFormat("\u00A4#,##0.00"); 
      System.out.println("Pattern: " + df.toPattern()
         + " -> " + df.format(myNumber));

      // speical negative subpattern
      df = new DecimalFormat("\u00A4#,##0.00;(\u00A4#,##0.00)"); 
      System.out.println("Pattern: " + df.toPattern()
                  + " -> " + df.format(myNumber));

      // percentage
      df = new DecimalFormat("#,##0.00%"); 
      System.out.println("Pattern: " + df.toPattern()
         + " -> " + df.format(myNumber));

      // locale sensitive currency symbole
      Locale.setDefault(Locale.JAPAN);
      df = new DecimalFormat("\u00A4#,##0.00"); 
      System.out.println("Pattern (Japan): " + df.toPattern()
         + " -> " + df.format(myNumber));
      Locale.setDefault(Locale.JAPAN);
   }
}

(Continued on next part...)

Part:   1   2