ArrayList in java with example programs – Collections Framework

Arraylist class implements List interface. It is widely used because of the functionality and flexibility it offers. Most of the developers choose Arraylist over Array as it’s a very good alternative of traditional java arrays. ArrayList is a resizable-array implementation of the List interface. It implements all optional list operations, and permits all elements, including null.

The issue with arrays is that they are of fixed length so if it is full you cannot add any more elements to it, likewise if there are number of elements gets removed from it the memory consumption would be the same as it doesn’t shrink. On the other ArrayList can dynamically grow and shrink after addition and removal of elements. Apart from these benefits ArrayList class enables us to use predefined methods of it which makes our task easy. Let’s see the ArrayList example first then we will discuss it’s methods and their usage.

There is a list of several ArrayList tutorials at the end of this guide, refer it to understand ArrayList concept fully.

ArrayList Example in Java

import java.util.*;

public class ArrayListExample {
   public static void main(String args[]) {
      /*Creation of ArrayList: I'm going to add String
       *elements so I made it of string type */
	  ArrayList<String> obj = new ArrayList<String>();

	  /*This is how elements should be added to the array list*/
	  obj.add("Ajeet");
	  obj.add("Harry");
	  obj.add("Chaitanya");
	  obj.add("Steve");
	  obj.add("Anuj");

	  /* Displaying array list elements */
	  System.out.println("Currently the array list has following elements:"+obj);

	  /*Add element at the given index*/
	  obj.add(0, "Rahul");
	  obj.add(1, "Justin");

	  /*Remove elements from array list like this*/
	  obj.remove("Chaitanya");
	  obj.remove("Harry");

	  System.out.println("Current array list is:"+obj);

	  /*Remove element from the given index*/
	  obj.remove(1);

	  System.out.println("Current array list is:"+obj);
   }
}

Output:

Currently the array list has following elements:[Ajeet, Harry, Chaitanya, Steve, Anuj]
Current array list is:[Rahul, Justin, Ajeet, Steve, Anuj]
Current array list is:[Rahul, Ajeet, Steve, Anuj]

Methods of ArrayList class

In the above example we have used methods such as add and remove. However there are number of methods available which can be used directly using object of ArrayList class. Let’s discuss few of the important methods.

1) add( Object o): This method adds an object o to the arraylist.

obj.add("hello");

This statement would add a string hello in the arraylist at last position.

2) add(int index, Object o): It adds the object o to the array list at the given index.

obj.add(2, "bye");

It will add the string bye to the 2nd index (3rd position as the array list starts with index 0) of array list.

3) remove(Object o): Removes the object o from the ArrayList.

obj.remove("Chaitanya");

This statement will remove the string “Chaitanya” from the ArrayList.

4) remove(int index): Removes element from a given index.

obj.remove(3);

It would remove the element of index 3 (4th element of the list – List starts with o).

5) set(int index, Object o): Used for updating an element. It replaces the element present at the specified index with the object o.

obj.set(2, "Tom");

It would replace the 3rd element (index =2 is 3rd element) with the value Tom.

6) int indexOf(Object o): Gives the index of the object o. If the element is not found in the list then this method returns the value -1.

int pos = obj.indexOf("Tom");

This would give the index (position) of the string Tom in the list.

7) Object get(int index): It returns the object of list which is present at the specified index.

String str= obj.get(2);

Function get would return the string stored at 3rd position (index 2) and would be assigned to the string “str”. We have stored the returned value in string variable because in our example we have defined the ArrayList is of String type. If you are having integer array list then the returned value should be stored in an integer variable.

8) int size(): It gives the size of the ArrayList – Number of elements of the list.

int numberofitems = obj.size();

9) boolean contains(Object o): It checks whether the given object o is present in the array list if its there then it returns true else it returns false.

obj.contains("Steve");

It would return true if the string “Steve” is present in the list else we would get false.

10) clear(): It is used for removing all the elements of the array list in one go. The below code will remove all the elements of ArrayList whose object is obj.

obj.clear();

ArrayList Tutorials

Here is the list of ArrayList tutorials published on beginnersbook.com.

ArrayList Basics

• Initialize ArrayList
• Loop ArrayList
• Find length of ArrayList

Sorting

• Sort ArrayList
• Sort ArrayList in Descending order
• Sort ArrayList of Objects using Comparable and Comparator

Add/Remove

• Add element to ArrayList
• Add element at particular index of ArrayList
• Append Collection elements to ArrayList
• Copy All List elements to ArrayList
• Insert all the collection elements to the specified position in ArrayList
• Remove element from the specified index in ArrayList
• Remove specified element from ArrayList

Get/Search

• Get Sub List of ArrayList
• Get the index of last occurrence of the element in the ArrayList
• Get element from ArrayList
• Get the index of  first occurrence of the element in the ArrayList
• Check whether element exists in ArrayList

Other Tutorials on ArrayList

• Compare two ArrayList
• Synchronize ArrayList
• Swap two elements in ArrayList
• Override toString() method – ArrayList
• Serialize ArrayList
• Join two ArrayList
• Clone ArrayList to another ArrayList
• Make ArrayList Empty
• Check whether ArrayList is empty or not
• Trim the Size of ArrayList
• Replace the value of existing element in ArrayList
• Increase the capacity(size) of ArrayList

Conversions:

• Convert LinkedList to ArrayList
• Convert Vector to ArrayList
• Convert ArrayList to String Array
• Convert Array to ArrayList
• Convert HashSet to ArrayList

Differences:

• ArrayList vs Vector
• ArrayList vs HashMap
• ArrayList vs LinkedList

Reference

• ArrayList Documentation