You can use the ________ method to replace an item at a specific location in an arraylist.

Replace existing element in ArrayList

To replace an existing element, we must find the exact position (index) of the element in arraylist. Once we have the index, we can use set() method to update the replace the old element with new element.

Table of Contents Show

Replace existing element in ArrayList
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Further Reading:
Java.util.ArrayList.add() Method in Java
List Interface in Java with Examples
Operations in a List interface
Iterating over List
Methods of the List Interface
Java List vs Set
Classes Association with a List Interface
Class 1: ArrayList
Class 2: Vector
Class 3: Stack
Class 4: LinkedList

Find index of existing element using indexOf() method.
Use set(index, object) to update new element.

ArrayList set() method example

Java program to update arraylist object and set value at index.

public class ArrayListExample { public static void main(String[] args) { ArrayList<String> list = new ArrayList<>(); list.add("A"); list.add("B"); list.add("C"); list.add("D"); System.out.println(list); //Replace C with C_NEW //1 - In multiple steps int index = list.indexOf("C"); list.set(index, "C_NEW"); System.out.println(list); //2 - In single step replace D with D_NEW list.set( list.indexOf("D") , "D_NEW"); System.out.println(list); } }

Program output.

[A, B, C, D] [A, B, C_NEW, D] [A, B, C_NEW, D_NEW]

Happy Learning !!

A Guide to Java ArrayList
ArrayList Java Docs

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Java.util.ArrayList.add() Method in Java

Below are the add() methods of ArrayList in Java:

boolean add(Object o) : This method appends the specified element to the end of this list. Parameters: object o: The element to be appended to this list. Exception: NA

// Java code to illustrate add(Object o) 

import java.io.*;

import java.util.ArrayList;

public class ArrayListDemo {

public static void main(String[] args)

{

// create an empty array list with an initial capacity

ArrayList<Integer> arrlist = new ArrayList<Integer>(5);

// use add() method to add elements in the list

arrlist.add(15);

arrlist.add(20);

arrlist.add(25);

// prints all the elements available in list

for (Integer number : arrlist) {

System.out.println("Number = " + number);

}

}
}

Output:

Number = 15 Number = 20 Number = 25

void add(int index, Object element) : This method inserts the specified element E at the specified position in this list.It shifts the element currently at that position (if any) and any subsequent elements to the right (will add one to their indices). Parameters: index : The index at which the specified element is to be inserted. element : The element to be inserted. Exception: Throws IndexOutOfBoundsException if the specified index is out of range (index size()).

// Java code to illustrate 
// void add(int index, Object element) 

import java.io.*;

import java.util.ArrayList;

public class ArrayListDemo {

public static void main(String[] args)

{

// create an empty array list with an initial capacity

ArrayList<Integer> arrlist = new ArrayList<Integer>(5);

// use add() method to add elements in the list

arrlist.add(10);

arrlist.add(22);

arrlist.add(30);

arrlist.add(40);

// adding element 35 at fourth position

arrlist.add(3, 35);

// let us print all the elements available in list

for (Integer number : arrlist) {

System.out.println("Number = " + number);

}

}
}

Output:

Number = 10 Number = 22 Number = 30 Number = 35 Number = 40

This article is contributed by Shambhavi Singh. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to . See your article appearing on the GeeksforGeeks main page and help other Geeks.

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Article Tags :

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List Interface in Java with Examples

The List interface in Java provides a way to store the ordered collection. It is a child interface of Collection. It is an ordered collection of objects in which duplicate values can be stored. Since List preserves the insertion order, it allows positional access and insertion of elements.

The List interface is found in java.util package and inherits the Collection interface. It is a factory of ListIterator interface. Through the ListIterator, we can iterate the list in forward and backward directions. The implementation classes of the List interface are ArrayList, LinkedList, Stack, and Vector. ArrayList and LinkedList are widely used in Java programming. The Vector class is deprecated since Java 5.

Declaration: The List interface is declared as:

public interface List<E> extends Collection<E>;

Let us elaborate on creating objects or instances in a List class. Since List is an interface, objects cannot be created of the type list. We always need a class that implements this List in order to create an object. And also, after the introduction of Generics in Java 1.5, it is possible to restrict the type of object that can be stored in the List. Just like several other user-defined ‘interfaces’ implemented by user-defined ‘classes’, List is an ‘interface’, implemented by the ArrayList class, pre-defined in the java.util package.

Syntax: This type of safelist can be defined as:

List<Obj> list = new ArrayList<Obj> ();

Note: Obj is the type of the object to be stored in List

Example:

Java

// Java program to Demonstrate List Interface
// Importing all utility classes

import java.util.*;
// Main class
// ListDemo class

class GFG {

// Main driver method

public static void main(String[] args)

{

// Creating an object of List interface

// implemented by the ArrayList class

List<Integer> l1 = new ArrayList<Integer>();

// Adding elements to object of List interface

// Custom inputs

l1.add(0, 1);

l1.add(1, 2);

// Print the elements inside the object

System.out.println(l1);

// Now creating another object of the List

// interface implemented ArrayList class

// Declaring object of integer type

List<Integer> l2 = new ArrayList<Integer>();

// Again adding elements to object of List interface

// Custom inputs

l2.add(1);

l2.add(2);

l2.add(3);

// Will add list l2 from 1 index

l1.addAll(1, l2);

System.out.println(l1);

// Removes element from index 1

l1.remove(1);

// Printing the updated List 1

System.out.println(l1);

// Prints element at index 3 in list 1

// using get() method

System.out.println(l1.get(3));

// Replace 0th element with 5

// in List 1

l1.set(0, 5);

// Again printing the updated List 1

System.out.println(l1);

}
}

Output [1, 2] [1, 1, 2, 3, 2] [1, 2, 3, 2] 2 [5, 2, 3, 2]

Now let us perform various operations using List Interface to have a better understanding of the same. We will be discussing the following operations listed below and later on implementing via clean java codes.

Operations in a List interface

Since List is an interface, it can be used only with a class that implements this interface. Now, let’s see how to perform a few frequently used operations on the List.

Operation 1: Adding elements to List class using add() method
Operation 2: Updating elements in List class using set() method
Operation 3: Removing elements using remove() method

Now let us discuss the operations individually and implement the same in the code to grasp a better grip over it.

Operation 1: Adding elements to List class using add() method

In order to add an element to the list, we can use the add() method. This method is overloaded to perform multiple operations based on different parameters.

Parameters: It takes 2 parameters, namely:

add(Object): This method is used to add an element at the end of the List.
add(int index, Object): This method is used to add an element at a specific index in the List

Example:

Java

// Java Program to Add Elements to a List
// Importing all utility classes

import java.util.*;
// Main class

class GFG {

// Main driver method

public static void main(String args[])

{

// Creating an object of List interface,

// implemented by ArrayList class

List<String> al = new ArrayList<>();

// Adding elements to object of List interface

// Custom elements

al.add("Geeks");

al.add("Geeks");

al.add(1, "For");

// Print all the elements inside the

// List interface object

System.out.println(al);

}
}

Output [Geeks, For, Geeks]

Operation 2: Updating elements

After adding the elements, if we wish to change the element, it can be done using the set() method. Since List is indexed, the element which we wish to change is referenced by the index of the element. Therefore, this method takes an index and the updated element which needs to be inserted at that index.

Example:

Java

// Java Program to Update Elements in a List
// Importing utility classes

import java.util.*;

// Main class

class GFG {

// Main driver method

public static void main(String args[])

{

// Creating an object of List interface

List<String> al = new ArrayList<>();

// Adding elements to object of List class

al.add("Geeks");

al.add("Geeks");

al.add(1, "Geeks");

// Display theinitial elements in List

System.out.println("Initial ArrayList " + al);

// Setting (updating) element at 1st index

// using set() method

al.set(1, "For");

// Print and display the updated List

System.out.println("Updated ArrayList " + al);

}
}

Output Initial ArrayList [Geeks, Geeks, Geeks] Updated ArrayList [Geeks, For, Geeks]

Operation 3: Removing Elements

In order to remove an element from a list, we can use the remove() method. This method is overloaded to perform multiple operations based on different parameters. They are:

Parameters:

remove(Object): This method is used to simply remove an object from the List. If there are multiple such objects, then the first occurrence of the object is removed.
remove(int index): Since a List is indexed, this method takes an integer value which simply removes the element present at that specific index in the List. After removing the element, all the elements are moved to the left to fill the space and the indices of the objects are updated.

Example:

Java

// Java Program to Remove Elements from a List
// Importing List and ArrayList classes
// from java.util package

import java.util.ArrayList;

import java.util.List;
// Main class

class GFG {

// Main driver method

public static void main(String args[])

{

// Creating List class object

List<String> al = new ArrayList<>();

// Adding elements to the object

// Custom inputs

al.add("Geeks");

al.add("Geeks");

// Adding For at 1st indexes

al.add(1, "For");

// Print the initialArrayList

System.out.println("Initial ArrayList " + al);

// Now remove element from the above list

// present at 1st index

al.remove(1);

// Print the List after removal of element

System.out.println("After the Index Removal " + al);

// Now remove the current object from the updated

// List

al.remove("Geeks");

// Finally print the updated List now

System.out.println("After the Object Removal "

+ al);

}
}

Output Initial ArrayList [Geeks, For, Geeks] After the Index Removal [Geeks, Geeks] After the Object Removal [Geeks]

Iterating over List

Till now we are having a very small input size and we are doing operations manually for every entity. Now let us discuss various ways by which we can iterate over the list to get them working for a larger sample set.

Methods: There are multiple ways to iterate through the List. 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 a loop.

Example:

Java

// Java program to Iterate the Elements
// in an ArrayList
// Importing java utility classes

import java.util.*;
// Main class

public class GFG {

// main driver method

public static void main(String args[])

{

// Creating an empty Arraylist of string type

List<String> al = new ArrayList<>();

// Adding elements to above object of ArrayList

al.add("Geeks");

al.add("Geeks");

// Adding element at specified position

// inside list object

al.add(1, "For");

// Using for loop for iteration

for (int i = 0; i < al.size(); i++) {

// Using get() method to

// access particular element

System.out.print(al.get(i) + " ");

}

// New line for better readability

System.out.println();

// Using for-each loop for iteration

for (String str : al)

// Printing all the elements

// which was inside object

System.out.print(str + " ");

}
}

Output Geeks For Geeks Geeks For Geeks

Methods of the List Interface

Since the main concept behind the different types of the lists is the same, the list interface contains the following methods:

Method	Description
add(int index, element)	This method is used to add an element at a particular index in the list. When a single parameter is passed, it simply adds the element at the end of the list.
addAll(int index, Collection collection)	This method is used to add all the elements in the given collection to the list. When a single parameter is passed, it adds all the elements of the given collection at the end of the list.
size()	This method is used to return the size of the list.
clear()	This method is used to remove all the elements in the list. However, the reference of the list created is still stored.
remove(int index)	This method removes an element from the specified index. It shifts subsequent elements(if any) to left and decreases their indexes by 1.
remove(element)	This method is used to remove the first occurrence of the given element in the list.
get(int index)	This method returns elements at the specified index.
set(int index, element)	This method replaces elements at a given index with the new element. This function returns the element which was just replaced by a new element.
indexOf(element)	This method returns the first occurrence of the given element or -1 if the element is not present in the list.
lastIndexOf(element)	This method returns the last occurrence of the given element or -1 if the element is not present in the list.
equals(element)	This method is used to compare the equality of the given element with the elements of the list.
hashCode()	This method is used to return the hashcode value of the given list.
isEmpty()	This method is used to check if the list is empty or not. It returns true if the list is empty, else false.
contains(element)	This method is used to check if the list contains the given element or not. It returns true if the list contains the element.
containsAll(Collection collection)	This method is used to check if the list contains all the collection of elements.
sort(Comparator comp)	This method is used to sort the elements of the list on the basis of the given comparator.

Java List vs Set

Both the List interface and the Set interface inherits the Collection interface. However, there exists some differences between them.

List	Set
The List is an ordered sequence.	The Set is an unordered sequence.
List allows duplicate elements	Set doesn’t allow duplicate elements.
Elements by their position can be accessed.	Position access to elements is not allowed.
Multiple null elements can be stored.	The null element can store only once.
List implementations are ArrayList, LinkedList, Vector, Stack	Set implementations are HashSet, LinkedHashSet.

Classes Association with a List Interface

Now let us discuss the classes that implement the List Interface for which first do refer to the pictorial representation below to have a better understanding of the List interface. It is as follows:

AbstractList, CopyOnWriteArrayList, and the AbstractSequentialList are the classes that implement the List interface. A separate functionality is implemented in each of the mentioned classes. They are as follows:

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.
AbstractSequentialList: This class implements the Collection interface and the AbstractCollection class. 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.

We will proceed in this manner.

ArrayList
Vector
Stack
LinkedList

Let us discuss them sequentially and implement the same to figure out the working of the classes with the List interface.

Class 1: ArrayList

An ArrayList class which is implemented in the collection framework provides us with dynamic arrays in Java. Though, it may be slower than standard arrays but can be helpful in programs where lots of manipulation in the array is needed. Let’s see how to create a list object using this class.

Example:

Java

// Java program to demonstrate the
// creation of list object using the
// ArrayList class

import java.io.*;

import java.util.*;

class GFG {

public static void main(String[] args)

{

// Size of ArrayList

int n = 5;

// Declaring the List with initial size n

List<Integer> arrli

= new ArrayList<Integer>(n);

// Appending the new elements

// at the end of the list

for (int i = 1; i <= n; i++)

arrli.add(i);

// Printing elements

System.out.println(arrli);

// Remove element at index 3

arrli.remove(3);

// Displaying the list after deletion

System.out.println(arrli);

// Printing elements one by one

for (int i = 0; i < arrli.size(); i++)

System.out.print(arrli.get(i) + " ");

}
}

Output [1, 2, 3, 4, 5] [1, 2, 3, 5] 1 2 3 5

Class 2: Vector

Vector is a class that is implemented in the collection framework implements a growable array of objects. Vector implements a dynamic array that means it can grow or shrink as required. Like an array, it contains components that can be accessed using an integer index. Vectors basically fall in legacy classes but now it is fully compatible with collections. Let’s see how to create a list object using this class.

Example:

Java

// Java program to demonstrate the
// creation of list object using the
// Vector class

import java.io.*;

import java.util.*;

class GFG {

public static void main(String[] args)

{

// Size of the vector

int n = 5;

// Declaring the List with initial size n

List<Integer> v = new Vector<Integer>(n);

// Appending the new elements

// at the end of the list

for (int i = 1; i <= n; i++)

v.add(i);

// Printing elements

System.out.println(v);

// Remove element at index 3

v.remove(3);

// Displaying the list after deletion

System.out.println(v);

// Printing elements one by one

for (int i = 0; i < v.size(); i++)

System.out.print(v.get(i) + " ");

}
}

Output [1, 2, 3, 4, 5] [1, 2, 3, 5] 1 2 3 5

Class 3: Stack

Stack is a class that is implemented in the collection framework and extends the vector class models and implements the Stack data structure. The class is based on the basic principle of last-in-first-out. In addition to the basic push and pop operations, the class provides three more functions of empty, search and peek. Let’s see how to create a list object using this class.

Example:

Java

// Java program to demonstrate the
// creation of list object using the
// Stack class

import java.io.*;

import java.util.*;

class GFG {

public static void main(String[] args)

{

// Size of the stack

int n = 5;

// Declaring the List

List<Integer> s = new Stack<Integer>();

// Appending the new elements

// at the end of the list

for (int i = 1; i <= n; i++)

s.add(i);

// Printing elements

System.out.println(s);

// Remove element at index 3

s.remove(3);

// Displaying the list after deletion

System.out.println(s);

// Printing elements one by one

for (int i = 0; i < s.size(); i++)

System.out.print(s.get(i) + " ");

}
}

Output [1, 2, 3, 4, 5] [1, 2, 3, 5] 1 2 3 5

Class 4: LinkedList

LinkedList is a class that is implemented in the collection framework which inherently implements the linked list data structure. It 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 address part. The elements are linked using pointers and addresses. Each element is known as a node. Due to the dynamicity and ease of insertions and deletions, they are preferred over the arrays. Let’s see how to create a list object using this class.

Example:

Java

// Java program to demonstrate the
// creation of list object using the
// LinkedList class

import java.io.*;

import java.util.*;

class GFG {

public static void main(String[] args)

{

// Size of the LinkedList

int n = 5;

// Declaring the List with initial size n

List<Integer> ll = new LinkedList<Integer>();

// Appending the new elements

// at the end of the list

for (int i = 1; i <= n; i++)

ll.add(i);

// Printing elements

System.out.println(ll);

// Remove element at index 3

ll.remove(3);

// Displaying the list after deletion

System.out.println(ll);

// Printing elements one by one

for (int i = 0; i < ll.size(); i++)

System.out.print(ll.get(i) + " ");

}
}