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

Formatter class

First, let's look at the new java.util.Formatter class. You probably won't use this class directly much, but it provides the guts for the formatting you'll be doing. In the Javadoc for this class, you'll find a table describing the supported formatting options. These options range from something like %7.4f for specifying the precision and width of a floating point number to %tT for formatting a time to %3$s for formatting the third argument.

Using Formatter to format output involves two steps: creating an Appendable object to store the output and using the format() method to put formatted content into that object. Here's a list of implementers for the Appendable interface:

  • BufferedWriter
  • CharArrayWriter
  • CharBuffer
  • FileWriter
  • FilterWriter
  • LogStream
  • OutputStreamWriter
  • PipedWriter
  • PrintStream
  • PrintWriter
  • StringBuffer
  • StringBuilder
  • StringWriter
  • Writer

An object implementing this interface can be used as the destination when using a Formatter class by passing the object into the Formatter constructor. Most of these classes should look familiar, except for the StringBuilder class. StringBuilder is nearly identical to the StringBuffer class, with one big exception: It isn't thread safe. If you know you are going to build up a string in a single thread, use StringBuilder. If the building can cross thread bounds, use StringBuffer. Listing 1 shows how you'd typically start using Formatter:

Listing 1. Typical formatter usage

   StringBuilder sb = new StringBuilder();
   Formatter formatter = new Formatter(sb, Locale.US);

After creating a Formatter class, you call its format() method with format strings and arguments. If you need to use a different Locale than that sent into the constructor for part of the formatted output, you can also pass in a Locale object to the format() method. Listing 2 shows the two varieties of format():

Listing 2. format() methods of Formatter

public Formatter format(String format,
                        Object... args)
public Formatter format(Locale l,
                        String format,
                        Object... args)

If you want to get the value of Pi to 10 digits of precision, the code in Listing 3 will put that value into the StringBuilder and print the output. Printing the formatter object will display the content of the Appendable object.

Listing 3. Demonstrating a Formatter

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

public class Build {
  public static void main(String args[]) {
   StringBuilder sb = new StringBuilder();
   Formatter formatter = new Formatter(sb, Locale.US);
   formatter.format("PI = %12.10f", Math.PI);

Don't forget to compile with the -source 1.5 option or the compiler won't recognize the variable argument list. Because formatting output and sending it to the console are common tasks, there are conveniences available to this behavior. We'll look at those next.

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First published by IBM developerWorks

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