Java ArrayList initialize default values

This Tutorial Explains How to Declare, Initialize & Print Java ArrayList with Code Examples. You will also learn about 2D Arraylist & Implementation of ArrayList in Java:

Java Collections Framework and the List interface were explained in detail in our previous tutorials. ArrayList is a data structure that is part of the Collections Framework and can be viewed as similar to arrays and vectors.

ArrayList can be perceived as a dynamic array that allows you to add or remove elements from it any time or simply said, dynamically.

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In other words, its size can increase or decrease dynamically unlike arrays whose size remains static once declared.

ArrayList Class In Java

The ArrayList data structure in Java is represented by the ArrayList class which is a part of the java.util package.

The hierarchy for the ArrayList class is shown below.

As you can see, the ArrayList class implements the List interface which in turn extends from the Collection interface.

The general definition of the ArrayList class is given below:

public class ArrayList<E> extends AbstractList<E> implements List<E>,RandomAccess, Cloneable, Serializable

Here are some of the distinguishing characteristics of ArrayList:

• The ArrayList class of Java stores elements by maintaining the insertion order.
• The ArrayList allows duplicate elements stored in it.
• ArrayList is not synchronized, the major point that differentiates the ArrayList from Vector class in Java.
• ArrayList in Java is more identical to Vectors in C++.
• The ArrayList in Java also uses indices like arrays and supports random access.
• The operations that manipulate elements in the ArrayList are slow as a lot of shifting of elements needs to be done if any element is to be removed from the ArrayList.
• The ArrayList class cannot contain primitive types but only objects. In this case, we usually call it as ArrayList of objects. So if you want to store integer type of elements, then you have to use the Integer object of the wrapper class and not primitive type int.

Create And Declare ArrayList

In order to use the ArrayList class in your program, you need to include it first in your program using the import directive as shown below:

import java.util.ArrayList;

OR

import java.util.*; //this will include all classes from java.util package

Once you import the ArrayList class in your program, you can create an ArrayList object.

The general ArrayList creation syntax is:

ArrayList<data_type> arrayList = new ArrayList<> ();

Apart from the above statement that uses default constructor, the ArrayList class also provides other overloaded constructors that you can use to create the ArrayList.

Constructor Methods

The ArrayList class in Java provides the following constructor methods to create the ArrayList.

Method #1: ArrayList()

This method uses the default constructor of the ArrayList class and is used to create an empty ArrayList.

The general syntax of this method is:

ArrayList<data_type> list_name = new ArrayList<>();

For Example, you can create a generic ArrayList of type String using the following statement.

ArrayList<String> arraylist = new ArrayList<>();

This will create an empty ArrayList named arraylist of type String.

Method #2: ArrayList (int capacity)

This overloaded constructor can be used to create an ArrayList with the specified size or capacity provided as an argument to the constructor.

The general syntax for this method is:

ArrayList<data_type> list_name = new ArrayList<>(int capacity);

Example:

ArrayList<Integer> arraylist = new ArrayList<>(10);

The above statement creates an empty ArrayList named arraylist of type Integer with capacity 10.

Method #3: ArrayList (Collection<? Extends E> c)

The third overloaded constructor for the ArrayList class takes an already existing collection as an argument and creates an ArrayList with the elements from the specified collection c as its initial elements.

The general syntax for the ArrayList initialization using this constructor is:

ArrayList<data_type> list_name = new ArrayList<> (Collection c)

For Example, if intList is an existing collection with elements {10,20,30,40,50}, then the following statement will create a list arraylist with the contents of intList as its initial elements.

ArrayList<Integer> ArrayList = new ArrayList<>(intList);

The ArrayList class also supports various methods that can be used to manipulate the contents of the list. We will discuss these methods in detail in our upcoming tutorial ArrayList methods in Java.

Initialize ArrayList In Java

Once the ArrayList is created, there are multiple ways to initialize the ArrayList with values. In this section, we will discuss these ways.

#1) Using Arrays.asList

Here, you can pass an Array converted to List using the asList method of Arrays class to initialize the ArrayList.

General Syntax:

ArrayList<data_type> arrayListName = new ArrayList<data_type>( Arrays.asList (Object o1, Object o2, , Object on));

Example:

import java.util.*; public class Main { public static void main(String args[]) { //create and initialize ArrayList object myList with Arrays.asList method ArrayList<String> myList = new ArrayList<String>( Arrays.asList("One", "Two", "Three")); //print the ArrayList System.out.println("List contents:"+myList); } }

Output:

#2) Using Anonymous inner class Method

Here we use the anonymous inner class to initialize the ArrayList to values.

The general syntax for using an anonymous inner class for ArrayList initialization is as follows:

ArrayList<data_type>arraylistName = new ArrayList<data_type>(){{ add(Object o1); add (Object o2); add (Object on);}};

Example:

import java.util.*; public class Main { public static void main(String args[]) { //create and initialize ArrayList with anonymous inner class calls ArrayList<String> colors = new ArrayList<String>(){{ add("Red"); add("Blue"); add("Purple"); }}; //print the ArrayList System.out.println("Content of ArrayList:"+colors); } }

Output:

#3) Using add Method

This is the common method to add elements to any collection.

The general syntax for using add method to add elements to ArrayList is:

ArrayList<data_type>ArraylistName = new ArrayList<data_type>(); ArraylistName.add(value1); ArraylistName.add(value2); ArraylistName.add(value3);

Programming Example:

import java.util.*; public class Main { public static void main(String args[]) { //create ArrayList ArrayList<String> colors = new ArrayList<String>(); //add elements to the ArrayList using add method colors.add("Red"); colors.add("Green"); colors.add("Blue"); colors.add("Orange"); //print the ArrayList System.out.println("Content of ArrayList:"+colors); }

Output:

#4) Using Collection.nCopies Method

This method is used to initialize the ArrayList with the same values. We provide the count of elements to be initialized and the initial value to the method.

The general syntax of initialization is:

ArrayList<data_type> arrayListName = new ArrayList<data_type>(Collections.nCopies(count, element));

The below example demonstrates Array initialization using Collections.nCopies method.

import java.util.*; public class Main { public static void main(String args[]) { //create ArrayList with 10 elements //initialized to value 10 using Collections.nCopies ArrayList<Integer> intList = new ArrayList<Integer>(Collections.nCopies(10,10)); //print the ArrayList System.out.println("Content of ArrayList:"+intList); } }

Output:

Iterating Through ArrayList

We have the following ways to traverse through or loop through the ArrayList:

1. Using for loop
2. By for-each loop (enhanced for-loop).
3. Using the Iterator interface.
4. By ListIterator interface.
5. By forEachRemaining() method.

In fact, these methods are used to iterate through collections in general. We will see examples of each of the methods with respect to ArrayList in this tutorial.

#1) Using for loop

An index-based for loop can be used to traverse the ArrayList and print its elements.

Following is an example to traverse and print the ArrayList using for loop.

import java.util.*; public class Main { public static void main(String[] args) { //create a list List<Integer> intList = new ArrayList<>(); intList.add(10); intList.add(20); intList.add(30); intList.add(40); intList.add(50); //create & initialize a new ArrayList with previous list ArrayList<Integer> arraylist = new ArrayList<>(intList); System.out.println("Contents of ArrayList using for-loop:"); //use for loop to traverse through its elements and print it for(int i=0;i<intList.size();i++){ System.out.print(intList.get(i) + " "); } } }

Output:

This is the simplest and easiest way to traverse and print the elements of ArrayList and works the same way in case of other collections as well.

#2) By for-each loop (enhanced for loop)

You can also traverse the ArrayList using a for-each loop or the enhanced for loop. Prior to Java 8, it did not include lambda expressions. But from Java 8 onwards, you can also include Lambda expressions in the for-each loop.

Further reading =>> How to use Lambda Expressions in Python

The program below demonstrates the traversal and printing of ArrayList using for each loop and lambda expression.

import java.util.*; public class Main { public static void main(String[] args) { //create a list List<Integer> intList = new ArrayList<>(); intList.add(10); intList.add(20); intList.add(30); intList.add(40); intList.add(50); //create & initialize a new ArrayList with previous list ArrayList<Integer> arraylist = new ArrayList<>(intList); System.out.println("Contents of ArrayList using for-each loop:"); //use for-each loop to traverse through its elements and print it intList.forEach(val ->{ System.out.print(val + " "); }); } }

Output:

#3) Using Iterator Interface

We have seen the Iterator interface in detail in our previous topics. Iterator interface can be used to iterate through the ArrayList and print its values.

The following program shows this.

import java.util.*; public class Main { public static void main(String[] args) { //create a list List<Integer> intList = new ArrayList<>(); intList.add(5); intList.add(10); intList.add(15); intList.add(20); intList.add(25); //create & initialize a new ArrayList with previous list ArrayList<Integer> arraylist = new ArrayList<>(intList); System.out.println("Contents of ArrayList using Iterator interface:"); //Traverse through the ArrayList using iterator Iterator iter=arraylist.iterator(); while(iter.hasNext()){ System.out.print(iter.next() + " "); } } }

Output:

#4) By ListIterator Interface

You can also traverse the ArrayList using ListIterator. ListIterator can be used to traverse the ArrayList in forward as well as backward direction.

Lets implement a Java program that demonstrates an example of using ListIterator.

import java.util.*; class Main{ public static void main(String args[]){ //create a list and initiliaze it List<String> colors_list=new ArrayList<String>();//Creating arraylist colors_list.add("Red"); colors_list.add("Green"); colors_list.add("Blue"); colors_list.add("Cyan"); colors_list.add("Magenta"); colors_list.add("Yellow"); System.out.println("The contents of the list using ListIterator:"); //Traverse the list using ListIterator ListIterator<String> color_iter=colors_list.listIterator(colors_list.size()); while(color_iter.hasPrevious()) { String str=color_iter.previous(); System.out.print(str + " "); } } }

Output:

As you can see from the output, in the above program the ArrayList is traversed in backward direction using hasPrevious () and previous () methods of ListIterator.

#5) By forEachRemaining () Method

This is one of the methods to traverse the ArrayList and is available since Java 8.

The following program demonstrates the forEachRemaining () method to traverse ArrayList.

import java.util.*; class Main{ public static void main(String args[]){ //create a list and initiliaze it List<String> colors_list=new ArrayList<String>(); colors_list.add("Red"); colors_list.add("Green"); colors_list.add("Blue"); colors_list.add("Cyan"); colors_list.add("Magenta"); colors_list.add("Yellow"); System.out.println("The contents of the list using forEachRemaining() method:"); //Traverse the list using forEachRemaining () method Iterator<String> itr=colors_list.iterator(); itr.forEachRemaining(val-> //lambda expression { System.out.print(val + " "); }); } }

Output:

We use the forEachRemaining () method along with an Iterator. It is similar to each and we use lambda expression inside this method.

ArrayList Java Example

In this section, we will see the ArrayList implementation in Java. As an example, we will implement a complete example from creating, initializing and using Java ArrayList to perform various manipulations.

import java.util.ArrayList; class Main { public static void main(String[] args) { //Creating a generic ArrayList ArrayList<String> newList = new ArrayList<String>(); //Size of arrayList System.out.println("Original size of ArrayList at creation: " + newList.size()); //add elements to it newList.add("IND"); newList.add("USA"); newList.add("AUS"); newList.add("UK"); //print the size after adding elements System.out.println("ArrayList size after adding elements: " + newList.size()); //Print ArrayList contents System.out.println("Contents of the ArrayList: " + newList); //Remove an element from the list newList.remove("USA"); System.out.println("ArrayList contents after removing element(USA): " + newList); //Remove another element by index newList.remove(2); System.out.println("ArrayList contents after removing element at index 2: " + newList); //print new size System.out.println("Size of arrayList: " + newList.size()); //print list contents System.out.println("Final ArrayList Contents: " + newList); } }

Output:

Two-dimensional ArrayList In Java

We know that an ArrayList does not have dimensions like Arrays. But we can have nested ArrayLists which are also called 2D ArrayLists or ArrayList of ArrayLists.

The simple idea behind these nested ArrayLists is that given an ArrayList, each element of this ArrayList is another ArrayList.

Let us understand this using the following program.

import java.util.*; public class Main { public static void main(String[] args) { int num = 3; // declare an arrayList of ArrayLists or 2D ArrayList ArrayList<ArrayList<Integer>> intList = new ArrayList<ArrayList<Integer>>(num); // Create individual elements or ArrayLists and add them to intList as elements ArrayList<Integer> list_elem1 = new ArrayList<Integer>(); list_elem1.add(10); intList.add(list_elem1); ArrayList<Integer> list_elem2 = new ArrayList<Integer>(); list_elem2.add(20); list_elem2.add(30); intList.add(list_elem2); ArrayList<Integer> list_elem3 = new <Integer>(); list_elem3.add(40); list_elem3.add(50); list_elem3.add(60); intList.add(list_elem3); System.out.println("Contents of 2D ArrayList(Nested ArrayList):"); //print the 2D ArrayList or nested ArrayList for (int i = 0; i <intList.size(); i++) { for (int j = 0; j <intList.get(i).size(); j++) { System.out.print(intList.get(i).get(j) + " "); } System.out.println(); } } }

Output:

The above program shows 2D ArrayList. Here, first, we declare an ArrayList of ArrayLists. Then we define individual ArrayLists that will serve as individual elements of nested ArrayList when we add each of these ArrayLists to Nested ArrayList.

To access each element of the ArrayList, we need to call get method two times. First to access the row of the Nested ArrayList and then to access the individual intersection of row and column.

Note that you can increase the nested levels of ArrayList to define multi-dimensional ArrayLists. For example, 3D ArrayList will have 2D ArrayLists as its elements and so on.

Frequently Asked Questions

Q #1) What is the ArrayList in Java?

Answer: An ArrayList in Java is a dynamic array. It is resizable in nature i.e. it increases in size when new elements are added and shrinks when elements are deleted.

Q #2) What is the difference between Array and ArrayList?

Answer: An Array is in static structure and its size cannot be altered once declared. An ArrayList is a dynamic array and changes its size when elements are added or removed.

The array is a basic structure in Java whereas an ArrayList is a part of the Collection Framework in Java. Another difference is that while Array uses subscript ([]) to access elements, ArrayList uses methods to access its elements.

Q #3) Is ArrayList a list?

Answer: ArrayList is a subtype of the list. ArrayList is a class while List is an interface.

Q #4) Is ArrayList a collection?

Answer: No. ArrayList is an implementation of Collection which is an interface.

Q #5) How does ArrayList increase its size?

Answer: Internally ArrayList is implemented as an Array. ArrayList has a size parameter. When the elements are added to the ArrayList and size value is reached, ArrayList internally adds another array to accommodate new elements.

Conclusion

This was the tutorial on the basics of the ArrayList class in Java. We have seen the creation and initialization of the ArrayList class along with a detailed programming implementation of ArrayList.

We also discussed 2D and multidimensional ArrayLists. The ArrayList class supports the various methods that we can use to manipulate the elements. In our upcoming tutorials, we will take up these methods.

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