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Java Collections Algorithms

What you'll learn

  • Understand the main ideas in Java Algorithms.
  • See a working example and explanation to help you learn by doing.
  • Follow a clear path to the next lesson in this topic.

When to use this lesson

Use this lesson when you want to understand the key concepts behind Java Algorithms.

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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.

Understand the underlying data structures in our Java Collections guide.

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.

Learn more about implementing behavior in our Java Interfaces guide.

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);
  }
}

Exercise

?

Which method from the Collections class is used to sort a List?

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