ArrayList in Java 8 – A Complete Guide to JAVA ArrayList

Java Arraylist is datastructure used to store elements/objects in java. This object can be of same or different type depending on the declaration of arrayList. ArrayList is part of collection framework of java.util package of java. It is an implementation of List interface of java which inturn implements Collection and Iterable.

Important points about Java ArrayList

• ArrayList in java was introduced in 1.2 version of java as part of the collection framework.
• ArrayList is a class of java which extends AbstractList class which inturn implements List interface as shown in diagram.
• Java ArrayList can be used to store heterogenous or homogenous elements.
• ArrayList can grow its size dynamically as its made up of dynamic array.
• Java ArrayList maintains the order of insertion elements.
• ArrayList is randomly accessible as it implements RandomAccess Interface.
• ArrayList cannot be declared of primitive type (int, long etc). It should be declared using Wrraper classes such as Integer or Long

Wrong Way : ArrayList<int> list = new ArrayList();

Correct Way: ArrayList<Integer> list = new ArrayList();

• ArrayList in java can contain duplicate elements.
• ArrayList class in java is not synchronized.
• You can Synchronize given array list using Collections.synchronizedList

List list = Collections.synchronizedList(new ArrayList(...));

• ArrayList are more efficient than LinkList while searching an element in the list but are non-efficient than LinkList when it comes to insertion and deletion of an element in between the list.
• ArrayList works on an index-based system that means you can get/delete/add an element at its specific index just like an array.

Now, Let’s learn some technicality of ArrayList in java.

ArrayList Construtors

ArrayList provide 3 constructor.

//create a new list of default size 10
List<Integer> list = new ArrayList<>();

List<Integer> list = new ArrayList<>();
//creates a newList with all the elements from list
List<Integer> newList = new ArrayList<>(list);

//creates a list of initial capacity of  20
List<Integer> list = new ArrayList<>(20);

ArrayList Methods

How to add element to ArrayList in java?

ArrayList in java provides variants of ways to add elements in java.

Examples for adding elements to ArrayList in java

package com.techijournal.collection;

import java.util.ArrayList;
import java.util.List;

public class MyArrayList {

	public static void main(String atgs []) {
		arrayListExample();
	}
	private static void arrayListExample() {
		//Creating a arraylist of default size i.e 10
		List<Integer> defaultList = new ArrayList<>();
		//Creating a ArrayList of capacity 5
		List<Integer> listOfCapaciyFive = new ArrayList<>(5);
		
		//Example for add()
		defaultList.add(33);
		defaultList.add(54);
		defaultList.add(66);
		defaultList.add(12);
		
		System.out.print("printing  elements of defaultList");
		System.out.println("defaultList elements are :"+defaultList);
		System.out.print("Creating ArrayList from existing list");
		List<Integer> listFromExistingList = new ArrayList<Integer>(defaultList);
		System.out.println("printing element of listFromExistingList");
		System.out.println("listFromExistingList elements are : " + listFromExistingList);
		
		
		System.out.println("adding element at index 3 in listFromExistingList");
		listFromExistingList.add(3,454);
		System.out.println("listFromExistingList elements after update are : " + listFromExistingList);
		
		System.out.println("Adding listFromExistingList to listOfCapaciyFive");
		listOfCapaciyFive.addAll(listFromExistingList);
		
		System.out.println("listOfCapaciyFive elements are :"+listOfCapaciyFive);
		
		
		System.out.println("Adding defaultList to listOfCapaciyFive at index 4");
		listOfCapaciyFive.addAll(4,defaultList);
		
		System.out.println("listOfCapaciyFive elements are :"+listOfCapaciyFive);
		
	}
}

OUTPUT: 
printing elements of defaultListdefaultList elements are :[33, 54, 66, 12]
Creating ArrayList from existing listprinting element of listFromExistingList
listFromExistingList elements are : [33, 54, 66, 12]
adding element at index 3 in listFromExistingList
listFromExistingList elements after update are : [33, 54, 66, 454, 12]
Adding listFromExistingList to listOfCapaciyFive
listOfCapaciyFive elements are :[33, 54, 66, 454, 12]
Adding defaultList to listOfCapaciyFive at index 4
listOfCapaciyFive elements are :[33, 54, 66, 454, 33, 54, 66, 12, 12]

Removing or Deleting Elements from ArrayList in Java

ArrayList provides different methods to delete or remove elements from the list. These methods are listed bellow.

Deleting an element from ArrayList using remove(int index)

List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("Initial List :"+ list);
		System.out.println("Removing element from list using list.remove(index)");
		String removedElement = list.remove(3);
		System.out.println(removedElement+" is element was present at index 3 got removed");

Remove Object element from ArrayList using remove(Object o)

List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("Initial List :"+ list);
System.out.println("Removing element using list.remove(object)");
		list.remove("US"); //returns boolean if successful

Remove an element from ArrayList using remove(Collection c)

list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		
		System.out.println("Re-Initializing List with elements :"+ list);
		List<String> listToRemoved = new ArrayList<>();
		listToRemoved.add("US");
		listToRemoved.add("India");
		System.out.println("List to be removed from list is "+ listToRemoved);
		System.out.print("Removing elements from arraylist using list.remove(collection)");
		list.removeAll(listToRemoved);

Removing an element from ArrayList using removeIf(Predicte p)

list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		
		System.out.println("Re-Initializing List with elements :"+ list);

		System.out.println("Removing elements from list using predicate i.e using list.removeIf(predicate)");
		list.removeIf(s-> s.equals("Rome") || s.equals("India"));
		System.out.println("List after removing is "+ list);

Remove all element from ArrayList using clear()

list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
System.out.println("Remvoing all elements using list.clear()");
		list.clear();
		System.out.print("List after calling clear is "+list);

Using retainAll( Collection c) to remove elements in ArrayList

We can use reatinAll() method to retain all elements of the passed collection and remove all other collection.

list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("removing elements from list using list.retainAll()");
		System.out.println("Re-Initializing List with elements :"+ list);
		List<String> listToRetain = new ArrayList<>();
		listToRetain.add("US");
		listToRetain.add("India");
		System.out.println(" List to retain is :"+ list);
		list.retainAll(listToRetain);
		System.out.println("Now, updated list is "+ list);

Example Code for Removing an Element from ArrayList

package com.techijournal.collection;

import java.util.ArrayList;
import java.util.List;

public class MyArrayList {

	public static void main(String atgs []) {
		remove();
	}

	private static void remove() {
		List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("Initial List :"+ list);
		System.out.println("Removing element from list using list.remove(index)");
		String removedElement = list.remove(3);
		System.out.println(removedElement+" is element was present at index 3 got removed");
		
		
		System.out.println("Removing element using list.remove(object)");
		list.remove("US"); //returns boolean if successful
		System.out.println("US object got removed");
		
		list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		
		System.out.println("Re-Initializing List with elements :"+ list);
		List<String> listToRemoved = new ArrayList<>();
		listToRemoved.add("US");
		listToRemoved.add("India");
		System.out.println("List to be removed from list is "+ listToRemoved);
		System.out.print("Removing elements from arraylist using list.remove(collection)");
		list.removeAll(listToRemoved);
		
		System.out.println("List after removing collection "+ listToRemoved+ " is "+ list);
		list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		
		System.out.println("Re-Initializing List with elements :"+ list);

		System.out.println("Removing elements from list using predicate i.e using list.removeIf(predicate)");
		list.removeIf(s-> s.equals("Rome") || s.equals("India"));
		System.out.println("List after removing is "+ list);

		System.out.println("Remvoing all elements using list.clear()");
		list.clear();
		System.out.println("List after calling clear is "+list);
		
		list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("removing elements from list using list.retainAll()");
		System.out.println("Re-Initializing List with elements :"+ list);
		List<String> listToRetain = new ArrayList<>();
		listToRetain.add("US");
		listToRetain.add("India");
		System.out.println(" List to retain is :"+ list);
		list.retainAll(listToRetain);
		System.out.println("Now, updated list is "+ list);
		
	}
}

Output of above code :

Initial List :[US, China, India, Rome, Cannada]
Removing element from list using list.remove(index)
Rome is element was present at index 3 got removed
Removing element using list.remove(object)
US object got removed
Re-Initializing List with elements :[US, China, India, Rome, Cannada]
List to be removed from list is [US, India]
Removing elements from arraylist using list.remove(collection)List after removing collection [US, India] is [China, Rome, Cannada]
Re-Initializing List with elements :[US, China, India, Rome, Cannada]
Removing elements from list using predicate i.e using list.removeIf(predicate)
List after removing is [US, China, Cannada]
Remvoing all elements using list.clear()
List after calling clear is []
removing elements from list using list.retainAll()
Re-Initializing List with elements :[US, China, India, Rome, Cannada]
List to retain is :[US, China, India, Rome, Cannada]
Now, updated list is [US, India]

Fetching/Getting Elements from ArrayList

ArrayList in java only two methods required to fetch elements from list. those are

Example : Code for fetching element from ArrayList

package com.techijournal.collection;

import java.util.ArrayList;
import java.util.List;

public class MyArrayList {

	public static void main(String atgs []) {
		fetchingElement();
	}
	
	private static void fetchingElement() {
		List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("Initial List :"+ list);
		
		System.out.println("fetching element from list using list.get(index)");
		System.out.println("Element at index 3 of list is : "+list.get(3));
		
		System.out.println("fetching element from list using list.sublist(fromIndex, toIndex)");
		System.out.println("Element from index 1 to index 3 of list is : "+list.subList(1, 3));
	}
}

Output for above code:

Initial List :[US, China, India, Rome, Cannada]
fetching element from list using list.get(index)
Element at index 3 of list is : Rome
fetching element from list using list.sublist(fromIndex, toIndex)
Element from index 1 to index 3 of list is : [China, India]

Iterating ArrayList in Java

Java provide multiple way to Iterate ArrayList. like

• We can use for loop for iterating in ArrayList
• Or from Java 8, ArrayList provides forEach() to iterate.
• Iterator() or listIterator() can be used to iterate over ArrayList in java .
• If we want to Iterate from specific index we can use listIterator(int index)
• From Java 8 We can use spliterator(). SplitIterator can help in parallel Processing.

Iterating ArrayList in Java using for loop

Java legacy for loop can be used to traverse array list as show below

List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("Initial List :"+ list);
		
		System.out.print("Using for Loop to Iterate ArrayList");
		for(int i = 0; i< list.size(); i++) {
			System.out.print(list.get(i)+ " ");
		}

Iterating ArrayList in Java using forEach()

	List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("Initial List :"+ list);
		
		System.out.println("Iterating ArrayList using list.forEach");
		list.forEach(country -> System.out.print(country+ " "));
		

Iterating ArrayList in Java using iterator()

Java ArrayList class provides Iterator which can be used to Iterate over ArrayList elements as shown in the below code. This method returns Iterator Interface from java.util package.

List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("Initial List :"+ list);
		
		System.out.println("Iterating ArrayList using list.Iterator()");
		Iterator<String> iterator = list.iterator();
		
		while (iterator.hasNext()) {
			System.out.println(iterator.next());
		}

Iterating ArrayList in Java using iteratorList() and iteratorList(int Index)

Java ArrayList listIterator() and listIterator( int index) to traverse through list elements. Traversing through list Iterator has many advantages like

• You can traverse reverse direction as well as front direction using ListIterator's method previous() and next().
• Even you can previous and next Index using ListIterator previousIndex() and nextIndex().
• You can add an element or Replace elements to list iterator while traversing using ListIterator’s add(E e) or set (E e)

System.out.println("Iterating ArrayList using list.ListIterator()");
		ListIterator<String> listIterator = list.listIterator();
		System.out.println("Traversing in front direction");
		while(listIterator.hasNext()) {
			String country = listIterator.next();
			System.out.println("current element: " +country);
			
		} 
		System.out.println("Traversing in backword direction");
		while(listIterator.hasPrevious()) {
			String country = listIterator.previous();
			System.out.println("current element: " +country);
			
		} 

Adding and Updating ArrayList while traversing or Iterating in java

ListIterator of ArrayList provides set(E e) and add( E e) methods to update or add elements to current ArrayList while traversing. Below code shows adding of country Japan when Us and updating China to Nepal

ListIterator<String> listIterator = list.listIterator();
		System.out.println("Traversing in front direction");
		while(listIterator.hasNext()) {
			String country = listIterator.next();
			System.out.println("current element: " +country);
			if(country.equals("US"))
			  listIterator.add("Japan");
			if(country.equals("China"))
				  listIterator.set("Nepal");
			
		} 
		System.out.println("Updated List: " + list);

Example code for iterating Arraylist in different ways.

package com.techijournal.collection;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

import java.util.ListIterator;

public class MyArrayList {

	public static void main(String atgs []) {
		IteratingOverList();
	}
	private static void IteratingOverList() {
		List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("Initial List :"+ list);
                System.out.println("Iterating ArrayList using list.forEach");
		list.forEach(country -> System.out.print(country+ " "));
		
		System.out.println();
		
		System.out.print("Using for Loop to Iterate ArrayList");
		for(int i = 0; i< list.size(); i++) {
			System.out.println(list.get(i)+ " ");
		}
		System.out.println("Iterating ArrayList using list.Iterator()");
		Iterator<String> iterator = list.iterator();
		
		while (iterator.hasNext()) {
			System.out.println(iterator.next());
			
		}
		
		System.out.println("Iterating ArrayList using list.ListIterator()");
		ListIterator<String> listIterator = list.listIterator();
		System.out.println("Traversing in front direction");
		while(listIterator.hasNext()) {
			String country = listIterator.next();
			System.out.println("current element: " +country);
			if(country.equals("US"))
			  listIterator.add("Japan");
			if(country.equals("China"))
				  listIterator.set("Nepal");
			
		} 
		System.out.println("Updated List: " + list);

		
		System.out.println("Traversing in backword direction");
		while(listIterator.hasPrevious()) {
			String country = listIterator.previous();
			System.out.println("current element: " +country);
			
		} 
		
		
		
	}
}

Output for above code is:

Initial List :[US, China, India, Rome, Cannada]
Iterating ArrayList using list.forEach
US China India Rome Cannada
Using for Loop to Iterate ArrayList
US
China
India
Rome
Cannada
Iterating ArrayList using list.Iterator()
US
China
India
Rome
Cannada
Iterating ArrayList using list.ListIterator()
Traversing in front direction
current element: US
current element: China
current element: India
current element: Rome
current element: Cannada
Updated List: [US, Japan, Nepal, India, Rome, Cannada]
Traversing in backword direction
current element: Cannada
current element: Rome
current element: India
current element: Nepal
current element: Japan
current element: US

ArrayList Clone Method in Java

Java ArrayList provide a way to create clone for ArrayList using shallow Copy using its clone() method.

A shallow copy of the given ArrayList means that you have created a duplicate of the list, not the elements in it. Example In the below code we have ArrayList of type Country with some countries in it. Now, we will create a clone of it to duplicateList.

Now what is happening is your list is cloned and reference of the country in list is passed to duplicateList elements. Means whenever you update any Country properties it any of the list (duplicateList or list) it will reflected in both list.

In the below code at line no 14 we updated duplicateList’s index 0 country to USD. The changes are reflected in both lists.

Example of clone() in ArrayList

	private static void cloningArrayList() {
		
		ArrayList<Country> list = new ArrayList<>();
		list.add(new Country("US"));
		list.add(new Country("China"));
		list.add(new Country("India"));
		list.add(new Country("Rome"));
		list.add(new Country("Cannada"));
		System.out.println("Initial List :"+ list);
		
		List<Country> duplicateList = new ArrayList<>();
		duplicateList = (List<Country>) list.clone();
		System.out.println("Cloned list is :"+ duplicateList);
		duplicateList.get(0).setName("USD");
		System.out.println("Updated Initial List :"+ list);
		System.out.println("Updated Cloned List :"+ duplicateList);
	}

Output for above code:

Initial List :[Country [name=US], Country [name=China], Country [name=India], Country [name=Rome], Country [name=Cannada]]
Cloned list is :[Country [name=US], Country [name=China], Country [name=India], Country [name=Rome], Country [name=Cannada]]
Initial List :[Country [name=USD], Country [name=China], Country [name=India], Country [name=Rome], Country [name=Cannada]]
Cloned List :[Country [name=USD], Country [name=China], Country [name=India], Country [name=Rome], Country [name=Cannada]]

Getting size of ArrayList

ArrayList provide size() to get size of the arrayList.

List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
System.out.println(list.size());

output for above code is:

5

Converting List to Array

ArrayList provides toArray() method to convert arrayList to array.

Example code for Converting ArrayList to Array

private static void toArray() {
		List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("Initial List :"+ list);
		String [] objectArr = (String[]) list.toArray(new String[10]);
		System.out.println("Converted arraylist to array is : ");
		list.forEach(e -> System.out.println(String.valueOf(e)+" ") );
		String [] arr = (String[]) list.toArray(new String[10]);
		System.out.println("Converted arraylist to array is : ");
		list.forEach(e -> System.out.print(e+" ") );
	}

Output for the above code:

Initial List :[US, China, India, Rome, Cannada]
Converted arraylist to array is :
US
China
India
Rome
Cannada
Converted arraylist to array is :
US China India Rome Cannada

Sorting the ArrayList

From java 8 ArrayList have sort() which can sort the elements on basis of passed comparator to it.

Example of sorting array list using sort()

private static void sortArrayList() {
		List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("Initial List :"+ list);
		
		list.sort((o1,o2) ->{
			return o1.compareTo(o2);
		});
		System.out.println("Sorted list is : "+list);
	}

Output for the above code is:

Initial List :[US, China, India, Rome, Cannada]
Sorted list is : [Cannada, China, India, Rome, US]

Extra methods of ArrayList

Difference between set() and add() methods of ArrayList

ArrayList set( int index, E element ) is used to replace element at given index in array list. And it return the replaced element.

Where as,

ArraylList add( int index, E element ) is used to add element at given index This method will also update the existing old element at index. But it returns a boolean value.

	private static void setAndAdd() {
		List<String> list = new ArrayList<>();
		list.add("US");
		list.add("China");
		list.add("India");
		list.add("Rome");
		list.add("Cannada");
		System.out.println("Initial List :"+ list);
		//list.add(5,"Japan");
		System.out.println(list.set(4, "Russia"));
	}

output for above code is:

Initial List :[US, China, India, Rome, Cannada]
Cannada

ArrayList Vs Array

ArrayList Vs LinkedList

Learn about linkedList in more detail here at Java LinkedList

ArrayList Vs Vector

Conclusion

In this post, we saw different ways to use ArrayList in Java. We used add(E element ) and add( int index, E element ) to add elements to list and also we saw different ways to create an ArrayList example: new ArrayList().

We learned different flavours of method to remove elements from ArrayList like removeAll(Collection c) .

Thank’s Note,

Thanks for spending you valuable time on this post hope this post was use full to you and you learned something new today. If you like our post please share our post.

Reference: Oracles Docs