JavaAlgorithms
Java Algorithms
In the previous chapters, you learned how data structures (likeArrayList,HashMap, etc.) are used to store and organize data.
Algorithms are used to solve problems by sorting, searching, and manipulating data structures.
In Java, many useful algorithms are already built into theCollections class (found in thejava.util package), so you don't have to write them from scratch.
Searching
To find elements in a list, Java provides helper methods. The most common isCollections.binarySearch(), which searches in asorted list:
Example
Search for an element in a sortedArrayList:
import java.util.*;public class Main { public static void main(String[] args) { ArrayList<String> names = new ArrayList<>(); names.add("Liam"); names.add("Jenny"); names.add("Kasper"); names.add("Angie"); Collections.sort(names); // must be sorted first int index = Collections.binarySearch(names, "Angie"); System.out.println("Angie is at index: " + index); }}Sorting
Sorting is one of the most common algorithms. WithArrayList, you can useCollections.sort() to sort the elements:
Example
Sort a list of numbers:
import java.util.*;public class Main { public static void main(String[] args) { ArrayList<Integer> numbers = new ArrayList<>(); numbers.add(5); numbers.add(1); numbers.add(7); numbers.add(3); numbers.add(9); Collections.sort(numbers); System.out.println(numbers); // [1, 3, 5, 7, 9] }}You can also sort in reverse order withCollections.sort(list, Collections.reverseOrder()):
Example
Sort anArrayList in descending order:
import java.util.*;public class Main { public static void main(String[] args) { ArrayList<Integer> numbers = new ArrayList<>(); numbers.add(5); numbers.add(1); numbers.add(7); numbers.add(3); numbers.add(9); Collections.sort(numbers, Collections.reverseOrder()); System.out.println(numbers); // [9, 7, 5, 3, 1] }}Iterating
Iterating (looping) through elements is another common algorithm. You can use thefor-each loop or theIterator interface:
Example
Loop through anArrayList using for-each:
import java.util.*;public class Main { public static void main(String[] args) { ArrayList<String> colors = new ArrayList<>(); colors.add("Red"); colors.add("Green"); colors.add("Blue"); for (String c : colors) { System.out.println(c); } }}Example
Loop through anArrayList using anIterator:
import java.util.*;public class Main { public static void main(String[] args) { ArrayList<String> colors = new ArrayList<>(); colors.add("Red"); colors.add("Green"); colors.add("Blue"); Iterator<String> it = colors.iterator(); while (it.hasNext()) { System.out.println(it.next()); } }}Other Useful Algorithms
TheCollections class contains many more algorithms, such as:
Collections.max()- find the largest elementCollections.min()- find the smallest elementCollections.shuffle()- randomly shuffle elementsCollections.frequency()- count how many times an element appearsCollections.swap()- swap two elements in a list
In this example, we useCollections.max() andCollections.min() to find the largest and smallest element in anArrayList:
Example
import java.util.*;public class Main { public static void main(String[] args) { ArrayList<Integer> numbers = new ArrayList<>(); numbers.add(5); numbers.add(1); numbers.add(7); numbers.add(3); numbers.add(9); System.out.println("Max: " + Collections.max(numbers)); System.out.println("Min: " + Collections.min(numbers)); }}Randomly shuffle anArrayList:
Example
import java.util.*;public class Main { public static void main(String[] args) { ArrayList<String> cards = new ArrayList<>(); cards.add("Ace"); cards.add("King"); cards.add("Queen"); cards.add("Jack"); Collections.shuffle(cards); System.out.println(cards); }}Collections.frequency() counts how many times an element appears in a list:
Example
import java.util.*;public class Main { public static void main(String[] args) { ArrayList<String> fruits = new ArrayList<>(); fruits.add("Apple"); fruits.add("Banana"); fruits.add("Orange"); fruits.add("Banana"); fruits.add("Mango"); int count = Collections.frequency(fruits, "Banana"); System.out.println("Banana appears: " + count + " times"); }}Collections.swap() swaps two elements in a list:
Example
import java.util.*;public class Main { public static void main(String[] args) { ArrayList<String> fruits = new ArrayList<>(); fruits.add("Apple"); fruits.add("Banana"); fruits.add("Orange"); fruits.add("Mango"); Collections.swap(fruits, 0, 2); // Swap first and third element System.out.println(fruits); }}Summary
- Analgorithm is a procedure to solve a problem.
- Java provides built-in algorithms in the
Collectionsclass. - Common algorithms includesearching,sorting,iterating, and findingmin/max.
- Algorithms work together with data structures (like
ArrayList,HashSet, etc.) to make your programs more powerful and efficient.
Complete Collections Reference
For a complete reference of all Collections methods, go to ourJava Collections Reference.
