How do I create a Linked List Data Structure in Java?

Question

What's the best way to make a linked list in Java?

The best way to make a linked list is to use the built in linked list. Don't re-write built in classes. — Peter Lawrey, Dec 26 '09 at 17:26
this question is legitimate and very constructive for programmers discussion — anshulkatta, May 25 '14 at 09:36

score 220 · Accepted Answer · edited May 14 '17 at 14:02

The obvious solution to developers familiar to Java is to use the LinkedList class already provided in java.util. Say, however, you wanted to make your own implementation for some reason. Here is a quick example of a linked list that inserts a new link at the beginning of the list, deletes from the beginning of the list and loops through the list to print the links contained in it. Enhancements to this implementation include making it a double-linked list, adding methods to insert and delete from the middle or end, and by adding get and sort methods as well.

Note: In the example, the Link object doesn't actually contain another Link object - nextLink is actually only a reference to another link.

class Link {
    public int data1;
    public double data2;
    public Link nextLink;

    //Link constructor
    public Link(int d1, double d2) {
        data1 = d1;
        data2 = d2;
    }

    //Print Link data
    public void printLink() {
        System.out.print("{" + data1 + ", " + data2 + "} ");
    }
}

class LinkList {
    private Link first;

    //LinkList constructor
    public LinkList() {
        first = null;
    }

    //Returns true if list is empty
    public boolean isEmpty() {
        return first == null;
    }

    //Inserts a new Link at the first of the list
    public void insert(int d1, double d2) {
        Link link = new Link(d1, d2);
        link.nextLink = first;
        first = link;
    }

    //Deletes the link at the first of the list
    public Link delete() {
        Link temp = first;
        if(first == null){
         return null;
         //throw new NoSuchElementException(); // this is the better way. 
        }
        first = first.nextLink;
        return temp;
    }

    //Prints list data
    public void printList() {
        Link currentLink = first;
        System.out.print("List: ");
        while(currentLink != null) {
            currentLink.printLink();
            currentLink = currentLink.nextLink;
        }
        System.out.println("");
    }
}  

class LinkListTest {
    public static void main(String[] args) {
        LinkList list = new LinkList();

        list.insert(1, 1.01);
        list.insert(2, 2.02);
        list.insert(3, 3.03);
        list.insert(4, 4.04);
        list.insert(5, 5.05);

        list.printList();

        while(!list.isEmpty()) {
            Link deletedLink = list.delete();
            System.out.print("deleted: ");
            deletedLink.printLink();
            System.out.println("");
        }
        list.printList();
    }
}

you could also quite easily improve this code to use generics for the data type rather than storing an int and a double. — shsteimer, May 29 '09 at 13:36
@shsteimer: quite definitely, but since pretty much the only good use of this code is to demonstrate the technique, it wouldn't help anyone. It would only diffuse the basic idea. — Joachim Sauer, Dec 28 '09 at 15:07
It isn't good OO approach to have `public Link nextLink` and operating on it outside the class. It could be respectable when `Link` would be an internal class of `LinkList`. It is another bunch of code written as Java was only another-version-of-c. — Bart, Sep 03 '10 at 11:29
When you Insert, your first item will never get a nextLink - unless I'm missing something with Java references — Chris S, Mar 06 '11 at 22:39
If you delete from an empty list, you'll get a NullPointerException, specifically when you try to access `first.nextLink` — Bad Request, May 11 '13 at 22:04

score 55 · Answer 2 · answered Aug 13 '08 at 16:35

55

Java has a LinkedList implementation, that you might wanna check out. You can download the JDK and it's sources at java.sun.com.

answered Aug 13 '08 at 16:35

dlinsin

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Does Java's Linkedlist doesn't allow you insert and remove elements at arbitrary positions? – Seun Osewa Feb 25 '10 at 17:40
10

Isn't that the whole point of a linked list? – jrockway May 17 '10 at 03:48
2

@Seun Osewa if you want to add at an arbitrary position pls use an ArrayList :) – headgrowe Feb 28 '13 at 13:41
3

Instead of downloading the JDK to view its implementation of `LinkedList`, you can just [view its `LinkedList.java` online here](http://grepcode.com/file/repository.grepcode.com/java/root/jdk/openjdk/7-b147/java/util/LinkedList.java). That page even syntax-highlights the code and renders the Javadoc comments inline. – Rory O'Kane Dec 13 '13 at 05:20

Juha Syrjälä · Answer 3 · 2008-08-20T06:45:06.857

22

Use java.util.LinkedList. Like this:

list = new java.util.LinkedList()

edited Aug 20 '08 at 06:45

answered Aug 15 '08 at 16:11

Juha Syrjälä

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score 18 · Answer 4 · edited Dec 28 '09 at 14:44

18

The above linked list display in opposite direction. I think the correct implementation of insert method should be

public void insert(int d1, double d2) { 
    Link link = new Link(d1, d2); 

    if(first==null){
        link.nextLink = null;
        first = link; 
        last=link;
    }
    else{
        last.nextLink=link;
        link.nextLink=null;
        last=link;
    }
}

edited Dec 28 '09 at 14:44

sth

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answered Dec 26 '09 at 17:13

arnab sarkar

181
1
2

1

Add new at end unless otherwise stated. :-) – Jul 19 '11 at 18:02

score 9 · Answer 5 · answered Mar 27 '09 at 17:14

9

Its much better to use java.util.LinkedList, because it's probably much more optimized, than the one that you will write.

answered Mar 27 '09 at 17:14

Jakub Arnold

79,807
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16

And it will work first time. – Peter Lawrey May 29 '09 at 21:33

score 7 · Answer 6 · edited Jul 13 '11 at 10:34

//slightly improved code without using collection framework

package com.test;

public class TestClass {

    private static Link last;
    private static Link first;

    public static void main(String[] args) {

        //Inserting
        for(int i=0;i<5;i++){
            Link.insert(i+5);
        }
        Link.printList();

        //Deleting
        Link.deletefromFirst();
        Link.printList();
    }


    protected  static class Link {
        private int data;
        private Link nextlink;

        public Link(int d1) {
            this.data = d1;
        }

        public static void insert(int d1) {
            Link a = new Link(d1);
            a.nextlink = null;
            if (first != null) {
                last.nextlink = a;
                last = a;
            } else {
                first = a;
                last = a;
            }
            System.out.println("Inserted -:"+d1);
        }

        public static void deletefromFirst() {
            if(null!=first)
            {
                System.out.println("Deleting -:"+first.data);
                first = first.nextlink;
            }
            else{
                System.out.println("No elements in Linked List");
            }
        }

        public static void printList() {
            System.out.println("Elements in the list are");
            System.out.println("-------------------------");
            Link temp = first;
            while (temp != null) {
                System.out.println(temp.data);
                temp = temp.nextlink;
            }
        }
    }
}

How do I create a Linked List Data Structure in Java?

6 Answers6

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