ArrayList Class in Java Collection Framework

The ArrayList class extends AbstractList and implements the List interface. ArrayList supports dynamic arrays that can grow as needed.

Standard Java arrays are of a fixed length. After arrays are created, they cannot grow or shrink, which means that you must know in advance how many elements an array will hold.

Array lists are created with an initial size. When this size is exceeded, the collection is automatically enlarged. When objects are removed, the array may be shrunk.

ArrayList class

• ArrayList class extends AbstractList class and implements the List interface.
• ArrayList supports dynamic array that can grow as needed. ArrayList has three constructors.

//constructor builds an empty array list
ArrayList()
//The following constructor builds an array list that is initialized with the elements of the collection c.
ArrayList( Collection C )
/*The following constructor builds an array list that has the specified initial capacity. The capacity is the size of the underlying array that is used to store the elements.

The capacity grows automatically as elements are added to an array list.*/
ArrayList( int capacity )

• ArrayLists are created with an initial size, when this size is exceeded, it gets enlarged automatically.
• It can coontain Duplicate elements and maintains the insertion order.
• ArrayLists are not synchronized.

Methods in the ArrayList Class:

Example of ArrayList:

import java.util.*;

public class ArrayListDemo {
   public static void main(String args[]) {
      // create an array list
      ArrayList list = new ArrayList();
      System.out.println("Initial size of list: " + list.size());

      // add elements to the array list
      list.add("Dinesh");
      list.add("Sweety");
      list.add("Adesh");
      list.add("Vinesh");
      list.add("Mom");
      list.add("DAD");
      
      System.out.println("Size of list after additions: " + list.size());

      // display the array list
      System.out.println("Contents of list: " + list);
     
       System.out.println("Iterating list");
       Iterator itr = list.iterator();
       while(itr.hasNext()){
           System.out.println(itr.next());
       }
   }
}

output:

Two ways to iterate the elements of collection:

1. By Iterator interface.
2. By for-each loop.

Iterating the elements of Collection by for-each loop:

import java.util.*;

public class ArrayListDemo {
   public static void main(String args[]) {
      // create an array list
      ArrayList list = new ArrayList();
      
      // add elements to the array list
      list.add("Dinesh");
      list.add("Sweety");
      list.add("Adesh");
      list.add("Vinesh");
      list.add("Mom");
      list.add("DAD");
      System.out.println("Iterating list using for each loop");
      for(Object obj : list){
           System.out.println(obj);
      }
   }
}

output:

Storing User-Defined classes:
In the above example we are storing only string object in ArrayList collection. But You can store any type of object, including object of class that you create in Collection classes.
Example of storing User-Defined object

Employee class

class Employee
{
    String firstName;
    String lastName;
    String phone;

    public Employee(String firstName,String lastName,String phone) 
    {
        this.firstName = firstName ;
        this.lastName = lastName;
        this.phone = phone;
    }
    
    public String toString()
    {
        return firstName +" "+lastName +" ("+phone +") ";
    }
}

Storing Employee class

public class EmployeeListDemo
{
    
   public static void main(String[] args) 
   {
       Employee e1 = new Employee("Dinesh", "Rajput","999100091");
       Employee e2 = new Employee("Sandeep", "Sharma","998392819");
       Employee e3 = new Employee("Vinesh", "Kohli","998131319");
       
      ArrayList al = new ArrayList(); 
      al.add(e1);
      al.add(e2);
      al.add(e3);
      System.out.println(al);
   }
    
}

output:

Example of addAll(Collection c) method:

import java.util.*;
class SimpleListDemo{
 public static void main(String args[]){
 
  ArrayList al = new ArrayList();
  al.add("Dinesh");
  al.add("Anamika");
  al.add("Sweety");
  
  ArrayList al2 = new ArrayList();
  al2.add("Adesh");
  al2.add("Vinesh");
  
  al.addAll(al2);  

  Iterator itr=al.iterator();
  while(itr.hasNext()){
   System.out.println(itr.next());
  }
 }
}

output:

Example of removeAll() method:

import java.util.*;
class SimpleListDemo{
 public static void main(String args[]){
 
  ArrayList al = new ArrayList();
  al.add("Dinesh");
  al.add("Anamika");
  al.add("Sweety");
  
  ArrayList al2 = new ArrayList();
  al2.add("Anamika");
  al2.add("Vinesh");

  al.removeAll(al2);  

  Iterator itr=al.iterator();
  while(itr.hasNext()){
   System.out.println(itr.next());
  }
 }
}

output:

Example of retainAll() method:

import java.util.*;
class SimpleListDemo{
 public static void main(String args[]){
 
  ArrayList al = new ArrayList();
  al.add("Dinesh");
  al.add("Anamika");
  al.add("Sweety");
  
  ArrayList al2 = new ArrayList();
  al2.add("Anamika");
  al2.add("Vinesh");
  System.out.println("iterating the elements after retaining the elements of al2...");
  al.retainAll(al2);  

  Iterator itr=al.iterator();
  while(itr.hasNext()){
   System.out.println(itr.next());
  }
 }
}

output: