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The Java Collections Framework is a collection of Interfaces and Classes which helps in storing and processing that data efficiently
This framework has several useful classes which have tons of useful functions which makes a programmer task super easy
Arraylist class implements List interface and it is based on an Array data structure.
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.
Why ArrayList is better than Array?
The limitation with array is that it has a 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 (See the images below). Apart from these benefits ArrayList class enables us to use predefined methods of it which makes our task easy. Let’s see the diagrams to understand the addition and removal of elements from ArrayList and then we will see the programs.
How to create an ArrayList?
We can create an ArrayList by writing a simple statement like this:
This statement creates an ArrayList with the name alist with type “String”. The type determines which type of elements the list will have. Since this list is of “String” type, the elements that are going to be added to this list will be of type “String”.
ArrayList<String> alist=new ArrayList<String>();
Similarly we can create ArrayList that accepts int elements.
ArrayList<Integer> list=new ArrayList<Integer>();
How to add elements to an ArrayList?
We add elements to an ArrayList by using add() method, this method has couple of variations, which we can use based on the requirement. For example: If we want to add the element at the end of the List then simply do it like this:
alist.add("Steve"); //This will add "Steve" at the end of List
To add the element at the specified location in ArrayList, we can specify the index in the add method like this:
alist.add(3, "Steve"); //This will add "Steve" at the fourth position
Lets write the complete code:
import java.util.*;
class JavaExample{
public static void main(String args[]){
ArrayList<String> alist=new ArrayList<String>();
alist.add("Steve");
alist.add("Tim");
alist.add("Lucy");
alist.add("Pat");
alist.add("Angela");
alist.add("Tom");
//displaying elements
System.out.println(alist);
//Adding "Steve" at the fourth position
alist.add(3, "Steve");
//displaying elements
System.out.println(alist);
}
}
Output:
[Steve, Tim, Lucy, Pat, Angela, Tom]
[Steve, Tim, Lucy, Steve, Pat, Angela, Tom]
Note: Since the index starts with 0, index 3 would represent fourth position not 3.
Change an element in ArrayList
We can use the set method to change an element in ArrayList. We provide the index and new element, this method then updates the element present at the given index with the new given element. In the following example, we have given the index as 0 and new element as “Lucy” in the set() method, so the method updated the element present at the index 0 (which is the first element “Jim” in this example) with the new String element “Lucy”, which we can see in the output.
import java.util.ArrayList;
public class JavaExample {
public static void main(String[] args) {
ArrayList<String> names = new ArrayList<String>();
names.add("Jim");
names.add("Jack");
names.add("Ajeet");
names.add("Chaitanya");
names.set(0, "Lucy");
System.out.println(names);
}
}
Output:
How to remove elements from ArrayList?
We use remove() method to remove elements from an ArrayList, Same as add() method, this method also has few variations.
For example:
import java.util.*;
class JavaExample{
public static void main(String args[]){
ArrayList<String> alist=new ArrayList<String>();
alist.add("Steve");
alist.add("Tim");
alist.add("Lucy");
alist.add("Pat");
alist.add("Angela");
alist.add("Tom");
//displaying elements
System.out.println(alist);
//Removing "Steve" and "Angela"
alist.remove("Steve");
alist.remove("Angela");
//displaying elements
System.out.println(alist);
//Removing 3rd element
alist.remove(2);
//displaying elements
System.out.println(alist);
}
}
Output:
[Steve, Tim, Lucy, Pat, Angela, Tom]
[Tim, Lucy, Pat, Tom]
[Tim, Lucy, Tom]
Iterating ArrayList
In the above examples, we have displayed the ArrayList elements just by referring the ArrayList instance, which is definitely not the right way to displays the elements. The correct way of displaying the elements is by using an advanced for loop like this.
import java.util.*;
class JavaExample{
public static void main(String args[]){
ArrayList<String> alist=new ArrayList<String>();
alist.add("Gregor Clegane");
alist.add("Khal Drogo");
alist.add("Cersei Lannister");
alist.add("Sandor Clegane");
alist.add("Tyrion Lannister");
//iterating ArrayList
for(String str:alist)
System.out.println(str);
}
}
Output:
Gregor Clegane
Khal Drogo
Cersei Lannister
Sandor Clegane
Tyrion Lannister
ArrayList Size
We can use size() method of ArrayList to find the number of elements in an ArrayList.
import java.util.ArrayList;
public class JavaExample {
public static void main(String[] args) {
ArrayList<Integer> numbers = new ArrayList<Integer>();
numbers.add(1);
numbers.add(7);
numbers.add(5);
numbers.add(6);
System.out.println("Number of elements in ArrayList: "+numbers.size());
}
}
Output:
ArrayList Example in Java
This example demonstrates how to create, initialize, add and remove elements from ArrayList. In this example we have an ArrayList of type “String”. We have added 5 String element in the ArrayList using the method add(String E), this method adds the element at the end of the ArrayList.
We are then adding two more elements in the ArrayList using method add(int index, String E), this method adds the specified element at the specified index, index 0 indicates first position and 1 indicates second position.
We are then removing the elements “Chaitanya” and “Harry” from the ArrayList and then we are removing the second element of the ArrayList using method remove(int index). Since we have specified the index as 1 (remove(1)), it would remove the second element.
import java.util.*;
public class JavaExample {
public static void main(String args[]) {
/* Creating ArrayList of type "String" which means
* we can only add "String" elements
*/
ArrayList<String> obj = new ArrayList<String>();
/*This is how we add elements to an ArrayList*/
obj.add("Ajeet");
obj.add("Harry");
obj.add("Chaitanya");
obj.add("Steve");
obj.add("Anuj");
// Displaying elements
System.out.println("Original ArrayList:");
for(String str:obj)
System.out.println(str);
/* Add element at the given index
* obj.add(0, "Rahul") - Adding element "Rahul" at first position
* obj.add(1, "Justin") - Adding element "Justin" at second position
*/
obj.add(0, "Rahul");
obj.add(1, "Justin");
// Displaying elements
System.out.println("ArrayList after add operation:");
for(String str:obj)
System.out.println(str);
//Remove elements from ArrayList like this
obj.remove("Chaitanya"); //Removes "Chaitanya" from ArrayList
obj.remove("Harry"); //Removes "Harry" from ArrayList
// Displaying elements
System.out.println("ArrayList after remove operation:");
for(String str:obj)
System.out.println(str);
//Remove element from the specified index
obj.remove(1); //Removes Second element from the List
// Displaying elements
System.out.println("Final ArrayList:");
for(String str:obj)
System.out.println(str);
}
}
Output:
Original ArrayList:
Ajeet
Harry
Chaitanya
Steve
Anuj
ArrayList after add operation:
Rahul
Justin
Ajeet
Harry
Chaitanya
Steve
Anuj
ArrayList after remove operation:
Rahul
Justin
Ajeet
Steve
Anuj
Final ArrayList:
Rahul
Ajeet
Steve
Anuj
To continue please refer Part 2 for Java Collections in Sort ArrayList, ArrayList Class and HashMap
Hope you liked it!!
Happy Coding :)