Collections: binarySearch needs a sorted list

Collections.binarySearch is logarithmic but assumes sorted input. On unsorted data the result is meaningless, and a miss returns a negative insertion point.

Code

var sorted = new ArrayList<>(List.of(10, 20, 30, 40, 50));

System.out.println("index of 30 = " + Collections.binarySearch(sorted, 30));
int miss = Collections.binarySearch(sorted, 35);
System.out.println("miss returns " + miss + ", insertion point = " + (-miss - 1));

sorted.add(-miss - 1, 35);
System.out.println("inserted in order: " + sorted);

Output

index of 30 = 2
miss returns -4, insertion point = 3
inserted in order: [10, 20, 30, 35, 40, 50]

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Published 2026-07-30