Java Reference
In-Depth Information
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 re-
cipe's for you. There is quite a variety of methods, depending on which collection class you
have. The methods in
Table 7-4
can be used.
Table 7-4. Finding objects in a collection
Method(s)
Meaning
Implementing classes
Fairly fast search
Arrays
,
Collections
binarySearch()
Search
ArrayList
,
HashSet
,
Hashtable
,
LinkList
,
Properties
,
Vector
contains( )
containsKey( )
,
con-
tainsValue( )
Checks if the collection contains the object
as a
Key
or as a
Value
HashMap
,
Hashtable
,
Properties
,
TreeMap
Returns location where object is found
ArrayList
,
LinkedList
,
List
,
Stack
,
Vect-
or
indexOf( )
Search
search( )
Stack
The methods whose names start with
contains
will use a linear search if the collection is a
collection (
List
,
Set
), but will be quite fast if the collection is hashed (
HashSet
,
HashMap
).
So you do have to know what implementation is being used to think about performance, par-
ticularly when the collection is (or is likely to grow) large.
The next example plays a little game of “find the hidden number” (or “needle in a hay-
stack”): 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 in
Generating Random Numbers
)
. The ar-
ray to be used with
Arrays.binarySearch()
must be in sorted order, but because we just
