Collections and Generics - SCJP Sun Certified Programmer for Java Platform SE6 - page 473

We just discussed how to use a Comparator object for sorting arrays either to alter the
natural ordering or for sorting an array of objects that do not implement the Comparable
interface. Now let's explore how to use the binarySearch method of Arrays to search
arrays sorted either in the natural order or with a Comparator .
Searching Arrays
The Arrays class contains a collection of binarySearch methods for searching arrays of
primitive types and objects. There is a pair of binarySearch methods for each Java array
type (except arrays of boolean s). For example, the following binarySearch methods are for
searching arrays of longs :
public static int binarySearch(long[] a, long key) searches the entire array for
the specified long . The array must be sorted first.
public static int binarySearch(long[] a, int fromIndex, int toIndex, long
key) searches a subset of the array for the specified long . The array must be sorted
first.
Similarly, there are two binarySearch methods for byte , short , int , float , double , and
char arrays. There is also a pair of binarySearch methods for Object arrays:
public static int binarySearch(Object[] a, Object key)
public static int binarySearch(Object[] a, int fromIndex, int toIndex,
Object key)
The elements in the Object array must implement Comparable and be mutually
comparable. The Arrays class also contains a generics version of binarySearch for arrays
sorted using a Comparator , as follows:
public static <T> int binarySearch(T[] a, T key, Comparator<? super T> c)
public static <T> int binarySearch(T[] a, int fromIndex, int toIndex, T
key, Comparator<? super T> c)
All of the binarySearch methods require the array to be sorted prior to searching.
If the array is not sorted, then the result of the search is undefi ned. The return value of
binarySearch is the index in the array where the key was found or negative if the given key
does not appear in the array.
Let's look at an example. The following statements search an array of long elements.
Study the code and see if you can determine its output:
6. long [] values = {432432L, 2342323L, 1244L, 89349L, 7898239L};
7. Arrays.sort(values);
8. long key = 432432L;
9. int index = Arrays.binarySearch(values, key);
10. System.out.println(key + “ found at index “ + index);
11. long key2 = 55555L;
12. int index2 = Arrays.binarySearch(values, key2);
13. System.out.println(key2 + “ found at index “ + index2);