Java ArrayList

Java ArrayList class hierarchy

Java ArrayList class uses a dynamic array for storing the elements. It is like an array, but there is no size limit. We can add or remove elements anytime. So, it is much more flexible than the traditional array. It is found in the java.util package. It is like the Vector in C++.

The ArrayList in Java can have the duplicate elements also. It implements the List interface so we can use all the methods of the List interface here. The ArrayList maintains the insertion order internally.

It inherits the AbstractList class and implements List interface.

The important points about the Java ArrayList class are:

Java ArrayList class can contain duplicate elements.
Java ArrayList class maintains insertion order.
Java ArrayList class is non synchronized.
Java ArrayList allows random access because the array works on an index basis.
In ArrayList, manipulation is a little bit slower than the LinkedList in Java because a lot of shifting needs to occur if any element is removed from the array list.
We can not create an array list of the primitive types, such as int, float, char, etc. It is required to use the required wrapper class in such cases. For example:

  ArrayList al = ArrayList(); // does not work  ArrayList al = new ArrayList(); // works fine  

Java ArrayList gets initialized by the size. The size is dynamic in the array list, which varies according to the elements getting added or removed from the list.

Hierarchy of ArrayList class

As shown in the above diagram, the Java ArrayList class extends AbstractList class which implements the List interface. The List interface extends the Collection and Iterable interfaces in hierarchical order.

ArrayList class declaration

Let’s see the declaration for java.util.ArrayList class.

  public class ArrayList extends AbstractList implements List, RandomAccess, Cloneable, Serializable  

Constructors of ArrayList

Constructor	Description
ArrayList()	It is used to build an empty array list.
ArrayList(Collection<? extends E> c)	It is used to build an array list that is initialized with the elements of the collection c.
ArrayList(int capacity)	It is used to build an array list that has the specified initial capacity.

Methods of ArrayList

Method	Description
void add(int index, E element)	It is used to insert the specified element at the specified position in a list.
boolean add(E e)	It is used to append the specified element at the end of a list.
boolean addAll(Collection<? extends E> c)	It is used to append all of the elements in the specified collection to the end of this list, in the order that they are returned by the specified collection’s iterator.
boolean addAll(int index, Collection<? extends E> c)	It is used to append all the elements in the specified collection, starting at the specified position of the list.
void clear()	It is used to remove all of the elements from this list.
void ensureCapacity(int requiredCapacity)	It is used to enhance the capacity of an ArrayList instance.
E get(int index)	It is used to fetch the element from the particular position of the list.
boolean isEmpty()	It returns true if the list is empty, otherwise false.
Iterator()
listIterator()
int lastIndexOf(Object o)	It is used to return the index in this list of the last occurrence of the specified element, or -1 if the list does not contain this element.
Object[] toArray()	It is used to return an array containing all of the elements in this list in the correct order.
<T> T[]toArray(T[] a)	It is used to return an array containing all of the elements in this list in the correct order.
Object clone()	It is used to return a shallow copy of an ArrayList.
boolean contains(Object o)	It returns true if the list contains the specified element.
int indexOf(Object o)	It is used to return the index in this list of the first occurrence of the specified element, or -1 if the List does not contain this element.
E remove(int index)	It is used to remove the element present at the specified position in the list.
boolean remove(Object o)	It is used to remove the first occurrence of the specified element.
boolean removeAll(Collection<?> c)	It is used to remove all the elements from the list.
boolean removeIf(Predicate<? super E> filter)	It is used to remove all the elements from the list that satisfies the given predicate.
protected void removeRange(int fromIndex, int toIndex)	It is used to remove all the elements lies within the given range.
void replaceAll(UnaryOperator<E> operator)	It is used to replace all the elements from the list with the specified element.
void retainAll(Collection<?> c)	It is used to retain all the elements in the list that are present in the specified collection.
E set(int index, E element)	It is used to replace the specified element in the list, present at the specified position.
void sort(Comparator<? super E> c)	It is used to sort the elements of the list on the basis of the specified comparator.
Spliterator<E> spliterator()	It is used to create a spliterator over the elements in a list.
List<E> subList(int fromIndex, int toIndex)	It is used to fetch all the elements that lies within the given range.
int size()	It is used to return the number of elements present in the list.
void trimToSize()	It is used to trim the capacity of this ArrayList instance to be the list’s current size.

Java Non-generic Vs. Generic Collection

Java collection framework was non-generic before JDK 1.5. Since 1.5, it is generic.

Java new generic collection allows you to have only one type of object in a collection. Now it is type-safe, so typecasting is not required at runtime.

Let’s see the old non-generic example of creating a Java collection.

Let’s see the new generic example of creating java collection.

In a generic collection, we specify the type in angular braces. Now ArrayList is forced to have the only specified type of object in it. If you try to add another type of object, it gives a compile-time error.

For more information on Java generics, click here Java Generics Tutorial.

Java ArrayList Example

FileName: ArrayListExample1.java

  import java.util.*;   public class ArrayListExample1{   public static void main(String args[]){    ArrayList list=new ArrayList();//Creating arraylist      list.add(“Mango”);//Adding object in arraylist      list.add(“Apple”);      list.add(“Banana”);      list.add(“Grapes”);      //Printing the arraylist object     System.out.println(list);   }  }  

Test it Now

Output:

[Mango, Apple, Banana, Grapes]

Iterating ArrayList using Iterator

Let’s see an example to traverse ArrayList elements using the Iterator interface.

FileName: ArrayListExample2.java

  import java.util.*;  public class ArrayListExample2{   public static void main(String args[]){    ArrayList list=new ArrayList();//Creating arraylist    list.add(“Mango”);//Adding object in arraylist      list.add(“Apple”);      list.add(“Banana”);      list.add(“Grapes”);      //Traversing list through Iterator    Iterator itr=list.iterator();//getting the Iterator    while(itr.hasNext()){//check if iterator has the elements     System.out.println(itr.next());//printing the element and move to next    }   }  }  

Test it Now

Output:

Mango  Apple  Banana  Grapes

Iterating ArrayList using For-each loop

Let’s see an example to traverse the ArrayList elements using the for-each loop

FileName: ArrayListExample3.java

  import java.util.*;  public class ArrayListExample3{   public static void main(String args[]){    ArrayList list=new ArrayList();//Creating arraylist    list.add(“Mango”);//Adding object in arraylist      list.add(“Apple”);      list.add(“Banana”);      list.add(“Grapes”);      //Traversing list through for-each loop    for(String fruit:list)        System.out.println(fruit);       }  }  

Output:

Test it Now

Mango  Apple  Banana  Grapes

Get and Set ArrayList

The get() method returns the element at the specified index, whereas the set() method changes the element.

FileName: ArrayListExample4.java

  import java.util.*;  public class ArrayListExample4{   public static void main(String args[]){    ArrayList al=new ArrayList();    al.add(“Mango”);    al.add(“Apple”);    al.add(“Banana”);    al.add(“Grapes”);    //accessing the element      System.out.println(“Returning element: “+al.get(1));//it will return the 2nd element, because index starts from 0    //changing the element    al.set(1,”Dates”);    //Traversing list    for(String fruit:al)        System.out.println(fruit);       }  }  

Test it Now

Output:

Returning element: Apple  Mango  Dates  Banana  Grapes

How to Sort ArrayList

The java.util package provides a utility class Collections, which has the static method sort(). Using the Collections.sort() method, we can easily sort the ArrayList.

FileName: SortArrayList.java

  import java.util.*;  class SortArrayList{   public static void main(String args[]){    //Creating a list of fruits    List list1=new ArrayList();    list1.add(“Mango”);    list1.add(“Apple”);    list1.add(“Banana”);    list1.add(“Grapes”);    //Sorting the list    Collections.sort(list1);     //Traversing list through the for-each loop    for(String fruit:list1)      System.out.println(fruit);         System.out.println(“Sorting numbers…”);    //Creating a list of numbers    List list2=new ArrayList();    list2.add(21);    list2.add(11);    list2.add(51);    list2.add(1);    //Sorting the list    Collections.sort(list2);     //Traversing list through the for-each loop    for(Integer number:list2)      System.out.println(number);   }     }  

Output:

Apple  Banana  Grapes  Mango  Sorting numbers...  1  11  21  51

Ways to iterate the elements of the collection in Java

There are various ways to traverse the collection elements:

By Iterator interface.
By for-each loop.
By ListIterator interface.
By for loop.
By forEach() method.
By forEachRemaining() method.

Iterating Collection through remaining ways

Let’s see an example to traverse the ArrayList elements through other ways

FileName: ArrayList4.java

  import java.util.*;  class ArrayList4{   public static void main(String args[]){      ArrayList list=new ArrayList();//Creating arraylist     list.add(“Ravi”);//Adding object in arraylist     list.add(“Vijay”);     list.add(“Ravi”);     list.add(“Ajay”);         System.out.println(“Traversing list through List Iterator:”);     //Here, element iterates in reverse order        ListIterator list1=list.listIterator(list.size());        while(list1.hasPrevious())        {        String str=list1.previous();        System.out.println(str);        }      System.out.println(“Traversing list through for loop:”);        for(int i=0;i{ //Here, we are using lambda expression          System.out.println(a);        });              System.out.println(“Traversing list through forEachRemaining() method:”);        Iterator itr=list.iterator();        itr.forEachRemaining(a-> //Here, we are using lambda expression        {      System.out.println(a);        });   }  }  ();i++)>

Output:

Traversing list through List Iterator:  Ajay  Ravi  Vijay  Ravi  Traversing list through for loop:  Ravi  Vijay  Ravi  Ajay  Traversing list through forEach() method:  Ravi  Vijay  Ravi  Ajay  Traversing list through forEachRemaining() method:  Ravi  Vijay  Ravi  Ajay

User-defined class objects in Java ArrayList

Let’s see an example where we are storing Student class object in an array list.

FileName: ArrayList5.java

  class Student{    int rollno;    String name;    int age;    Student(int rollno,String name,int age){     this.rollno=rollno;     this.name=name;     this.age=age;    }  }  

  import java.util.*;   class ArrayList5{   public static void main(String args[]){    //Creating user-defined class objects    Student s1=new Student(101,”tutor”,23);    Student s2=new Student(102,”Ravi”,21);    Student s2=new Student(103,”Hanumat”,25);    //creating arraylist    ArrayList al=new ArrayList();    al.add(s1);//adding Student class object    al.add(s2);    al.add(s3);    //Getting Iterator    Iterator itr=al.iterator();    //traversing elements of ArrayList object    while(itr.hasNext()){      Student st=(Student)itr.next();      System.out.println(st.rollno+” “+st.name+” “+st.age);    }   }  }  

Output:

       101 tutor 23         102 Ravi 21         103 Hanumat 25

Java ArrayList Serialization and Deserialization Example

Let’s see an example to serialize an ArrayList object and then deserialize it.

FileName: ArrayList6.java

  import java.io.*;  import java.util.*;   class ArrayList6 {        public static void main(String [] args)      {        ArrayList al=new ArrayList();        al.add(“Ravi”);          al.add(“Vijay”);          al.add(“Ajay”);                  try        {                //Serialization        FileOutputStream fos=new FileOutputStream(“file”);        ObjectOutputStream oos=new ObjectOutputStream(fos);        oos.writeObject(al);        fos.close();        oos.close();        //Deserialization        FileInputStream fis=new FileInputStream(“file”);        ObjectInputStream ois=new ObjectInputStream(fis);      ArrayList  list=(ArrayList)ois.readObject();      System.out.println(list);          }catch(Exception e)        {        System.out.println(e);        }     }  }           

Output:

       [Ravi, Vijay, Ajay]

Java ArrayList example to add elements

Here, we see different ways to add an element.

FileName: ArrayList7.java

  import java.util.*;   class ArrayList7{   public static void main(String args[]){    ArrayList al=new ArrayList();     System.out.println(“Initial list of elements: “+al);     //Adding elements to the end of the list     al.add(“Ravi”);     al.add(“Vijay”);     al.add(“Ajay”);     System.out.println(“After invoking add(E e) method: “+al);     //Adding an element at the specific position     al.add(1, “Gaurav”);     System.out.println(“After invoking add(int index, E element) method: “+al);     ArrayList al2=new ArrayList();     al2.add(“tutor”);     al2.add(“Hanumat”);     //Adding second list elements to the first list     al.addAll(al2);     System.out.println(“After invoking addAll(Collection<? extends E> c) method: “+al);     ArrayList al3=new ArrayList();     al3.add(“John”);     al3.add(“Rahul”);     //Adding second list elements to the first list at specific position     al.addAll(1, al3);     System.out.println(“After invoking addAll(int index, Collection<? extends E> c) method: “+al);        }  }  

Output:

Initial list of elements: []  After invoking add(E e) method: [Ravi, Vijay, Ajay]  After invoking add(int index, E element) method: [Ravi, Gaurav, Vijay, Ajay]  After invoking addAll(Collection<? extends E> c) method:   [Ravi, Gaurav, Vijay, Ajay, tutor, Hanumat]  After invoking addAll(int index, Collection<? extends E> c) method:   [Ravi, John, Rahul, Gaurav, Vijay, Ajay, tutor, Hanumat]

Java ArrayList example to remove elements

Here, we see different ways to remove an element.

FileName: ArrayList8.java

  import java.util.*;   class ArrayList8 {        public static void main(String [] args)      {        ArrayList al=new ArrayList();        al.add(“Ravi”);          al.add(“Vijay”);          al.add(“Ajay”);         al.add(“Anuj”);        al.add(“Gaurav”);        System.out.println(“An initial list of elements: “+al);         //Removing specific element from arraylist        al.remove(“Vijay”);        System.out.println(“After invoking remove(object) method: “+al);         //Removing element on the basis of specific position        al.remove(0);        System.out.println(“After invoking remove(index) method: “+al);                 //Creating another arraylist        ArrayList al2=new ArrayList();          al2.add(“Ravi”);          al2.add(“Hanumat”);          //Adding new elements to arraylist        al.addAll(al2);        System.out.println(“Updated list : “+al);         //Removing all the new elements from arraylist        al.removeAll(al2);        System.out.println(“After invoking removeAll() method: “+al);         //Removing elements on the basis of specified condition        al.removeIf(str -> str.contains(“Ajay”));   //Here, we are using Lambda expression         System.out.println(“After invoking removeIf() method: “+al);        //Removing all the elements available in the list        al.clear();        System.out.println(“After invoking clear() method: “+al);      }  }                   

Output:

An initial list of elements: [Ravi, Vijay, Ajay, Anuj, Gaurav]  After invoking remove(object) method: [Ravi, Ajay, Anuj, Gaurav]  After invoking remove(index) method: [Ajay, Anuj, Gaurav]  Updated list : [Ajay, Anuj, Gaurav, Ravi, Hanumat]  After invoking removeAll() method: [Ajay, Anuj, Gaurav]  After invoking removeIf() method: [Anuj, Gaurav]  After invoking clear() method: []

Java ArrayList example of retainAll() method

FileName: ArrayList9.java

  import java.util.*;  class ArrayList9{   public static void main(String args[]){    ArrayList al=new ArrayList();    al.add(“Ravi”);    al.add(“Vijay”);    al.add(“Ajay”);    ArrayList al2=new ArrayList();    al2.add(“Ravi”);    al2.add(“Hanumat”);    al.retainAll(al2);    System.out.println(“iterating the elements after retaining the elements of al2”);    Iterator itr=al.iterator();    while(itr.hasNext()){     System.out.println(itr.next());    }   }  }  

Output:

       iterating the elements after retaining the elements of al2         Ravi

Java ArrayList example of isEmpty() method

FileName: ArrayList4.java

    import java.util.*;   class ArrayList10{        public static void main(String [] args)      {        ArrayList al=new ArrayList();        System.out.println(“Is ArrayList Empty: “+al.isEmpty());        al.add(“Ravi”);          al.add(“Vijay”);          al.add(“Ajay”);          System.out.println(“After Insertion”);        System.out.println(“Is ArrayList Empty: “+al.isEmpty());      }  }      

Output:

Is ArrayList Empty: true  After Insertion  Is ArrayList Empty: false

Java ArrayList Example: Book

Let’s see an ArrayList example where we are adding books to the list and printing all the books.

FileName: ArrayListExample20.java

  import java.util.*;  class Book {  int id;  String name,author,publisher;  int quantity;  public Book(int id, String name, String author, String publisher, int quantity) {  this.id = id;  this.name = name;  this.author = author;  this.publisher = publisher;  this.quantity = quantity;  }  }  public class ArrayListExample20 {  public static void main(String[] args) {  //Creating list of Books  List list=new ArrayList();  //Creating Books  Book b1=new Book(101,”Let us C”,”Yashwant Kanetkar”,”BPB”,8);  Book b2=new Book(102,”Data Communications and Networking”,”Forouzan”,”Mc Graw Hill”,4);  Book b3=new Book(103,”Operating System”,”Galvin”,”Wiley”,6);  //Adding Books to list  list.add(b1);  list.add(b2);  list.add(b3);  //Traversing list  for(Book b:list){  System.out.println(b.id+” “+b.name+” “+b.author+” “+b.publisher+” “+b.quantity);  }  }  }  

Test it Now

Output:

101 Let us C Yashwant Kanetkar BPB 8  102 Data Communications and Networking Forouzan Mc Graw Hill 4  103 Operating System Galvin Wiley 6

Size and Capacity of an ArrayList

Size and capacity of an array list are the two terms that beginners find confusing. Let’s understand it in this section with the help of some examples. Consider the following code snippet.

FileName: SizeCapacity.java

  import java.util.*;    public class SizeCapacity  {    public static void main(String[] args) throws Exception  {             ArrayList al = new ArrayList();            System.out.println(“The size of the array is: ” + al.size());  }  }  

Output:

The size of the array is: 0

Explanation: The output makes sense as we have not done anything with the array list. Now observe the following program.

FileName: SizeCapacity1.java

  import java.util.*;    public class SizeCapacity1  {    public static void main(String[] args) throws Exception  {             ArrayList al = new ArrayList(10);            System.out.println(“The size of the array is: ” + al.size());  }  }  

Output:

The size of the array is: 0

Explanation: We see that the size is still 0, and the reason behind this is the number 10 represents the capacity no the size. In fact, the size represents the total number of elements present in the array. As we have not added any element, therefore, the size of the array list is zero in both programs.

Capacity represents the total number of elements the array list can contain. Therefore, the capacity of an array list is always greater than or equal to the size of the array list. When we add an element to the array list, it checks whether the size of the array list has become equal to the capacity or not. If yes, then the capacity of the array list increases. So, in the above example, the capacity will be 10 till 10 elements are added to the list. When we add the 11^th element, the capacity increases. Note that in both examples, the capacity of the array list is 10. In the first case, the capacity is 10 because the default capacity of the array list is 10. In the second case, we have explicitly mentioned that the capacity of the array list is 10.

Note: There is no any standard method to tell how the capacity increases in the array list. In fact, the way the capacity increases vary from one GDK version to the other version. Therefore, it is required to check the way capacity increases code is implemented in the GDK. There is no any pre-defined method in the ArrayList class that returns the capacity of the array list. Therefore, for better understanding, use the capacity() method of the Vector class. The logic of the size and the capacity is the same in the ArrayList class and the Vector class.

ArrayList in Java

Java ArrayList

Hierarchy of ArrayList class

ArrayList class declaration

Constructors of ArrayList

Methods of ArrayList

Java Non-generic Vs. Generic Collection

Java ArrayList Example

Iterating ArrayList using Iterator

Iterating ArrayList using For-each loop

Get and Set ArrayList

How to Sort ArrayList

Ways to iterate the elements of the collection in Java

Iterating Collection through remaining ways

User-defined class objects in Java ArrayList

Java ArrayList Serialization and Deserialization Example

Java ArrayList example to add elements

Java ArrayList example to remove elements

Java ArrayList example of retainAll() method

Java ArrayList example of isEmpty() method

Java ArrayList Example: Book

Size and Capacity of an ArrayList

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