Arraylist of students java

ArrayList in Java With Examples

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

Array vs arraylist in java

The main difference between array and arraylist is that arraylist can grow and shrink dynamically while an array cannot.

An array has a fixed length so if it is full you cannot add any more elements to it. Similarly, if number of elements are removed from ArrayList, the memory consumption remains same as it doesn’t shrink.

On the other hand, ArrayList can dynamically grow and shrink after addition and removal of elements. ArrayList class has several useful methods that can make our task easy.

ArrayList in Java

• ArrayList can grow and shrink automatically based on the addition and removal of elements.
• ArrayList can contain duplicate elements
• ArrayList maintains the insertion order, which means the elements appear in the same order in which they are inserted.
• ArrayList is non synchronized. However you can make it synchronized.

Hierarchy of ArrayList class in Java

ArrayList class implements List interface and List interface extends Collection interface.

Arraylist in Java declaration

This is how you can declare an ArrayList of String type:

ArrayList list=new ArrayList<>();

This is how you can declare an ArrayList of Integer type:

ArrayList list=new ArrayList<>();

Adding elements to Arraylist in java

Adding Element in ArrayList at specified position:

You can add elements to an ArrayList by using add() method. This method has couple of variations, which you can use based on the requirement.

For example: If you want to add the element at the end of the List then you can simply call the add() method like this:

arrList.add("Steve"); //This will add "Steve" at the end of List

To add the element at the specified location in ArrayList, you can specify the index in the add() method like this:

arrList.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[])< ArrayListarrList=new ArrayList(); arrList.add("Steve"); arrList.add("Tim"); arrList.add("Lucy"); arrList.add("Pat"); arrList.add("Angela"); arrList.add("Tom"); //displaying elements System.out.println(arrList); //Adding "Steve" at the fourth position arrList.add(3, "Steve"); //displaying elements System.out.println(arrList); > >

[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

You can use the set method to change an element in ArrayList. You need to 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. The method updated the element present at the index 0 (“Jim”) with the new String element “Lucy”.

import java.util.ArrayList; public class JavaExample < public static void main(String[] args) < ArrayListnames = new ArrayList(); names.add("Jim"); names.add("Jack"); names.add("Ajeet"); names.add("Chaitanya"); names.set(0, "Lucy"); System.out.println(names); > >

Output:

How to remove element from Arraylist in Java?

Removing Element from ArrayList:

You can use remove() method to remove elements from an ArrayList. Similar to add() method, this method also has couple of variations.

For example:

import java.util.*; class JavaExample < public static void main(String args[])< ArrayListalist=new ArrayList(); 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); > >

[Steve, Tim, Lucy, Pat, Angela, Tom] [Tim, Lucy, Pat, Tom] [Tim, Lucy, Tom]

Iterating ArrayList

Here, we are using enhanced for loop to iterate ArrayList elements. This one of the best ways to iterate an ArrayList of string type.

import java.util.*; class JavaExample < public static void main(String args[])< ArrayListalist=new ArrayList(); 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); > >

Gregor Clegane Khal Drogo Cersei Lannister Sandor Clegane Tyrion Lannister

There are several other ways to loop an ArrayList:

• Using iterator
• Using enhanced for-each loop.
• Using list iterator
• Using for loop
• Using forEachRemaining() method.

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) < ArrayListnumbers = new ArrayList(); numbers.add(1); numbers.add(7); numbers.add(5); numbers.add(6); System.out.println("Number of elements in ArrayList: "+numbers.size()); > >

Output:

Sort ArrayList

You can use the sort() method of the Collections utility class to sort an ArrayList. This class is is a part of java.util package. In the following example we are sorting a list of String type alphabetically. This method also works on numeric lists (such as Integer type ArrayList).

import java.util.ArrayList; import java.util.Collections; public class JavaExample < public static void main(String[] args) < ArrayListfruits = new ArrayList(); fruits.add("Orange"); fruits.add("Apple"); fruits.add("Banana"); fruits.add("Pineapple"); Collections.sort(fruits); for (String str : fruits) < System.out.println(str); >> >

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 “String” type. We are adding 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, 1 indicates second position and so on.

We are then removing the elements “Chaitanya” and “Harry” from the ArrayList. We again removed the second element of the ArrayList using method remove(int index).

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 */ ArrayListobj = new ArrayList(); /*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

All methods of Arraylist in Java

In the above examples, we have used methods such as add() and remove(). However there are number of other useful methods available in ArrayList class.

1) add( Object o): This method adds an object o at the end of the arraylist.

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

2) add(int index, Object o): It adds the object o at the specified index in the ArrayList.

It will add the string “bye” at the 2nd index (third element as array list starts with index 0) of array list.

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

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

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

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.

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.

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.

This would return the string stored at 3rd position (index 2) and would be assigned to the string “str”. We are using string variable to store the get() result because the list is of string type. If the list is of int type then we can use int variable to store the returned element.

8) int size(): It returns 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 the element is found it returns true else it returns false.

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.

Arraylist in Java Interview Questions

These are the popular questions asked during interviews. I have covered them in separate posts as this guide is already long enough and I do not want to make it any longer. Happy learning!

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About the Author

I have 15 years of experience in the IT industry, working with renowned multinational corporations. Additionally, I have dedicated over a decade to teaching, allowing me to refine my skills in delivering information in a simple and easily understandable manner.

Comments

Thank you very much for your Beginner’s Book. It is better than any other source I have found for a Java beginner. The examples are especially helpful. I wish you success.

Thank you so much for posting these contents. The examples given here are best to understand any concept. Keep adding more examples. Thanks again 🙂

Friend,I am not able to Command Array List showing me error: The type ArrayList is not generic; it cannot be parameterized with arguments

I suspect you imported some different ArrayList class in your classpath.Try import java.util.ArrayList, and I’m sure it would certainly work. Check your Java version as well, ArrayList is not a legacy class so it might support from JDK 1.5
Priya.

Example are clear and easy to understand quickly.
The way of presentation is really very nice.
Thanks ?

i just stumbled on your website on google, never knew this kind of well explanatory java website exist. Your topics are truly helpful for a java starter(Beginners). i will directed my beginners here.Thanks to you.

say there are duplicate elements in the list. If I call the method remove(), then will it remove all the duplicate elements?

I tried to do this and for example if you have it like this: If you have the ArrayList created let`s say like this: ArrayList list = new ArrayList(); and you set it like this: position 0 – “John”
1 – “Michael”
2 – “Mitch”
3 – “Gus”
4 – “John”
5 – “Johnny” if you say: list.remove(“John”); then it will remove its first occurance. So, the new elements will be like this: 0 – “Michael”
1 – “Mitch”
2 – “Gus”
3 – “John”
4 – “Johnny” I hope I was clear enough. 🙂

The element that was spotted first will be removed with remove() method.The rest of duplicate elements remain in the list.

No.It will not remove all the duplicate elements.remove() method will remove the element specified which occurs at the first instance.

I want to compare arraylist with my string input in if statement. if condition becomes true then it return me the element or string which i compared in if statement.
Please help.
thanks in advance

No words to say, That much your tutorials are impressing. i can get everything in single website, which is very great thing.
its really helpfull to add spring and hibernate with this page.

is there any method that removes all elements matching the given element(including duplicates)in collection framework.
please advise.

Thank you very very very much..for sharing ur knowledge with us with great style
#Respect_From_Palestine

Such A Lovely Explanations , i had suggested all my friends for Beginnerbooks.com
Thank you Team.
God Bless You

Really really very very good tutorial for beginners..Please read this tutorial if any one want to know clear idea about collections…After reading this tutorial 90% of ideas you can get in collections and am damn sure..

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Подробный разбор класса ArrayList [Часть 2]

К сожалению, в одну статью вся информация не поместилась, поэтому продолжаем разбираться с оставшимися методами класса ArrayList. Отсортировать коллекцию:

 public void sort(Comparator < ? super E>c) 

Осуществляет сортировку списка по заданному правилу. Правило сортировки представляет собой реализованный интерфейс Comparator с переопределенным методом compare() . Переопределение нужно, если коллекция содержит объекты собственного класса. При работе со стандартными классами (Integer, String и так далее) переопределение compare обычно требуется только для нестандартной сортировки. Создадим класс Student :

 class Student < String student_name; int id; Student(int id, String student_name)< this.id = id; this.student_name = student_name; >public String toString() < return id + " " + student_name; >> 

 class StudentIdComparator implements Comparator < public int compare(Student e1, Student e2) < return e1.id.compareTo(e2.id); >> 

 ArrayList myList = new ArrayList<> (); StudentIdComparator comparator = new StudentIdComparator(); 

В результате исходный список [4 David, 2 Tom, 5 Rohit, 1 Paul, 3 Vishal] превратится в [1 Paul, 2 Tom, 3 Vishal, 4 David, 5 Rohit]. Напоследок я оставил очень интересный, но редко кем используемый метод:

 public List subList(int fromIndex, int toIndex) 

Он возвращает не новый список, как может показаться, а вид (view) списка (подсписок), для которого этот метод был вызван, таким образом, что оба списка станут разделять хранимые элементы. subList — полнофункциональный список, он работает и на запись, внося соответствующие изменения в родительский список. Из этого вытекают прекрасные свойства:

В данном примере из списка someList будут удалены четыре элемента, с третьего по седьмой (не включительно). Указываем диапазон для работы со списком и вперед. Внутри метода, по сути, происходит обращение к классу SubList, у которого есть собственные реализации известных методов, а в результате работы метода возвращается объект этого класса. Саму реализацию класса можно посмотреть в исходном коде. Для закрепления материала я предлагаю Вам написать свою реализацию динамического массива. Это будет очень полезно в будущем. В качестве примера я представляю свою реализацию динамического массива только для чисел с комментариями в коде.

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