Java Cookbook

Finding an Object in a Collection

Problem

You need to see whether a given collection contains a particular value.

Solution

Ask the collection if it contains an object of the given value.

Discussion

If you have created the contents of a collection, you probably know what is in it and what is not. But if the collection is prepared by another part of a large application, or even if you’ve just been putting objects into it and now need to find out if a given value was found, this recipe’s for you. There is quite a variety of methods, depending on which class of collection you have. The following methods can be used:

Method	Meaning	Implementing classes
binarySearch( )	Fairly fast search	Arrays, Collections
contains( )	Linear search	ArrayList, HashSet, Hashtable, LinkList, Properties, Vector
containsKey( ), containsValue( )	Checks if the collection contains the object as a Key or as a Value	HashMap, Hashtable, Properties, TreeMap
indexOf( )	Returns location where object is found	ArrayList, LinkedList, List, Stack, Vector
search()	Linear search	Stack

This example plays a little game of “find the hidden number” (or “needle in a haystack”); the numbers to look through are stored in an array. As games go, it’s fairly pathetic: the computer plays against itself, so you probably know who’s going to win. I wrote it that way so I would know that the data array contains valid numbers. The interesting part is not the generation of the random numbers (discussed ...