Java 12_Linked List

The LinkedList class is a collection based on a Linked List instead of an array. They are particularly good when you expect to perform many additions and deletions with a collection. When using a linked list you don't have to move items around when you add or delete an item. They aren't particularly suited to providing access based off of index searches like an array though. Each object in a linked list contains a pointer to the objects that proceed and follow it, When you change an ArrayList a new array is created by it.

Import libraries

import java.util.Arrays;
import java.util.LinkedList; // LinkedList Library methods

Creates a LinkedList object

      LinkedList linkedListOne = new LinkedList();

Creates a LinkedList that contains Strings

        LinkedList<String> names = new LinkedList<String>();

Add items to the Linked List

        names.add("Ahmed Bennani");
        names.add("Ali Syed");

Place the object

addLast places the object last in the list

        names.addLast("Nathan Martin");

addFirst places the object first in the list

        names.addFirst("Joshua Smith");

Define a specific position to place the object in linked list

        names.add(0, "Noah Peeters");

Replace a value in an index with set()

names.set(2, "Paul Newman");

Remove

items either by providing the index, or the value

                names.remove(4);
        names.remove("Joshua Smith");

removeFirst() removes the first element
removeLast() removes the last element
removeFirstOccurrence(Object) removes the first Object that matches what you passed

Use the enhanced for to print them out

        for(String name : names)
        {
            System.out.println(name);
        }
        
        /* OUTPUT
         * Noah Peeters
         * Paul Newman
         * Ali Syed
         */

Retrieve values with get()

        System.out.println("\nFirst Index: " + names.get(0));
        
        /* OUTPUT
         * First Index: Noah Peeters
         */

Retrieve the first value with getFirst()

        System.out.println("\nFirst Index: " + names.getFirst());
        
        /* OUTPUT
         * First Index: Noah Peeters
         */

Retrieve the last value with getLast()

        System.out.println("\nLast Index: " + names.getLast());
        
        /* OUTPUT
         * Last Index: Ali Syed
         */

Copy in the linked list

        LinkedList<String> nameCopy = new LinkedList<String>(names);
        
        // When you print out the LinkedList itself the toString
        // method is called
        System.out.println("\nnameCopy: " + nameCopy);
        
        /* OUTPUT
         * nameCopy: [Noah Peeters, Paul Newman, Ali Syed]
         */

Check if an object is in the list with contains()

        if(names.contains("Noah Peeters"))
        {
            System.out.println("\nNoahs Here");
        }
        
        /* OUTPUT
         * Noahs Here
         */
        
        // You can check if everything in one list is in another
        if(names.containsAll(nameCopy))
        {
            System.out.println("\nCollections are the same");
        }
        
        /* OUTPUT
         * Collections are the same
         */

Return the index for an object with indexOf

        System.out.println("\nIndex of Ali is: " + names.indexOf("Ali Syed"));
        
        /* OUTPUT
         * Index of Ali is: 2
         */

Check if a list is empty with isEmpty()

        
        /* OUTPUT
         * List Empty: false
         */

Get the size of list

        System.out.println("How many values: " + names.size());
        
        /* OUTPUT
         * How many values: 3
         */

peek & poll & push & pop

peek() retrieves the first element, but doesn't throw an error if there is no element to retrieve

        System.out.println("Look without error: " + names.peek());
        
        /* OUTPUT
         * Look without error: Noah Peeters
         */

    // poll() returns the first value and deletes it from the list
    System.out.println("Remove first element: " + nameCopy.poll());
    
    /* OUTPUT
     * Remove first element: Noah Peeters
     */

poll() returns the last value and deletes it from the list

        System.out.println("Remove last element: " + nameCopy.pollLast());
        
        /* OUTPUT
         * Remove last element: Ali Syed
         */

push puts a new element on the front of the list

     nameCopy.push("Noah Peeters");

pop removes an element on the front of the list

        nameCopy.pop();     
        System.out.println("\nnameCopy: " + nameCopy);
        
        /* OUTPUT
         * nameCopy: [Paul Newman]
         */

Create a new array to hold values from the Linked List

        Object[] nameArray = new Object[4];
        
        // toArray converts the LinkedList into an array of objects
        nameArray = names.toArray();
        
        // toString converts items in the array into a String
        System.out.println(Arrays.toString(nameArray));
        
        /* OUTPUT
         * [Noah Peeters, Paul Newman, Ali Syed]
         */

clear() deletes everything in the Linked List

    names.clear();