Unlock the Secrets of Java Linked Lists: Learn How to Implement Them without Collections with Examples

Table of content

1. Introduction
2. Basics of Linked Lists
3. Types of Linked Lists
4. Implementing a Singly Linked List
5. Implementing a Doubly Linked List
6. Advantages and Disadvantages of Linked Lists
7. Exercises and Examples

Introduction

Linked lists are a fundamental data structure in computer science, used to store collections of data in a sequential order. In Java, linked lists are commonly implemented using the built-in Collections framework. However, understanding how to implement linked lists without Collections is an important skill, as it allows programmers to build more optimized solutions and gain a deeper understanding of how linked lists work.

In this article, we will explore how to implement linked lists in Java without using the Collections framework, including the basic concepts of linked lists and how they differ from arrays. We will also provide step-by-step examples to help you follow along with the implementation process. By the end of this article, you will have a solid understanding of how to implement linked lists in Java and be ready to apply this knowledge to create more efficient and effective solutions in your Java programming projects.

Basics of Linked Lists

A linked list is a data structure used in programming for storing and managing a collection of items. Unlike an array, a linked list does not have a fixed size and can dynamically grow or shrink as needed. A linked list consists of nodes, each of which contains a value and a reference to the next node in the list.

To create a basic linked list in Java, we first define a Node class with two instance variables: data to store the value and next to store a reference to the next node. We then create a LinkedList class that initializes an empty list and provides methods to add nodes to the list, remove nodes from the list, and traverse the list.

When adding a node to the list, we create a new Node object with the given value and set its next reference to the current head of the list. We then set the new node as the new head. When removing a node from the list, we first check if the node is the head of the list. If it is, we simply set the next node as the new head. Otherwise, we traverse the list until we find the node and update its previous node's next reference to skip over the node.

Understanding the is crucial for implementing more advanced data structures and algorithms in Java. By mastering this fundamental concept, you can unlock the full potential of Java programming and take your skills to the next level.

Types of Linked Lists

There are several in Java that can be implemented without relying on collections. The most common include singly linked lists, doubly linked lists, and circular linked lists.

A singly linked list is a simple type of linked list where each node contains a reference to the next node in the list. This means that each node only has one pointer, which points to the next node in the sequence. Singly linked lists are often used when we only need to traverse the list in one direction.

A doubly linked list, on the other hand, is a type of linked list where each node contains a reference to the next node in the list as well as the previous node in the list. This means that each node has two pointers, one that points to the next node in the sequence and another that points to the previous node in the sequence. This type of linked list is often used when we need to traverse the list in both forward and backward directions.

A circular linked list is another type of linked list where the last node in the list points back to the first node, creating a circular loop. This means that each node has a reference to the next node in the list, with the last node pointing back to the first node. Circular linked lists are often used in computer science to implement various data structures, such as hash tables and queue systems.

Overall, understanding the various available in Java and how to implement them without collections can greatly enhance one's ability to write efficient and effective code. By utilizing the appropriate type of linked list for a given problem, programmers can optimize their code and create more robust applications.

Implementing a Singly Linked List

To implement a singly linked list in Java, you first need to define a Node class that contains a data element and a next pointer. The data element can be of any data type, while the next pointer is a reference to the next Node in the list.

Here's an example Node class:

public class Node {
    int data;
    Node next;

    public Node(int data) {
        this.data = data;
        this.next = null;
    }
}

Now that we have our Node class, we can define our SinglyLinkedList class which has a head pointer referencing the first node in the list.

Here's an example SinglyLinkedList class:

public class SinglyLinkedList {
    Node head;

    public void add(int data) {
        Node newNode = new Node(data);
        if (head == null) {
            head = newNode;
        } else {
            Node current = head;
            while (current.next != null) {
                current = current.next;
            }
            current.next = newNode;
        }
    }

    public void print() {
        Node current = head;
        while (current != null) {
            System.out.print(current.data + " ");
            current = current.next;
        }
        System.out.println();
    }
}

In the add() method, we create a new Node containing the data and add it to the end of the list. If the list is empty, the new Node becomes the head. Otherwise, we iterate through the list until we find the last Node, and then set its next pointer to the new Node.

The print() method simply iterates through the list, printing the data of each Node.

With these two classes, you can create and manipulate a singly linked list in Java!

Implementing a Doubly Linked List

A doubly linked list is a type of linked list where each node contains both a reference to the next node in the list and a reference to the previous node in the list. This makes it possible to traverse the list in both directions.

To implement a doubly linked list in Java, we need to define a Node class that contains a prev and next reference, as well as a data field to store the actual data. The prev reference will point to the previous node in the list, and the next reference will point to the next node in the list. If a node is the first node in the list, its prev reference will be null, and if it is the last node in the list, its next reference will be null.

public class Node {
    public Object data;
    public Node prev;
    public Node next;
    
    public Node(Object data) {
        this.data = data;
        this.prev = null;
        this.next = null;
    }
}

Next, we need to define a DoublyLinkedList class that contains references to the first and last nodes in the list. This makes it easy to add or remove nodes from the front or back of the list. The DoublyLinkedList class also needs to keep track of the size of the list.

public class DoublyLinkedList {
    private Node head;
    private Node tail;
    private int size;
    
    public DoublyLinkedList() {
        this.head = null;
        this.tail = null;
        this.size = 0;
    }
    
    // methods to add and remove nodes from the list
}

To add a node to the front of the list, we create a new node with the given data, set its next reference to the current head of the list, and set the head of the list to the new node. We also need to update the prev reference of the old head node to point to the new node.

public void addFirst(Object data) {
    Node newNode = new Node(data);
    
    if (head == null) {
        // list is empty, new node is both head and tail
        head = newNode;
        tail = newNode;
    } else {
        // list is non-empty, add new node to front
        newNode.next = head;
        head.prev = newNode;
        head = newNode;
    }
    
    size++;
}

To remove a node from the front of the list, we simply update the next reference of the current head of the list to point to the next node in the list, and update the prev reference of the new head node (if one exists) to point to null.

public Object removeFirst() {
    if (head == null) {
        // list is empty, nothing to remove
        return null;
    } else if (head == tail) {
        // list contains only one node, remove it
        Object data = head.data;
        head = null;
        tail = null;
        size--;
        return data;
    } else {
        // list contains multiple nodes, remove first node
        Object data = head.data;
        head = head.next;
        head.prev = null;
        size--;
        return data;
    }
}

We can add similar methods to add or remove nodes from the back of the list. The basic idea is to update the prev and next references of the relevant nodes after adding or removing a node.

public void addLast(Object data) {
    // similar to addFirst, but updates tail instead of head
}

public Object removeLast() {
    // similar to removeFirst, but updates tail instead of head
}

With these basic methods in place, we can use a doubly linked list to store and manipulate data in a flexible and efficient way. Depending on the requirements of our program, we may need to add additional methods or data fields to the DoublyLinkedList class, but the basic structure and principles should remain the same.

Advantages and Disadvantages of Linked Lists

Linked lists offer several advantages and disadvantages when used for data storage and manipulation. One significant advantage of linked lists is their ability to efficiently insert and delete elements at any point in the list. This is because, unlike arrays, linked lists do not require shifting elements or allocating new memory when an element is added to or removed from the middle of the list.

Another advantage of linked lists is their dynamic size, as they can grow or shrink as needed without requiring pre-allocation of memory. This enables flexibility in data storage for applications where the number and size of elements may vary over time.

However, linked lists also have some disadvantages. One major drawback is their lack of efficient random access, as accessing elements in a linked list requires traversal from the head or tail of the list, which can be time-consuming for large lists.

Additionally, linked lists have greater memory overhead compared to arrays, as each element in the list requires a separate node with pointers to the next element. This can result in increased memory usage and slower performance for large lists that contain many small elements.

Overall, linked lists can be a powerful tool for data storage and manipulation in certain applications, but it is important to consider the advantages and disadvantages before deciding whether to use them.

Exercises and Examples

To help you better understand the implementation of Java Linked Lists without using Collections, we have provided some .

Exercise #1: Create a Simple Linked List

The first step in implementing a Linked List is to create a simple Linked List. Here are the steps:

1. Create a Node class that would represent each element in the Linked List. The Node class should have two instance variables: data (to store the value of the node) and next (to store the reference of the next node).
2. Create a Linked List class that would hold the reference of the first node in the Linked List. The Linked List class should have two instance variables: head (to store the reference of the first node) and size (to store the size of the Linked List).
3. Create methods that would allow you to add, remove, and get elements from the Linked List.

Here's an example:

class Node {
    int data;
    Node next;
 
    public Node(int data) {
        this.data = data;
        this.next = null;
    }
}
 
class LinkedList {
    Node head;
    int size;
 
    public LinkedList() {
        head = null;
        size = 0;
    }
 
    public void add(int data) {
        Node newNode = new Node(data);
        if (head == null) {
            head = newNode;
        } else {
            Node current = head;
            while (current.next != null) {
                current = current.next;
            }
            current.next = newNode;
        }
        size++;
    }
 
    public int get(int index) {
        Node current = head;
        for (int i = 0; i < index; i++) {
            current = current.next;
        }
        return current.data;
    }
 
    public void remove(int index) {
        if (index < 0 || index >= size) {
            throw new IndexOutOfBoundsException();
        }
 
        if (index == 0) {
            head = head.next;
        } else {
            Node current = head;
            for (int i = 0; i < index - 1; i++) {
                current = current.next;
            }
            current.next = current.next.next;
        }
 
        size--;
    }
}

Exercise #2: Reverse a Linked List

In this exercise, you will implement a method to reverse a Linked List. Here are the steps:

1. Traverse the Linked List and store each node in a stack.
2. Traverse the Linked List again and pop each node from the stack and add it to the reversed Linked List.

Here's an example:

public void reverse() {
    Stack<Node> stack = new Stack<>();
    Node current = head;
 
    while (current != null) {
        stack.push(current);
        current = current.next;
    }
 
    Node newHead = stack.pop();
    current = newHead;
 
    while (!stack.empty()) {
        Node nextNode = stack.pop();
        current.next = nextNode;
        current = nextNode;
    }
 
    current.next = null;
    head = newHead;
}

Exercise #3: Merge Two Sorted Linked Lists

In this exercise, you will merge two sorted Linked Lists into a single sorted Linked List. Here are the steps:

1. Traverse both Linked Lists simultaneously and add the smaller element to the new Linked List.
2. Once one of the Linked Lists is exhausted, add the remaining elements of the other Linked List to the new Linked List.

Here's an example:

public static LinkedList merge(LinkedList list1, LinkedList list2) {
    Node current1 = list1.head;
    Node current2 = list2.head;
 
    LinkedList mergedList = new LinkedList();
    while (current1 != null && current2 != null) {
        if (current1.data < current2.data) {
            mergedList.add(current1.data);
            current1 = current1.next;
        } else {
            mergedList.add(current2.data);
            current2 = current2.next;
        }
    }
 
    while (current1 != null) {
        mergedList.add(current1.data);
        current1 = current1.next;
    }
 
    while (current2 != null) {
        mergedList.add(current2.data);
        current2 = current2.next;
    }
 
    return mergedList;
}

We hope these have helped you better understand the implementation of Java Linked Lists without using Collections.