How do you insert an element at the beginning of the list Java?

How to insert an element at beginning of the ArrayList in Java?

When we add elements to the ArrayList using theadd method, they are inserted at the end of the ArrayList. To insert element at the beginning or start of the ArrayList, use the overloaded add method of ArrayList which also accepts anindex parameter wherethe element needs to be inserted.

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How to insert an element at beginning of the ArrayList in Java?
Java ArrayList insert an element at the beginning example
Adding an Element to the Front of LinkedList in Java

1	public void add(int index, E element)

This method inserts the element at the specified index in the ArrayList. If there is any element exists at the specified position, it shifts the existing elements along with any subsequent elements to the right (means increases their index by 1).

Java ArrayList insert an element at the beginning example

package com.javacodeexamples.collections.arraylist;

import java.util.ArrayList;

public class ArrayListInsertElementExample {

public static void main(String[] args) {

//create new ArrayList

ArrayList<String> aListNumbers

= new ArrayList<String>();

//we are going to add some numbers to ArrayList

aListNumbers.add("One");

aListNumbers.add("Two");

aListNumbers.add("Three");

* To insert element at beginning of ArrayList

* use add method of ArrayList class with index 0

aListNumbers.add(0, "Zero");

System.out.println("Element added at beginning of ArrayList");

//print ArrayList

System.out.println(aListNumbers);

}

Output

1 2	Element added at beginning of ArrayList [Zero, One, Two, Three]

As you can see from the output, we specified index as 0 in the add method to insert element at the start of the ArrayList. List automatically shifted all the existing elements by increasing their index by 1.

Adding an Element to the Front of LinkedList in Java

A Linked List is a linear data structure, in which the elements are not stored at contiguous memory locations. The elements are linked using pointers and addresses. Each element is known as a node. This article shows how to add an element to the front of LinkedList in Java.

Method 1: (Using user-defined method)

Allocate the memory to a new node.
Put in the element to be inserted in the allocated node.
Make the next of the new node as the head.
Move the head to point to the new node.

Example:

Java

// Java program to Add an Element 
// to the Front of LinkedList 

import java.io.*;

class LinkedList {

// head reference

Node head;

// Node class

class Node {

int data;

Node next;

Node(int d)

{

data = d;

next = null;

}

}

// Inserting node at the front

public void insertfront(int data)

{

// Allocating and inserting the data in that node

Node new_node = new Node(data);

// Make the next of the newly allocated node to be

// the head

new_node.next = head;

// Now make the head to be the newly allocated node

head = new_node;

}

// Printing the List

public void print()

{

Node temp = head;

while (temp != null) {

System.out.print(temp.data + " ");

temp = temp.next;

}

}

public static void main(String args[])

{

// create a linkedlist

LinkedList l = new LinkedList();

// insert elements at the front

l.insertfront(6);

l.insertfront(5);

l.insertfront(8);

l.insertfront(9);

// print the linkedlist

l.print();

}
}

Output 9 8 5 6

Method 2: (Using addFirst(E e) method of LinkedList)

Declaration:

void addFirst(Object element)

Syntax:

LinkedList.addFirst(e)

Parameters: This function accepts a single parameter element as shown in the above syntax. The element specified by this parameter is appended at beginning of the list.

Return Value: This method does not return any value.

Java

// Java program to Add an Element 
// to the Front of LinkedList 

import java.util.LinkedList;

class AddElementsAtTheFront {

public static void main(String args[])

{

// create a LinkedList

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

// add elements at the front

list.addFirst("HI");

list.addFirst("HOW");

list.addFirst("ARE");

list.addFirst("YOU");

// print LinkedList

System.out.print(list);

}
}

Output [YOU, ARE, HOW, HI]

Method 3: (Using offerFirst(E e))

This method also inserts the specified element at the front of the list.

Declaration:

public boolean offerFirst(E e)

Syntax:

LinkedList.offerFirst(e)

Parameters: Here, e is the element to add

Return Value: This method returns true

Example:

Java

// Java program to Add an Element 
// to the Front of LinkedList 

import java.util.LinkedList;

class AddingElementsAtTheFront {

public static void main(String args[])

{

// create a LinkedList

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

// add elements at the front

list.offerFirst("HI");

list.offerFirst("HOW");

list.offerFirst("ARE");

list.offerFirst("YOU");

// print the LinkedList

System.out.print(list);

}
}

Output [YOU, ARE, HOW, HI]

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Java List

Java List Tutorial Video
List vs. Set
List Implementations
Create a List
Generic Lists
Insert Elements in a Java List
- Insert null Values
- Insert Elements at Specific Index
Insert All Elements From One List Into Another
Get Elements From a Java List
Find Elements in a List
- Find Last Occurrence of Element in a List
Checking if List Contains Element
Remove Elements From a Java List
Remove All Elements From a Java List
Retain All Elements From One List in Another
List Size
Sublist of List
Convert List to Set
Convert List to Array
Convert Array to List
Sort List
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- Sort List Using Comparator
Iterate List
- Iterate List Using Iterator
- Iterate List Using For-Each Loop
- Iterate List Using For Loop
- Iterate List Using Java Stream API
More Details in the Java List JavaDoc

Jakob Jenkov
Last update: 2020-02-29

The Java List interface, java.util.List, represents an ordered sequence of objects. The elements contained in a Java List can be inserted, accessed, iterated and removed according to the order in which they appear internally in the Java List. The ordering of the elements is why this data structure is called a List.

Each element in a Java List has an index. The first element in the List has index 0, the second element has index 1 etc. The index means "how many elements away from the beginning of the list". The first element is thus 0 elements away from the beginning of the list - because it is at the beginning of the list.

You can add any Java object to a List. If the List is not typed, using Java Generics, then you can even mix objects of different types (classes) in the same List. Mixing objects of different types in the same List is not often done in practice, however.

The Java List interface is a standard Java interface, and it is a subtype of the Java Collection interface, meaning List inherits from Collection.