Collections and Arrays utility methods

For this lesson, we'll focus on the methods that show up in code reviews and interviews: sorting, binary search, reversing, shuffling, max/min, frequency, disjoint checks, fill, rotate, swap, wrapper factories, Arrays.asList, array sort/search/copy/deep operations, streams, generators and Java 9 comparison helpers. Interviewers ask this because the methods look like trivia, but the real signal is whether you know the preconditions and mutability traps.

01Picture it first: sort the toolbox by job

Think of these utilities as labelled drawers. One drawer changes order, one searches, one fills or copies, one compares structure, and one controls mutability at the boundary. The hard part is not remembering every method name. The hard part is picking the drawer that matches the job.

Fig. 1

Collections and Arrays methods grouped by job: ordering, searching, copying, comparing, wrapping, randomizing and generating

Notice the split. Collections rearranges objects through collection interfaces, so it can swap elements in a list but cannot resize an array. Arrays can use array length and component type directly, so it owns copyOf, primitive sorting and deep array operations.

02Collections under the hood: ordering, searching and randomizing

Sorting is the gateway method. If you sort a list with natural order, search it with natural order. If you sort with a comparator, search it with the same comparator. Otherwise the return value is not a little wrong, it is undefined for your data.

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class CollectionsSearchDemo {
    public static void main(String[] args) {
        List<String> statuses = new ArrayList<>(List.of("paid", "new", "refunded"));
        Collections.sort(statuses);

        int found = Collections.binarySearch(statuses, "paid");
        int missing = Collections.binarySearch(statuses, "delta");
        int first = Collections.binarySearch(statuses, "aa");

        System.out.println(statuses);                 // [new, paid, refunded]
        System.out.println(found);                    // 1
        System.out.println(missing);                  // -1, insertion point 0
        System.out.println(-missing - 1);             // 0
        System.out.println(first);                    // -1, insertion point 0

        statuses.sort(Comparator.reverseOrder());
        int wrong = Collections.binarySearch(statuses, "paid");
        System.out.println(wrong);                    // not meaningful: list order changed
    }
}