Linked List is a part of the Collection framework present in java.util package. This class is an implementation of the LinkedList data structure, which is a linear data structure where the elements are not stored in contiguous locations, and every element is a separate object with a data part and an address part. The elements are linked using pointers and addresses, and each element is known as a node.
Example:
Java
// Java program to add elements to a LinkedList
import java.util.LinkedList;
public class Geeks {
// Main driver method
public static void main(String[] args) {
// Creating a LinkedList
LinkedList<String> l = new LinkedList<String>();
// Adding elements to the LinkedList using add() method
l.add("One");
l.add("Two");
l.add("Three");
l.add("Four");
l.add("Five");
// Printing the LinkedList
System.out.println(l);
}
}
Output[One, Two, Three, Four, Five]
Note: The nodes cannot be accessed directly instead we have to start from the head and follow the link until we find the node that we want.
Internal Working of LinkedList
Since a LinkedList acts as a dynamic array and we do not have to specify the size while creating it, the size of the list automatically increases when we dynamically add and remove items. And also, the elements are not stored in a continuous fashion. Therefore, there is no need to increase the size. Internally, the LinkedList is implemented using the doubly linked list data structure.
Normal List vs Doubly LinkedList:
The main difference between a normal linked list and a doubly LinkedList is that Doubly linked list contains an extra pointer which is known as previous pointer. In this each node points to both the next and previous nodes.
Constructors in the LinkedList
In order to create a LinkedList, we need to create an object of the LinkedList class. The LinkedList class consists of various constructors that allow the possible creation of the list. The following are the constructors available in this class:
1. LinkedList(): This constructor is used to create an empty linked list. If we wish to create an empty LinkedList with the name ll, then it can be created as:
LinkedList ll = new LinkedList();
2. LinkedList(Collection C): This constructor is used to create an ordered list that contains all the elements of a specified collection, as returned by the collection's iterator. If we wish to create a LinkedList with the name ll, then, it can be created as:
LinkedList ll = new LinkedList(C);
Methods for Java LinkedList
| Method | Description |
|---|
| add(int index, E element) | This method Inserts the specified element at the specified position in this list. |
| add(E e) | This method Appends the specified element to the end of this list. |
| addAll(int index, Collection<E> c) | This method Inserts all of the elements in the specified collection into this list, starting at the specified position. |
| addAll(Collection<E> c) | This method Appends all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection's iterator. |
| addFirst(E e) | This method Inserts the specified element at the beginning of this list. |
| addLast(E e) | This method Appends the specified element to the end of this list. |
| clear() | This method removes all of the elements from this list. |
| clone() | This method returns a shallow copy of this LinkedList. |
| contains(Object o) | This method returns true if this list contains the specified element. |
| descendingIterator() | This method returns an iterator over the elements in this deque in reverse sequential order. |
| element() | This method retrieves but does not remove, the head (first element) of this list. |
| get(int index) | This method returns the element at the specified position in this list. |
| getFirst() | This method returns the first element in this list. |
| getLast() | This method returns the last element in this list. |
| indexOf(Object o) | This method returns the index of the first occurrence of the specified element in this list, or -1 if this list does not contain the element. |
| lastIndexOf(Object o) | This method returns the index of the last occurrence of the specified element in this list, or -1 if this list does not contain the element. |
| listIterator(int index) | This method returns a list-iterator of the elements in this list (in proper sequence), starting at the specified position in the list. |
| offer(E e) | This method Adds the specified element as the tail (last element) of this list. |
| offerFirst(E e) | This method Inserts the specified element at the front of this list. |
| offerLast(E e) | This method Inserts the specified element at the end of this list. |
| peek() | This method retrieves but does not remove, the head (first element) of this list. |
| peekFirst() | This method retrieves, but does not remove, the first element of this list, or returns null if this list is empty. |
| peekLast() | This method retrieves, but does not remove, the last element of this list, or returns null if this list is empty. |
| poll() | This method retrieves and removes the head (first element) of this list. |
| pollFirst() | This method retrieves and removes the first element of this list, or returns null if this list is empty. |
| pollLast() | This method retrieves and removes the last element of this list, or returns null if this list is empty. |
| pop() | This method Pops an element from the stack represented by this list. |
| push(E e) | This method pushes an element onto the stack represented by this list. |
| remove() | This method retrieves and removes the head (first element) of this list. |
| remove(int index) | This method removes the element at the specified position in this list. |
| remove(Object o) | This method removes the first occurrence of the specified element from this list if it is present. |
| removeFirst() | This method removes and returns the first element from this list. |
| removeFirstOccurrence(Object o) | This method removes the first occurrence of the specified element in this list (when traversing the list from head to tail). |
| removeLast() | This method removes and returns the last element from this list. |
| removeLastOccurrence(Object o) | This method removes the last occurrence of the specified element in this list (when traversing the list from head to tail). |
| set(int index, E element) | This method replaces the element at the specified position in this list with the specified element. |
| size() | This method returns the number of elements in this list. |
| spliterator() | This method creates a late-binding and fail-fast Spliterator over the elements in this list. |
| toArray() | This method returns an array containing all of the elements in this list in proper sequence (from first to last element). |
| toArray(T[] a) | This method returns an array containing all of the elements in this list in proper sequence (from first to last element); the runtime type of the returned array is that of the specified array. |
| toString() | This method returns a string containing all of the elements in this list in proper sequence (from first to the last element), each element is separated by commas and the String is enclosed in square brackets. |
Below is the implementation of the above operations:
Java
// Java Program to Demonstrate
// Implementation of LinkedList
// class
import java.util.*;
public class Geeks
{
public static void main(String args[])
{
// Creating object of the
// class linked list
LinkedList<String> ll = new LinkedList<String>();
// Adding elements to the linked list
ll.add("A");
ll.add("B");
ll.addLast("C");
ll.addFirst("D");
ll.add(2, "E");
System.out.println(ll);
ll.remove("B");
ll.remove(3);
ll.removeFirst();
ll.removeLast();
System.out.println(ll);
}
}
Output[D, A, E, B, C]
[A]
Hierarchy of Java List Interface
The image below demonstrated the inheritance and implementation hierarchy of Java List interface and its key classes.
In the above illustration, AbstractList, CopyOnWriteArrayList, and AbstractSequentialList are the classes that implement the list interface. A separate functionality is implemented in each of the mentioned classes. They are:
- AbstractList: This class is used to implement an unmodifiable list, for which one needs to only extend this AbstractList Class and implement only the get() and the size() methods.
- CopyOnWriteArrayList: This class implements the list interface. It is an enhanced version of ArrayList in which all the modifications(add, set, remove, etc.) are implemented by making a fresh copy of the list.
- Adding elements
- Updating elements
- Removing elements
- Iterating over elements
- To Array()
- Size();
- removeFirst()
- removeLast()
1. Adding Elements: With the help of add() method, we can add elements to an ArrayList. This method can perform multiple operations based on different parameters. They are:
- add(Object): This method is used to add an element at the end of the LinkedList.
- add(int index, Object): This method is used to add an element at a specific index in the LinkedList.
Example:
Java
// Java program to add elements
// to a LinkedList
import java.util.*;
public class Geeks {
public static void main(String args[])
{
LinkedList<String> ll = new LinkedList<>();
ll.add("Geeks");
ll.add("Geeks");
ll.add(1, "For");
System.out.println(ll);
}
}
Output[Geeks, For, Geeks]
2. Changing Elements: With the of set() method, we can change element in a LinkedList. This method takes an index and the updated element which needs to be inserted at that index.
Example:
Java
// Java program to change elements
// in a LinkedList
import java.util.*;
public class Geeks {
public static void main(String args[])
{
LinkedList<String> ll = new LinkedList<>();
ll.add("Geeks");
ll.add("Geeks");
ll.add(1, "Geeks");
System.out.println("Initial LinkedList " + ll);
ll.set(1, "For");
System.out.println("Updated LinkedList " + ll);
}
}
OutputInitial LinkedList [Geeks, Geeks, Geeks]
Updated LinkedList [Geeks, For, Geeks]
3. Removing Elements: With the help of remove() method we can remove element from a LinkedList. This method can perform multiple operations based on different parameters. They are:
- remove(Object): This method is used to remove an object from the LinkedList. If there are multiple objects, then the first occurrence of the object is removed.
- remove(int index): This method removes the element at the specific index in the LinkedList. After removing, the list updates so the elements shift, and the LinkedList reflects the change.
Example:
Java
// Java program to remove elements
// in a LinkedList
import java.util.*;
public class Geeks {
public static void main(String args[])
{
LinkedList<String> ll = new LinkedList<>();
ll.add("Geeks");
ll.add("Geeks");
ll.add(1, "For");
System.out.println("Initial LinkedList " + ll);
// Function call
ll.remove(1);
System.out.println("After the Index Removal " + ll);
ll.remove("Geeks");
System.out.println("After the Object Removal "
+ ll);
}
}
OutputInitial LinkedList [Geeks, For, Geeks]
After the Index Removal [Geeks, Geeks]
After the Object Removal [Geeks]
4. Iterating the LinkedList: There are multiple ways to iterate through LinkedList. The most famous ways are by using the basic for loop in combination with a get() method to get the element at a specific index and the advanced for-loop.
Example:
Java
// Java program to iterate the elements
// in an LinkedList
import java.util.*;
public class Geeks {
public static void main(String args[])
{
LinkedList<String> ll
= new LinkedList<>();
ll.add("Geeks");
ll.add("Geeks");
ll.add(1, "For");
// Using the Get method and the
// for loop
for (int i = 0; i < ll.size(); i++) {
System.out.print(ll.get(i) + " ");
}
System.out.println();
// Using the for each loop
for (String str : ll)
System.out.print(str + " ");
}
}
OutputGeeks For Geeks
Geeks For Geeks
5. Linked list to Array by using toArray(): It converts the elements of the linked list into a new array.
Example:
Java
// Converting linkedlist into an array
import java.util.*;
public class Geeks {
public static void main(String[] args)
{
LinkedList<Integer> list= new LinkedList<Integer>();
list.add(123);
list.add(12);
list.add(11);
list.add(1134);
System.out.println("LinkedList: "+ list);
Object[] a = list.toArray();
System.out.print("After converted LinkedList to Array: ");
for(Object element : a)
System.out.print(element+" ");
}
}
OutputLinkedList: [123, 12, 11, 1134]
After converted LinkedList to Array: 123 12 11 1134
6. size(): This method returns the total number of elements in the linked list.
Example:
Java
// Getting the size of a linked list
import java.io.*;
import java.util.LinkedList;
public class Geeks {
public static void main(String args[]) {
LinkedList<String> list = new LinkedList<String>();
list.add("Geeks for Geeks ");
list.add("is best ");
// Displaying the size of the list
System.out.println("The size of the linked list is: " + list.size());
}
}
OutputThe size of the linked list is: 2
7. removeFirst(): This method returns the first element of a linkedlist.
Example:
Java
// Removing first element from the linkedlist
import java.io.*;
import java.util.LinkedList;
public class Geeks {
public static void main(String args[]) {
LinkedList<Integer> list = new LinkedList<Integer>();
list.add(10);
list.add(20);
list.add(30);
System.out.println("LinkedList:" + list);
System.out.println("The remove first element is: " + list.removeFirst());
// Displaying the final list
System.out.println("Final LinkedList:" + list);
}
}
OutputLinkedList:[10, 20, 30]
The remove first element is: 10
Final LinkedList:[20, 30]
8. removelast(): This method removes the last element from the linkedlist.
Example:
Java
// Removing last element from the linked list
import java.io.*;
import java.util.LinkedList;
public class Geeks {
public static void main(String args[])
{
LinkedList<Integer> list = new LinkedList<Integer>();
list.add(10);
list.add(20);
list.add(30);
System.out.println("LinkedList:" + list);
// Remove the tail using removeLast()
System.out.println("The last element is removed: " + list.removeLast());
// Displaying the final list
System.out.println("Final LinkedList:" + list);
// Remove the tail using removeLast()
System.out.println("The last element is removed: " + list.removeLast());
// Displaying the final list
System.out.println("Final LinkedList:" + list);
}
}
OutputLinkedList:[10, 20, 30]
The last element is removed: 30
Final LinkedList:[10, 20]
The last element is removed: 20
Final LinkedList:[10]
The LinkedList class in Java is a part of the Java Collections Framework and provides a linked list implementation of the List interface. It allows for the storage and retrieval of elements in a doubly-linked list data structure, where each element is linked to its predecessor and successor elements.
Here is a simple example that demonstrates how to use a LinkedList in Java:
Java
// Demonstrating the working of LinkedList
import java.util.LinkedList;
public class Geeks{
public static void main(String[] args) {
// Create a new linked list
LinkedList<Integer> linkedList = new LinkedList<>();
// Add elements to the linked list
linkedList.add(1);
linkedList.add(2);
linkedList.add(3);
// Add an element to the beginning of the linked list
linkedList.addFirst(0);
// Add an element to the end of the linked list
linkedList.addLast(4);
// Print the elements of the linked list
for (int i : linkedList) {
System.out.println(i);
}
}
}
Advantages
- The linked list can increase or decrease it's size whenever we need, so there is no need to set the size of a linkedlist before using it.
- Adding or removing elements in the middle of a linkedlist is a very simple task. We just need to change the links between nodes without moving other elements.
- In linked list we can move in forward and backward direction because each element knows about the one before and after it.
Disadvantages
- Finding an element in a linked list takes more time because we have to fo through the list from the start.
- In linked list each element stires extra information that's why it takes more memory.
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