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Java Cookbook by Ian F. Darwin

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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 ...

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