Java Cookbook, Second Edition

Problem

You need to work on a range of integers.

Solution

For a contiguous set, use a for loop.

Discussion

To process a contiguous set of integers, Java provides a for loop.^[3] Loop control for the for loop is in three parts: initialize, test, and change. If the test part is initially false, the loop will never be executed, not even once.

^[3] If the set of numbers is in an array or collection (see Chapter 7), use a "foreach" loop (see Recipe 8.2).

For discontinuous ranges of numbers, use a java.util.BitSet .

The following program demonstrates all of these techniques:

import java.util.BitSet; /** Operations on series of numbers */ public class NumSeries { public static void main(String[] args) { // When you want an ordinal list of numbers, use a for loop // starting at 1. for (int i = 1; i <= months.length; i++) System.out.println("Month # " + i); // When you want a set of array indexes, use a for loop // starting at 0. for (int i = 0; i < months.length; i++) System.out.println("Month " + months[i]); // For a discontiguous set of integers, try a BitSet // Create a BitSet and turn on a couple of bits. BitSet b = new BitSet( ); b.set(0); // January b.set(3); // April // Presumably this would be somewhere else in the code. for (int i = 0; i<months.length; i++) { if (b.get(i)) System.out.println("Month " + months[i] + " requested"); } } /** The names of the months. See Dates/Times chapter for a better way */ protected static String months[] = { "January", "February", "March", "April", "May", "June", "July", "August", "September", "October", "November", "December" }; }