Collections.binarySearch() in Java with Examples

Collections.binarySearch() is a method in Java used to search for an element in a sorted List using the binary search algorithm. It returns the index of the element if found, otherwise it returns a negative value indicating the insertion point.

• Works on a sorted list only for accurate results.
• Supports both natural ordering and custom sorting using Comparator.

Syntax

Used for Natural Order:

• Searches the specified key in a list sorted in ascending order
• Elements must implement Comparable

public static <T> int binarySearch(List<? extends Comparable<? super T>> list, T key)

Used for Custom Order:

• Searches the key using a custom sorting order defined by Comparator
• The list must be sorted using the same comparator before calling this method

public static <T> int binarySearch(List<? extends T> list, T key, Comparator<? super T> c)

Return Value

• If the element is found -> returns its index (≥ 0)
• If the element is not found returns:

-(insertion point) - 1

• Negative value: Returned when the element is not found in the list.
• Insertion point: The index where the element would be inserted to maintain the sorted order of the list. It also helps preserve the correct position where the element should be placed.

Example: Search an int key in a list, sorted in ascending order

3
-5

Example: Search an int key in a list, sorted in decending order

Found at index 1

Example: Search in a list of user-defined class objects

Found at index 0
-1

Note: Before using binarySearch() with a Comparator, the list must be sorted using the same Comparator; otherwise, the result is undefined.

Complexity Analysis

Time Complexity

• O(log n) for ArrayList (random access supported)
• For LinkedList, it may degrade to O(n log n) due to sequential access overhead

Auxiliary Space: O(1) (no extra data structures used)

Arrays.binarySearch() vs Collections.binarySearch()