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Java Collections Algorithms

The Java Collections Framework includes a utility class named java.util.Collections (with an 's' at the end) that consists exclusively of static methods that operate on or return collections.

This class provides a set of powerful, reusable algorithms that can save you a lot of time and effort.

Sorting with Collections.sort()

The sort() method sorts the elements of a List into ascending order, according to the natural ordering of its elements. All elements in the list must implement the Comparable interface.

Sorting a List

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


public class Main {
  public static void main(String[] args) {
    List<String> names = new ArrayList<>();
    names.add("Charlie");
    names.add("Alice");
    names.add("Bob");
    System.out.println("Original list: " + names);
    Collections.sort(names);
    System.out.println("Sorted list: " + names);
  }
}

Shuffling with Collections.shuffle()

The shuffle() method randomly permutes the specified list. This is useful for games, simulations, or any application where you need to randomize the order of elements.

Shuffling a List

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


public class Main {
  public static void main(String[] args) {
    List<Integer> numbers = new ArrayList<>();
    for (int i = 1; i <= 5; i++) {
      numbers.add(i);
    }
    System.out.println("Original list: " + numbers);
    Collections.shuffle(numbers);
    System.out.println("Shuffled list: " + numbers);
  }
}

Other Useful Algorithms

The Collections class provides many other useful static methods.

Method Description
reverse(List<?> list) Reverses the order of the elements in the specified list.
binarySearch(List<?> list, T key) Searches the specified list for an object using the binary search algorithm. The list must be sorted beforehand.
max(Collection<?> coll) Returns the maximum element of the given collection, according to the natural ordering of its elements.
min(Collection<?> coll) Returns the minimum element of the given collection.
fill(List<?> list, T obj) Replaces all of the elements of the specified list with the specified element.
frequency(Collection<?> c, Object o) Returns the number of times the specified object occurs in the specified collection.

Using Other Algorithms

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


public class Main {
  public static void main(String[] args) {
    List<Integer> numbers = new ArrayList<>();
    numbers.add(10);
    numbers.add(50);
    numbers.add(20);
    System.out.println("Max element: " + Collections.max(numbers));
    System.out.println("Min element: " + Collections.min(numbers));
    Collections.reverse(numbers);
    System.out.println("Reversed list: " + numbers);
  }
}

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