Java ArrayList

Java ArrayList class uses a dynamic array for storing the elements. It is like an array, but there is no size limit. We can add or remove elements anytime. So, it is much more flexible than the traditional array. It is found in the java.util package. It is like the Vector in C++.

The ArrayList in Java can have the duplicate elements also. It implements the List interface so we can use all the methods of the List interface here. The ArrayList maintains the insertion order internally.

It inherits the AbstractList class and implements List interface.

The important points about the Java ArrayList class are:

Java ArrayList class can contain duplicate elements.
Java ArrayList class maintains insertion order.
Java ArrayList class is non synchronized.
Java ArrayList allows random access because the array works on an index basis.
In ArrayList, manipulation is a little bit slower than the LinkedList in Java because a lot of shifting needs to occur if any element is removed from the array list.
We can not create an array list of the primitive types, such as int, float, char, etc. It is required to use the required wrapper class in such cases. For example:

ArrayList<int> al = ArrayList<int>(); // does not work
ArrayList<Integer> al = new ArrayList<Integer>(); // works fine

Java ArrayList gets initialized by the size. The size is dynamic in the array list, which varies according to the elements getting added or removed from the list.

Hierarchy of ArrayList class

As shown in the above diagram, the Java ArrayList class extends AbstractList class which implements the List interface. The List interface extends the Collection and Iterable interfaces in hierarchical order.

ArrayList class declaration

Let's see the declaration for java.util.ArrayList class.

public class ArrayList<E> extends AbstractList<E> implements List<E>, RandomAccess, Cloneable, Serializable

Constructors of ArrayList

Constructor	Description
ArrayList()	It is used to build an empty array list.
ArrayList(Collection<? extends E> c)	It is used to build an array list that is initialized with the elements of the collection c.
ArrayList(int capacity)	It is used to build an array list that has the specified initial capacity.

Methods of ArrayList

Method	Description
void add(int index, E element)	It is used to insert the specified element at the specified position in a list.
boolean add(E e)	It is used to append the specified element at the end of a list.
boolean addAll(Collection<? extends E> c)	It is used to append 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.
boolean addAll(int index, Collection<? extends E> c)	It is used to append all the elements in the specified collection, starting at the specified position of the list.
void clear()	It is used to remove all of the elements from this list.
void ensureCapacity(int requiredCapacity)	It is used to enhance the capacity of an ArrayList instance.
E get(int index)	It is used to fetch the element from the particular position of the list.
boolean isEmpty()	It returns true if the list is empty, otherwise false.
Iterator()
listIterator()
int lastIndexOf(Object o)	It is used to return the index in this list of the last occurrence of the specified element, or -1 if the list does not contain this element.
Object[] toArray()	It is used to return an array containing all of the elements in this list in the correct order.
<T> T[] toArray(T[] a)	It is used to return an array containing all of the elements in this list in the correct order.
Object clone()	It is used to return a shallow copy of an ArrayList.
boolean contains(Object o)	It returns true if the list contains the specified element.
int indexOf(Object o)	It is used to return the index in this list of the first occurrence of the specified element, or -1 if the List does not contain this element.
E remove(int index)	It is used to remove the element present at the specified position in the list.
boolean remove(Object o)	It is used to remove the first occurrence of the specified element.
boolean removeAll(Collection<?> c)	It is used to remove all the elements from the list.
boolean removeIf(Predicate<? super E> filter)	It is used to remove all the elements from the list that satisfies the given predicate.
protected void removeRange(int fromIndex, int toIndex)	It is used to remove all the elements lies within the given range.
void replaceAll(UnaryOperator<E> operator)	It is used to replace all the elements from the list with the specified element.
void retainAll(Collection<?> c)	It is used to retain all the elements in the list that are present in the specified collection.
E set(int index, E element)	It is used to replace the specified element in the list, present at the specified position.
void sort(Comparator<? super E> c)	It is used to sort the elements of the list on the basis of the specified comparator.
Spliterator<E> spliterator()	It is used to create a spliterator over the elements in a list.
List<E> subList(int fromIndex, int toIndex)	It is used to fetch all the elements that lies within the given range.
int size()	It is used to return the number of elements present in the list.
void trimToSize()	It is used to trim the capacity of this ArrayList instance to be the list's current size.

Java Non-generic Vs. Generic Collection

Java collection framework was non-generic before JDK 1.5. Since 1.5, it is generic.

Java new generic collection allows you to have only one type of object in a collection. Now it is type-safe, so typecasting is not required at runtime.

Let's see the old non-generic example of creating a Java collection.

Let's see the new generic example of creating java collection.

In a generic collection, we specify the type in angular braces. Now ArrayList is forced to have the only specified type of object in it. If you try to add another type of object, it gives a compile-time error.

For more information on Java generics, click here Java Generics Tutorial.

Java ArrayList Example

FileName: ArrayListExample1.java

import java.util.*;
 public class ArrayListExample1{
 public static void main(String args[]){
  ArrayList<String> list=new ArrayList<String>();//Creating arraylist  
	  list.add("Mango");//Adding object in arraylist  
	  list.add("Apple");  
	  list.add("Banana");  
	  list.add("Grapes");  
	  //Printing the arraylist object 
	  System.out.println(list);
 }
}

Test it Now

Output:

[Mango, Apple, Banana, Grapes]

Iterating ArrayList using Iterator

Let's see an example to traverse ArrayList elements using the Iterator interface.

FileName: ArrayListExample2.java

import java.util.*;
public class ArrayListExample2{
 public static void main(String args[]){
  ArrayList<String> list=new ArrayList<String>();//Creating arraylist
  list.add("Mango");//Adding object in arraylist  
  list.add("Apple");  
  list.add("Banana");  
  list.add("Grapes");  
  //Traversing list through Iterator
  Iterator itr=list.iterator();//getting the Iterator
  while(itr.hasNext()){//check if iterator has the elements
   System.out.println(itr.next());//printing the element and move to next
  }
 }
}

Test it Now

Output:

Mango
Apple
Banana
Grapes

Iterating ArrayList using For-each loop

Let's see an example to traverse the ArrayList elements using the for-each loop

FileName: ArrayListExample3.java

import java.util.*;
public class ArrayListExample3{
 public static void main(String args[]){
  ArrayList<String> list=new ArrayList<String>();//Creating arraylist
  list.add("Mango");//Adding object in arraylist  
  list.add("Apple");  
  list.add("Banana");  
  list.add("Grapes");  
  //Traversing list through for-each loop
  for(String fruit:list)  
    System.out.println(fruit);  

 }
}

Output:

Test it Now

Mango
Apple
Banana
Grapes

Get and Set ArrayList

The get() method returns the element at the specified index, whereas the set() method changes the element.

FileName: ArrayListExample4.java

import java.util.*;
public class ArrayListExample4{
 public static void main(String args[]){
  ArrayList<String> al=new ArrayList<String>();
  al.add("Mango");
  al.add("Apple");
  al.add("Banana");
  al.add("Grapes");
  //accessing the element  
  System.out.println("Returning element: "+al.get(1));//it will return the 2nd element, because index starts from 0
  //changing the element
  al.set(1,"Dates");
  //Traversing list
  for(String fruit:al)  
    System.out.println(fruit);  

 }
}

Test it Now

Output:

Returning element: Apple
Mango
Dates
Banana
Grapes

How to Sort ArrayList

The java.util package provides a utility class Collections, which has the static method sort(). Using the Collections.sort() method, we can easily sort the ArrayList.

FileName: SortArrayList.java

import java.util.*;
class SortArrayList{
 public static void main(String args[]){
  //Creating a list of fruits
  List<String> list1=new ArrayList<String>();
  list1.add("Mango");
  list1.add("Apple");
  list1.add("Banana");
  list1.add("Grapes");
  //Sorting the list
  Collections.sort(list1);
   //Traversing list through the for-each loop
  for(String fruit:list1)
    System.out.println(fruit);
    
 System.out.println("Sorting numbers...");
  //Creating a list of numbers
  List<Integer> list2=new ArrayList<Integer>();
  list2.add(21);
  list2.add(11);
  list2.add(51);
  list2.add(1);
  //Sorting the list
  Collections.sort(list2);
   //Traversing list through the for-each loop
  for(Integer number:list2)
    System.out.println(number);
 }
 
}

Output:

Apple
Banana
Grapes
Mango
Sorting numbers...
1
11
21
51

Ways to iterate the elements of the collection in Java

There are various ways to traverse the collection elements:

By Iterator interface.
By for-each loop.
By ListIterator interface.
By for loop.
By forEach() method.
By forEachRemaining() method.

Iterating Collection through remaining ways

Let's see an example to traverse the ArrayList elements through other ways

FileName: ArrayList4.java

import java.util.*;
class ArrayList4{
 public static void main(String args[]){
    ArrayList<String> list=new ArrayList<String>();//Creating arraylist
		   list.add("Ravi");//Adding object in arraylist
		   list.add("Vijay");
		   list.add("Ravi");
		   list.add("Ajay");
		  
		   System.out.println("Traversing list through List Iterator:");
		   //Here, element iterates in reverse order
	    	  ListIterator<String> list1=list.listIterator(list.size());
	    	  while(list1.hasPrevious())
	    	  {
	    		  String str=list1.previous();
	    		  System.out.println(str);
	    	  }
	    System.out.println("Traversing list through for loop:");
	   	   for(int i=0;i<list.size();i++)
	   	   {
	   		System.out.println(list.get(i));   
	   	   }
	    	
	   	System.out.println("Traversing list through forEach() method:");
	   	//The forEach() method is a new feature, introduced in Java 8.
	    	list.forEach(a->{ //Here, we are using lambda expression
	    	    System.out.println(a);
	    	  });
	    	  
	    	System.out.println("Traversing list through forEachRemaining() method:");
	    	  Iterator<String> itr=list.iterator();
	    	  itr.forEachRemaining(a-> //Here, we are using lambda expression
	    	  {
	    	System.out.println(a);
	    	  });
 }
}

Output:

Traversing list through List Iterator:
Ajay
Ravi
Vijay
Ravi
Traversing list through for loop:
Ravi
Vijay
Ravi
Ajay
Traversing list through forEach() method:
Ravi
Vijay
Ravi
Ajay
Traversing list through forEachRemaining() method:
Ravi
Vijay
Ravi
Ajay

User-defined class objects in Java ArrayList

Let's see an example where we are storing Student class object in an array list.

FileName: ArrayList5.java

class Student{
  int rollno;
  String name;
  int age;
  Student(int rollno,String name,int age){
   this.rollno=rollno;
   this.name=name;
   this.age=age;
  }
}

import java.util.*;
 class ArrayList5{
 public static void main(String args[]){
  //Creating user-defined class objects
  Student s1=new Student(101,"Sonoo",23);
  Student s2=new Student(102,"Ravi",21);
  Student s2=new Student(103,"Hanumat",25);
  //creating arraylist
  ArrayList<Student> al=new ArrayList<Student>();
  al.add(s1);//adding Student class object
  al.add(s2);
  al.add(s3);
  //Getting Iterator
  Iterator itr=al.iterator();
  //traversing elements of ArrayList object
  while(itr.hasNext()){
    Student st=(Student)itr.next();
    System.out.println(st.rollno+" "+st.name+" "+st.age);
  }
 }
}

Output:

       101 Sonoo 23
       102 Ravi 21
       103 Hanumat 25

Java ArrayList Serialization and Deserialization Example

Let's see an example to serialize an ArrayList object and then deserialize it.

FileName: ArrayList6.java

import java.io.*;
import java.util.*;
 class ArrayList6 {

	    public static void main(String [] args)
	    {
	      ArrayList<String> al=new ArrayList<String>();
	      al.add("Ravi");  
	      al.add("Vijay");  
	      al.add("Ajay");  
	      
	      try
	      {
              //Serialization
	    	  FileOutputStream fos=new FileOutputStream("file");
	    	  ObjectOutputStream oos=new ObjectOutputStream(fos);
	    	  oos.writeObject(al);
	    	  fos.close();
	    	  oos.close();
	    	  //Deserialization
	    	  FileInputStream fis=new FileInputStream("file");
	    	  ObjectInputStream ois=new ObjectInputStream(fis);
	    	ArrayList  list=(ArrayList)ois.readObject();
	    	System.out.println(list);  
	      }catch(Exception e)
	      {
	    	  System.out.println(e);
	      }
	   }
	}       

Output:

       [Ravi, Vijay, Ajay]

Java ArrayList example to add elements

Here, we see different ways to add an element.

FileName: ArrayList7.java

import java.util.*;
 class ArrayList7{
 public static void main(String args[]){
  ArrayList<String> al=new ArrayList<String>();
		   System.out.println("Initial list of elements: "+al);
		   //Adding elements to the end of the list
		   al.add("Ravi");
		   al.add("Vijay");
		   al.add("Ajay");
		   System.out.println("After invoking add(E e) method: "+al);
		   //Adding an element at the specific position
		   al.add(1, "Gaurav");
		   System.out.println("After invoking add(int index, E element) method: "+al);
		   ArrayList<String> al2=new ArrayList<String>();
		   al2.add("Sonoo");
		   al2.add("Hanumat");
		   //Adding second list elements to the first list
		   al.addAll(al2);
		   System.out.println("After invoking addAll(Collection<? extends E> c) method: "+al);
		   ArrayList<String> al3=new ArrayList<String>();
		   al3.add("John");
		   al3.add("Rahul");
		   //Adding second list elements to the first list at specific position
		   al.addAll(1, al3);
		   System.out.println("After invoking addAll(int index, Collection<? extends E> c) method: "+al);
		   
 }
}

Output:

Initial list of elements: []
After invoking add(E e) method: [Ravi, Vijay, Ajay]
After invoking add(int index, E element) method: [Ravi, Gaurav, Vijay, Ajay]
After invoking addAll(Collection<? extends E> c) method: 
[Ravi, Gaurav, Vijay, Ajay, Sonoo, Hanumat]
After invoking addAll(int index, Collection<? extends E> c) method: 
[Ravi, John, Rahul, Gaurav, Vijay, Ajay, Sonoo, Hanumat]

Java ArrayList example to remove elements

Here, we see different ways to remove an element.

FileName: ArrayList8.java

import java.util.*;
 class ArrayList8 {

	    public static void main(String [] args)
	    {
	      ArrayList<String> al=new ArrayList<String>();
	      al.add("Ravi");  
	      al.add("Vijay");  
	      al.add("Ajay"); 
	      al.add("Anuj");
	      al.add("Gaurav");
	      System.out.println("An initial list of elements: "+al); 
	      //Removing specific element from arraylist
	      al.remove("Vijay");
	      System.out.println("After invoking remove(object) method: "+al); 
	      //Removing element on the basis of specific position
	      al.remove(0);
	      System.out.println("After invoking remove(index) method: "+al); 
	      
	      //Creating another arraylist
	      ArrayList<String> al2=new ArrayList<String>();  
	      al2.add("Ravi");  
	      al2.add("Hanumat");  
	      //Adding new elements to arraylist
	      al.addAll(al2);
	      System.out.println("Updated list : "+al); 
	      //Removing all the new elements from arraylist
	      al.removeAll(al2);
	      System.out.println("After invoking removeAll() method: "+al); 
	      //Removing elements on the basis of specified condition
	      al.removeIf(str -> str.contains("Ajay"));   //Here, we are using Lambda expression 
	      System.out.println("After invoking removeIf() method: "+al);
	      //Removing all the elements available in the list
	      al.clear();
	      System.out.println("After invoking clear() method: "+al); 
	   }
	}                 

Output:

An initial list of elements: [Ravi, Vijay, Ajay, Anuj, Gaurav]
After invoking remove(object) method: [Ravi, Ajay, Anuj, Gaurav]
After invoking remove(index) method: [Ajay, Anuj, Gaurav]
Updated list : [Ajay, Anuj, Gaurav, Ravi, Hanumat]
After invoking removeAll() method: [Ajay, Anuj, Gaurav]
After invoking removeIf() method: [Anuj, Gaurav]
After invoking clear() method: []

Java ArrayList example of retainAll() method

FileName: ArrayList9.java

import java.util.*;
class ArrayList9{
 public static void main(String args[]){
  ArrayList<String> al=new ArrayList<String>();
  al.add("Ravi");
  al.add("Vijay");
  al.add("Ajay");
  ArrayList<String> al2=new ArrayList<String>();
  al2.add("Ravi");
  al2.add("Hanumat");
  al.retainAll(al2);
  System.out.println("iterating the elements after retaining the elements of al2");
  Iterator itr=al.iterator();
  while(itr.hasNext()){
   System.out.println(itr.next());
  }
 }
}

Output:

       iterating the elements after retaining the elements of al2
       Ravi

Java ArrayList example of isEmpty() method

FileName: ArrayList4.java

import java.util.*;
 class ArrayList10{

	    public static void main(String [] args)
	    {
	      ArrayList<String> al=new ArrayList<String>();
	      System.out.println("Is ArrayList Empty: "+al.isEmpty());
	      al.add("Ravi");  
	      al.add("Vijay");  
	      al.add("Ajay");  
	      System.out.println("After Insertion");
	      System.out.println("Is ArrayList Empty: "+al.isEmpty()); 
	   }
	}    

Output:

Is ArrayList Empty: true
After Insertion
Is ArrayList Empty: false

Java ArrayList Example: Book

Let's see an ArrayList example where we are adding books to the list and printing all the books.

FileName: ArrayListExample20.java

import java.util.*;
class Book {
int id;
String name,author,publisher;
int quantity;
public Book(int id, String name, String author, String publisher, int quantity) {
	this.id = id;
	this.name = name;
	this.author = author;
	this.publisher = publisher;
	this.quantity = quantity;
}
}
public class ArrayListExample20 {
public static void main(String[] args) {
	//Creating list of Books
	List<Book> list=new ArrayList<Book>();
	//Creating Books
	Book b1=new Book(101,"Let us C","Yashwant Kanetkar","BPB",8);
	Book b2=new Book(102,"Data Communications and Networking","Forouzan","Mc Graw Hill",4);
	Book b3=new Book(103,"Operating System","Galvin","Wiley",6);
	//Adding Books to list
	list.add(b1);
	list.add(b2);
	list.add(b3);
	//Traversing list
	for(Book b:list){
		System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+" "+b.quantity);
	}
}
}

Test it Now

Output:

101 Let us C Yashwant Kanetkar BPB 8
102 Data Communications and Networking Forouzan Mc Graw Hill 4
103 Operating System Galvin Wiley 6

Size and Capacity of an ArrayList

Size and capacity of an array list are the two terms that beginners find confusing. Let's understand it in this section with the help of some examples. Consider the following code snippet.

FileName: SizeCapacity.java

import java.util.*;

public class SizeCapacity
{

public static void main(String[] args) throws Exception
{
     
    ArrayList<Integer> al = new ArrayList<Integer>();
    
    System.out.println("The size of the array is: " + al.size());
}
}

Output:

The size of the array is: 0

Explanation: The output makes sense as we have not done anything with the array list. Now observe the following program.

FileName: SizeCapacity1.java

import java.util.*;

public class SizeCapacity1
{

public static void main(String[] args) throws Exception
{
     
    ArrayList<Integer> al = new ArrayList<Integer>(10);
    
    System.out.println("The size of the array is: " + al.size());
}
}

Output:

The size of the array is: 0

Explanation: We see that the size is still 0, and the reason behind this is the number 10 represents the capacity no the size. In fact, the size represents the total number of elements present in the array. As we have not added any element, therefore, the size of the array list is zero in both programs.

Capacity represents the total number of elements the array list can contain. Therefore, the capacity of an array list is always greater than or equal to the size of the array list. When we add an element to the array list, it checks whether the size of the array list has become equal to the capacity or not. If yes, then the capacity of the array list increases. So, in the above example, the capacity will be 10 till 10 elements are added to the list. When we add the 11^th element, the capacity increases. Note that in both examples, the capacity of the array list is 10. In the first case, the capacity is 10 because the default capacity of the array list is 10. In the second case, we have explicitly mentioned that the capacity of the array list is 10.