Collections utility methods

Definition

The Collections class has ready-made helpers for lists and other collections: sorting, shuffling with a seed, counting, finding the largest, reversing, and creating read-only views.

Example code

Handy one-liners

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

class Demo {
    public static void main(String[] args) {
        List<Integer> numbers = new ArrayList<>(List.of(4, 8, 15, 16, 23, 42, 8));

        System.out.println(Collections.max(numbers) + " " + Collections.min(numbers));
        System.out.println(Collections.frequency(numbers, 8));            // how many 8s

        Collections.sort(numbers);
        System.out.println(numbers + " -> index of 23: " + Collections.binarySearch(numbers, 23));

        Collections.reverse(numbers);
        System.out.println(numbers);

        Collections.shuffle(numbers, new Random(1));                       // a seed makes it repeatable
        System.out.println(numbers);

        List<Integer> readOnly = Collections.unmodifiableList(numbers);
        System.out.println(readOnly.size());
        System.out.println(Collections.nCopies(3, "ha"));
    }
}

// Output:
// 42 4
// 2
// [4, 8, 8, 15, 16, 23, 42] -> index of 23: 5
// [42, 23, 16, 15, 8, 8, 4]
// [15, 42, 8, 23, 16, 4, 8]
// 7
// [ha, ha, ha]