All the formatting routines are in the java.text package.  The package 
is quite sophisticated, and includes things like displaying currency using 
the units and display format for a number of different countries.

Here's an example of simple decimal formatting:

double numberToFormat = 7.89543;
NumberFormat numberFormatter = NumberFormat.getInstance();
numberFormatter.setMaximumFractionDigits(2);
String formattedNumber = numberFormatter.format(numberToFormat);
System.out.println(formattedNumber);

Note:  The call to system.out.println passes in the String that 
represents the formatted number, not the original double variable.

Tom