How to Implement Generic LinkedList in Java?

Last Updated : 14 Feb, 2023

Linked List is Linear Data Structures that store values in nodes. As we do know here each Node possesses two properties namely the value of the node and link to the next node if present so. Linked List can not only be of Integer data type but String, boolean, Float, Character, etc. We can implement such a “generic” Linked List Data Type that can store values of any data type.

There are 6 primary member functions of a linked list: E

add (data): It adds an element at the end of the linked list
add (position, data): It adds an element to any valid position in the linked list
remove(key): It removes node that contains key from the linked list
clear() : it clears the entire linked list
empty(): It checks if the linked list is empty or not
length(): It returns the length of the linked list

Note: Time complexity is of order N for adding and removing operations and of order 1 for other operations.

Illustration: An Integer Linked List {100,200,300,400} is represented looks like

Implementation:

Example

Java

// Java Program to Implement Generic  Linked List
 
// Importing all input output classes
import java.io.*;
 
// Class 1
// Helper Class (Generic node class for LinkedList)
class node<T> {
 
    // Data members
    // 1. Storing value of node
    T data;
    // 2. Storing address of next node
    node<T> next;
 
    // Parameterized constructor to assign value
    node(T data)
    {
 
        // This keyword refers to current object itself
        this.data = data;
        this.next = null;
    }
}
 
// Class 2
// Helper class ( Generic LinkedList class)
class list<T> {
 
    // Generic node instance
    node<T> head;
    // Data member to store length of list
    private int length = 0;
 
    // Default constructor
    list() { this.head = null; }
    // Method
    // To add node at the end of List
    void add(T data)
    {
 
        // Creating new node with given value
        node<T> temp = new node<>(data);
 
        // Checking if list is empty
        // and assigning new value to head node
        if (this.head == null) {
            head = temp;
        }
 
        // If list already exists
        else {
 
            // Temporary node for traversal
            node<T> X = head;
 
            // Iterating till end of the List
            while (X.next != null) {
                X = X.next;
            }
 
            // Adding new valued node at the end of the list
            X.next = temp;
        }
 
        // Increasing length after adding new node
        length++;
    }
 
    // Method
    // To add new node at any given position
    void add(int position, T data)
    {
 
        // Checking if position is valid
        if (position > length + 1) {
 
            // Display message only
            System.out.println(
                "Position Unavailable in LinkedList");
            return;
        }
 
        // If new position is head then replace head node
        if (position == 1) {
 
            // Temporary node that stores previous head
            // value
            node<T> temp = head;
 
            // New valued node stored in head
            head = new node<T>(data);
 
            // New head node pointing to old head node
            head.next = temp;
 
            return;
        }
 
        // Temporary node for traversal
        node<T> temp = head;
 
        // Dummy node with null value that stores previous
        // node
        node<T> prev = new node<T>(null);
        // iterating to the given position
        while (position - 1 > 0) {
            // assigning previous node
            prev = temp;
            // incrementing next node
            temp = temp.next;
            // decreasing position counter
            position--;
        }
        // previous node now points to new value
        prev.next = new node<T>(data);
        // new value now points to former current node
        prev.next.next = temp;
    }
    // Method
    // To remove a node from list
    void remove(T key)
    {
 
        //  NOTE
        // dummy node is used to represent the node before
        // the current node Since in a Singly Linked-List we
        // cannot go backwards from a node, we use a dummy
        // node to represent the previous node. In case of
        // head node, since there is no previous node, the
        // previous node is assigned to null.
 
        // Dummy node with null value
        node<T> prev = new node<>(null);
 
        // Dummy node pointing to head node
        prev.next = head;
 
        // Next node that points ahead of current node
        node<T> next = head.next;
 
        // Temporary node for traversal
        node<T> temp = head;
 
        // Boolean value that checks whether value to be
        // deleted exists or not
        boolean exists = false;
 
        // If head node needs to be deleted
        if (head.data == key) {
            head = head.next;
 
            // Node to be deleted exists
            exists = true;
        }
 
        // Iterating over LinkedList
        while (temp.next != null) {
 
            // We convert value to be compared into Strings
            // and then compare using
            // String1.equals(String2) method
 
            // Comparing value of key and current node
            if (String.valueOf(temp.data).equals(
                    String.valueOf(key))) {
 
                // If node to be deleted is found previous
                // node now points to next node skipping the
                // current node
                prev.next = next;
                // node to be deleted exists
                exists = true;
 
                // As soon as we find the node to be deleted
                // we exit the loop
                break;
            }
 
            // Previous node now points to current node
            prev = temp;
 
            // Current node now points to next node
            temp = temp.next;
 
            // Next node points the node ahead of current
            // node
            next = temp.next;
        }
 
        // Comparing the last node with the given key value
        if (exists == false
            && String.valueOf(temp.data).equals(
                String.valueOf(key))) {
 
            // If found , last node is skipped over
            prev.next = null;
 
            // Node to be deleted exists
            exists = true;
        }
 
        // If node to be deleted exists
        if (exists) {
 
            // Length of LinkedList reduced
            length--;
        }
 
        // If node to be deleted does not exist
        else {
 
            // Print statement
            System.out.println(
                "Given Value is not present in linked list");
        }
    }
 
    // Method
    // To clear the entire LinkedList
    void clear()
    {
 
        // Head now points to null
        head = null;
        // length is 0 again
        length = 0;
    }
 
    // Method
    // Returns whether List is empty or not
    boolean empty()
    {
 
        // Checking if head node points to null
        if (head == null) {
            return true;
        }
        return false;
    }
    // Method
    // Returning the length of LinkedList
    int length() { return this.length; }
 
    // Method
    // To display the LinkedList
    // @Override
    public String toString()
    {
 
        String S = "{ ";
 
        node<T> X = head;
 
        if (X == null)
            return S + " }";
 
        while (X.next != null) {
            S += String.valueOf(X.data) + " -> ";
            X = X.next;
        }
 
        S += String.valueOf(X.data);
        return S + " }";
    }
}
 
// Class 3
// Main Class
public class GFG {
 
    // main driver method
    public static void main(String[] args)
    {
 
        // Integer List
 
        // Creating new empty Integer linked list
        list<Integer> list1 = new list<>();
        System.out.println(
            "Integer LinkedList created as list1 :");
        // Adding elements to the above List object
 
        // Element 1 - 100
        list1.add(100);
        // Element 2 - 200
        list1.add(200);
        // Element 3 - 300
        list1.add(300);
 
        // Display message only
        System.out.println(
            "list1 after adding 100,200 and 300 :");
 
        // Print and display the above List elements
        System.out.println(list1);
 
        // Removing 200 from list1
        list1.remove(200);
 
        // Display message only
        System.out.println("list1 after removing 200 :");
 
        // Print and display again updated List elements
        System.out.println(list1);
 
        // String LinkedList
 
        // Creating new empty String linked list
        list<String> list2 = new list<>();
        System.out.println(
            "\nString LinkedList created as list2");
        // Adding elements to the above List object
 
        // Element 1 - hello
        list2.add("hello");
 
        // Element 2 - world
        list2.add("world");
 
        // Display message only
        System.out.println(
            "list2 after adding hello and world :");
 
        // Print current elements only
        System.out.println(list2);
 
        // Now, adding element 3- "GFG" at position 2
        list2.add(2, "GFG");
 
        // Display message only
        System.out.println(
            "list2 after adding GFG at position 2 :");
 
        // now print the updated List again
        // after inserting element at second position
        System.out.println(list2);
 
        // Float LinkedList
 
        // Creating new empty Float linked list
        list<Float> list3 = new list<>();
 
        // Display message only
        System.out.println(
            "\nFloat LinkedList created as list3");
 
        // Adding elements to the above List
 
        // Element 1 - 20.25
        list3.add(20.25f);
        // Element 2 - 50.42
        list3.add(50.42f);
        // Element 3 - 30.99
        list3.add(30.99f);
 
        // Display message only
        System.out.println(
            "list3 after adding 20.25, 50.42 and 30.99 :");
 
        // Print and display List elements
        System.out.println(list3);
 
        // Display message only
        System.out.println("Clearing list3 :");
 
        // Now.clearing this list using clear() method
        list3.clear();
 
        // Now, print and display the above list again
        System.out.println(list3);
    }
}

Output :-

Integer LinkedList created as list1 :
list1 after adding 100,200 and 300 :
{ 100 -&gt; 200 -&gt; 300 }
list1 after removing 200 :
{ 100 -&gt; 300 }

String LinkedList created as list2
list2 after adding hello and world :
{ hello -&gt; world }
list2 after adding GFG at position 2 :
{ hello -&gt; GFG -&gt; world }

Float LinkedList created as list3
list3 after adding 20.25, 50.42 and 30.99 :
{ 20.25 -&gt; 50.42 -&gt; 30.99 }
Clearing list3 :
{  }

Time Complexity: O(n)
Auxiliary Space : O(n)

Suggest improvement

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Public vs Package Access Modifiers in Java

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