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Collections In Java and How to Implement Them?

Updated: Jun 13, 2023

What is the Java Collection Framework?

Java Collection Framework offers the capability to Java Collection to represent a group of elements in classes and Interfaces.

Java Collection Framework enables the user to perform various data manipulation operations like storing data, searching, sorting, insertion, deletion, and updating of data on the group of elements. Followed by the Java Collections Framework, you must learn and understand the Hierarchy of Java collections and various descendants or classes and interfaces involved in the Java Collections.


Java Collection Framework Hierarchy





Java List

In this tutorial, we will learn about the List interface in Java and its methods.

In Java, the List interface is an ordered collection that allows us to store and access elements sequentially. It extends the Collection interface. Classes that Implement List Since List is an interface, we cannot create objects from it. In order to use functionalities of the List interface, we can use these classes:


How to use List?

In Java, we must import java.util.List package in order to use List.



// ArrayList implementation of List
List<String> list1 = new ArrayList<>();

// LinkedList implementation of List
List<String> list2 = new LinkedList<>();


Here, we have created objects list1 and list2 of classes ArrayList and LinkedList. These objects can use the functionalities of the List interface.

Methods of List

The List interface includes all the methods of the Collection interface. Its because Collection is a super interface of List.

Some of the commonly used methods of the Collection interface that's also available in the List interface are:

  • add() - adds an element to a list

  • addAll() - adds all elements of one list to another

  • get() - helps to randomly access elements from lists

  • iterator() - returns iterator object that can be used to sequentially access elements of lists

  • set() - changes elements of lists

  • remove() - removes an element from the list

  • removeAll() - removes all the elements from the list

  • clear() - removes all the elements from the list (more efficient than removeAll())

  • size() - returns the length of lists

  • toArray() - converts a list into an array

  • contains() - returns true if a list contains specified element

Implementation of the List Interface

1. Implementing the ArrayList Class



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

class Main {

    public static void main(String[] args) {
        // Creating list using the ArrayList class
        List<Integer> numbers = new ArrayList<>();

        // Add elements to the list
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        System.out.println("List: " + numbers);

        // Access element from the list
        int number = numbers.get(2);
        System.out.println("Accessed Element: " + number);

        // Remove element from the list
        int removedNumber = numbers.remove(1);
        System.out.println("Removed Element: " + removedNumber);
    }
}

        

Output

List: [1, 2, 3]
Accessed Element: 3
Removed Element: 2

2. Implementing the LinkedList Class



import java.util.List;
import java.util.LinkedList;

class Main {

    public static void main(String[] args) {
        // Creating list using the LinkedList class
        List<Integer> numbers = new LinkedList<>();

        // Add elements to the list
        numbers.add(1);
        numbers.add(2);
        numbers.add(3);
        System.out.println("List: " + numbers);

        // Access element from the list
        int number = numbers.get(2);
        System.out.println("Accessed Element: " + number);

        // Using the indexOf() method
        int index = numbers.indexOf(2);
        System.out.println("Position of 3 is " + index);

        // Remove element from the list
        int removedNumber = numbers.remove(1);
        System.out.println("Removed Element: " + removedNumber);
    }
}
    


Output

List: [1, 2, 3]
Accessed Element: 3
Position of 3 is 1
Removed Element: 2

Java List vs. Set

Both the List interface and the Set interface inherits the Collection interface. However, there exists some difference between them.

  • Lists can include duplicate elements. However, sets cannot have duplicate elements.

  • Elements in lists are stored in some order. However, elements in sets are stored in groups like sets in mathematics.

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