Java ArrayList Collection

What is ArrayList Collection in java?

• ArrayList is one of the collection class in java.
• ArrayList class extends AbstractList and implements the List interface.
• ArrayList supports dynamic arrays that can grow as needed, but arrays in java are fixed length ( once array is created that cannot grow or shrink).
• ArrayList is a variable length array of object references.
• ArrayList can dynamically increase or decrease in size of the collection.
• ArrayList is created with an initial size, when the size is exceeded, the size is automatically increased. when the objects are removed in ArrayList, the size may be reduced.
• Legacy class 'Vector' is also supports the dynamic arrays.

ArrayList Constructors Types

ArrayList()

This constructor builds an empty array list.

ArrayList(Collection collection)

This constructor builds an array list that is initialized with the elements of the collection.

ArrayList(int capacity)

This constructor builds an array list that has specified the initial capacity. here capacity is the size of the underlying array that is used to store the elements. Capacity grows automatically as elements are added to an array list.

ArrayList Methods

• add method is used to add new object in the array list.

  boolean add(Object)
  

  adds new object in the array list.

  boolean add(int, Object)
  

  adds new object at the specified index of array list.
• addAll method is used to add the collection of objects in the array list.

  boolean addAll(Collection) 
  

  boolean addAll(int, Collection)
  

  adds the collection of objects starting from the specified index of array list. Returns true if object was added to the ArrayList, otherwise false.
• clear method is used to remove all the elements in the array list.

  void clear()
  

• contains method returns true if object is an element of the array list, otherwise returns false.

  boolean contails(Object)
  

• containsAll method returns true if array list contains all the elements of provided collection, otherwise returns false.

  boolean containsAll(Collection)
  

• equals method returns true if collection and object are equal.

  boolean equals(Object)
  

• hashCode method returns hash code for the collection.

  int hashCode()
  

• isEmpty method returns true if collection is empty. otherwise returns false.

  boolean isEmpty()
  

• iterator method returns an iterator for the collection.

  Iterator iterator()
  

• remove method removes one instance of object from the collection. returns true if element was removed, otherwise returns false.

  boolean remove(Object)
  

• removeAll method removes all elements of provided collection from the invoking collection. Returns true if the elements were removed, otherwise returns false.

  boolean removeAll(Collection)
  

• retainAll method removes all elements from the invoking collection except those elements in provided collection. Returns true if elements were removed, otherwise return false.

  boolean retainAll(Collection)
  

• size method returns the number of elements in the collection.

  int size()
  

• toArray mehod returns an array that contains all the elements in the collection.

  Object[] toArray()
  

Creating ArrayList and Adding Elements in Java Program

// Codingpointer Demonstrates ArrayList                                         
                                                                                
import java.util.*;                                                             
                                                                                
class ArrayListTest {                                                           
 public static void main(String args[]) {                                       
  // Creates ArrayList                                                          
  ArrayList arrList = new ArrayList();                                          
  System.out.println("Array size default: " + arrList.size());                  
                                                                                
  // add array list elements.                                                   
  arrList.add(1);                                                               
  arrList.add(2);                                                               
  arrList.add(3);                                                               
  arrList.add(4);                                                               
  arrList.add(5);                                                               
  arrList.add(6);                                                               
  arrList.add(2, 4.5);                                                          
                                                                                
  System.out.println("Size of array list: "+ arrList.size());                   
  // prints array list elements                                                 
  System.out.println("Elements in array list: " + arrList);                     
 }                                                                              
}    

Output:

$ javac ArrayListTest.java 
Note: ArrayListTest.java uses unchecked or unsafe operations.
Note: Recompile with -Xlint:unchecked for details.
$ java ArrayListTest 
Array size default: 0
Size of array list: 7
Elements in array list: [1, 2, 4.5, 3, 4, 5, 6]

Remove Elements in ArrayList Java Program

// Codingpointer Demonstrates ArrayList                                         
                                                                                
import java.util.*;                                                             
                                                                                
class ArrayListTest {                                                           
 public static void main(String args[]) {                                       
  // Creates ArrayList                                                          
  ArrayList arrList = new ArrayList();                                          
  System.out.println("Array size default: " + arrList.size());                  
                                                                                
  // add array list elements.                                                   
  arrList.add("1");                                                             
  arrList.add("1");                                                             
  arrList.add("3");                                                             
  arrList.add("4");                                                             
  arrList.add("5");                                                             
  arrList.add("6");                                                             
  arrList.add(2, "4.5");                                                        
                                                                                
  System.out.println("Size of array list: "+ arrList.size());                   
  // prints array list elements                                                 
  System.out.println("Elements in array list: " + arrList);                     
                                                                                
  // remove array list elements                                                 
  arrList.remove("1");                                                          
  arrList.remove(4);                                                            
  System.out.println("Size of array list after deletions: "+arrList.size());    
  System.out.println("Elements in array list: " + arrList);                     
 }                                                                              
} 

Output:

$ java ArrayListTest 
Array size default: 0
Size of array list: 7
Elements in array list: [1, 1, 4.5, 3, 4, 5, 6]
Size of array list after deletions: 5
Elements in array list: [1, 4.5, 3, 4, 6]

Obtain Array from ArrayList Java Program

// Codingpointer Demonstrates ArrayList                                         
                                                                                
import java.util.*;                                                             
                                                                                
class ArrayListTest {                                                           
 public static void main(String args[]) {                                       
  // Creates ArrayList                                                          
  ArrayList arrList = new ArrayList();                                          
  System.out.println("Array size default: " + arrList.size());                  
                                                                                
  // add array list elements.                                                   
  arrList.add(new Integer(5));                                                  
  arrList.add(new Integer(1));                                                  
  arrList.add(new Integer(3));                                                  
  arrList.add(new Integer(2));                                                  
  arrList.add(new Integer(4));                                                  
                                                                                
                                                                                
  System.out.println("Size of array list: "+ arrList.size());                   
  // prints array list elements                                                 
  System.out.println("Elements in array list: " + arrList);                     
                                                                                
  // get array                                                                   
  Object arr[] = arrList.toArray();                                             
  int sum = 0;                                                                  
  // sum array                                                                  
  for(int i=0; i<arr.length; i++) {                                             
   sum += ((Integer) arr[i]).intValue();                                        
  }                                                                             
  System.out.println("Sum is: " + sum);                                         
 }                                                                              
}    

Output:

$ java ArrayListTest 
Array size default: 0
Size of array list: 5
Elements in array list: [5, 1, 3, 2, 4]
Sum is: 15

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