Introduction to Arrays

It was simple when we used to store just a couple of numbers, we just make some variables for that and store respectively. But what if we want to store 50 numbers? Ofcource, we can make 50 variables and store but that would be very childish approach and inappropriate. And now assume we have to 5000 numbers, we can't make just 5000 variables.

To handle this situation, in almost all programming languages we have a data structure called Arrays.

What is an Array ?

An array is a data structure used to store a collection of data of the same data type. Normally, an array is a collection of similar type of elements which has contiguous memory location.

Java array is an object which contains elements of a similar data type. Additionally, The elements of an array are stored in a contiguous memory location. It is a data structure where we store similar elements. We can store only a fixed set of elements in a Java array.

Array in Java is index-based, the first element of the array is stored at the 0th index, 2nd element is stored on 1st index and so on.

Array Indicing.png

Unlike C/C++, we can get the length of the array using the length member. In C/C++, we need to use the sizeof operator.

In Java, array is an object of a dynamically generated class. Java array inherits the Object class, and implements the Serializable as well as Cloneable interfaces. We can store primitive values or objects in an array in Java. Like C/C++, we can also create single dimentional or multidimentional arrays in Java.

Moreover, Java provides the feature of anonymous arrays which is not available in C/C++.

datatype[] variablename = new datatype[size];

eg, we want to store roll numbers:

int[] rollnos = new int[5];

         OR

int[] rollnos = {51, 82, 13, 15, 16};

int represents the type of data, all the elements in the array should be of same data type, here it is int so the elements in the should be of int data type.

int[] rollnow; //declaration of arrays

//rollnos get defined in stack

There are two types of Arrays:

Single Dimensional Arrays
Multidimentional Arrays

Let's understand these in detail:

1.One-Dimensional Arrays

The general form of a one-dimensional array declaration is

type var-name[];
OR
type[] var-name;

An array declaration has two components: the type and the name. type declares the element type of the array. The element type determines the data type of each element that comprises the array. Like an array of integers, we can also create an array of other primitive data types like char, float, double, etc. or user-defined data types (objects of a class). Thus, the element type for the array determines what type of data the array will hold Example:

// both are valid declarations
int intArray[]; 
or int[] intArray; 

byte[] byteArray;
short[] shortsArray;
boolean[] booleanArray;
long[] longArray;
float[] floatArray;
double[] doubleArray;
char[] charArray;

// an array of references to objects of
// the class MyClass (a class created by
// user)
MyClass myClassArray[]; 

Object[]  ao,        // array of Object
Collection[] ca;  // array of Collection
                     // of unknown type

Although the first declaration above establishes the fact that intArray is an array variable, no actual array exists. It merely tells the compiler that this variable (intArray) will hold an array of the integer type. To link intArray with an actual, physical array of integers, you must allocate one using new and assign it to intArray.

Instantiating an Array in Java

When an array is declared, only a reference of array is created. To actually create or give memory to array, you create an array like this:The general form of new as it applies to one-dimensional arrays appears as follows:

var-name = new type [size];

Here, type specifies the type of data being allocated, size specifies the number of elements in the array, and var-name is the name of array variable that is linked to the array. That is, to use new to allocate an array, you must specify the type and number of elements to allocate.

Example:

int intArray[];    //declaring array
intArray = new int[20];  // allocating memory to array
OR

int[] intArray = new int[20]; // combining both statements in one

One Dimensional Array.jpeg

Note :

The elements in the array allocated by new will automatically be initialized to zero (for numeric types), false (for boolean), or null (for reference types).Refer Default array values in Java
Obtaining an array is a two-step process. First, you must declare a variable of the desired array type. Second, you must allocate the memory that will hold the array, using new, and assign it to the array variable. Thus, in Java all arrays are dynamically allocated.

rollnos = new int[5]; //initialisation

//Actual memory allocation happens here. Here, Object is being created in Heap memory.

Declaration of 
  array               Initialisation
(compile time)          (run-time)
      |                     |
 ____\ /_____           ___\ /____
|           |          |          |
 int[]   arr     =      new int[5];
   ^      ^               ^
  / \    / \             / \
   |      |               |
   |      |               |
   |   ref var            |
   |                      |
   |           creating Object in Heap memory
DataType

This above concept is known as Dynamic memory allocation which means at runtime OR execution time is allocated.

Array Literal

In a situation, where the size of the array and variables of array are already known, array literals can be used.

 int[] intArray = new int[]{ 1,2,3,4,5,6,7,8,9,10 }; 
 // Declaring array literal

The length of this array determines the length of the created array.
There is no need to write the new int[] part in the latest versions of Java

Accessing Java Array Elements using for Loop

Each element in the array is accessed via its index. The index begins with 0 and ends at (total array size)-1. All the elements of array can be accessed using Java for Loop.

// accessing the elements of the specified array
for (int i = 0; i < arr.length; i++)
  System.out.println("Element at index " + i + 
                                " : "+ arr[i]);

// Java program to illustrate creating an array
// of integers,  puts some values in the array,
// and prints each value to standard output.
  
class Main 
{
    public static void main (String[] args) 
    {         
      // declares an Array of integers.
      int[] arr;
          
      // allocating memory for 5 integers.
      arr = new int[5];
          
      // initialize the first elements of the array
      arr[0] = 10;
          
      // initialize the second elements of the array
      arr[1] = 20;
          
      //so on...
      arr[2] = 30;
      arr[3] = 40;
      arr[4] = 50;
          
      // accessing the elements of the specified array
      for (int i = 0; i < arr.length; i++)
         System.out.println("Element at index " + i + 
                                      " : "+ arr[i]);          
    }
}

Output:

Element at index 0 : 10
Element at index 1 : 20
Element at index 2 : 30
Element at index 3 : 40
Element at index 4 : 50

Arrays of Objects

An array of objects is created just like an array of primitive type data items in the following way.

 Student[] arr = new Student[7]; //student is a user-defined class

The Student array arr contains seven memory spaces each of the size of student class in which the address of seven Student objects can be stored. The Student objects have to be instantiated using the constructor of the Student class and their references should be assigned to the array elements in the following way.

Student[] arr = new Student[5];

// Java program to illustrate creating an array of
// objects
  
class Student
{
    public int roll_no;
    public String name;
    Student(int roll_no, String name)
    {
        this.roll_no = roll_no;
        this.name = name;
    }
}
  
// Elements of the array are objects of a class Student.
public class GFG
{
    public static void main (String[] args)
    {
        // declares an Array of integers.
        Student[] arr;
  
        // allocating memory for 5 objects of type Student.
        arr = new Student[5];
  
        // initialize the first elements of the array
        arr[0] = new Student(1,"aman");
  
        // initialize the second elements of the array
        arr[1] = new Student(2,"vaibhav");
  
        // so on...
        arr[2] = new Student(3,"shikar");
        arr[3] = new Student(4,"dharmesh");
        arr[4] = new Student(5,"mohit");
  
        // accessing the elements of the specified array
        for (int i = 0; i < arr.length; i++)
            System.out.println("Element at " + i + " : " +
                        arr[i].roll_no +" "+ arr[i].name);
    }
}

Output:

Element at 0 : 1 aman
Element at 1 : 2 vaibhav
Element at 2 : 3 shikar
Element at 3 : 4 dharmesh
Element at 4 : 5 mohit

What happens if we try to access element outside the array size?

JVM throws ArrayIndexOutOfBoundsException to indicate that array has been accessed with an illegal index. The index is either negative or greater than or equal to the size of an array.

class GFG
{
    public static void main (String[] args)
    {
        int[] arr = new int[2];
        arr[0] = 10;
        arr[1] = 20;
  
        for (int i = 0; i <= arr.length; i++)
            System.out.println(arr[i]);
    }
}

Runtime error

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 2
    at GFG.main(File.java:12)

Output:

10
20

2. Multidimensional Arrays

Multidimensional arrays are arrays of arrays with each element of the array holding the reference of other array. These are also known as Jagged Arrays. A multidimensional array is created by appending one set of square brackets ([]) per dimension. Examples:

int[][] intArray = new int[10][20]; //a 2D array or matrix
int[][][] intArray = new int[10][20][10]; //a 3D array

Two Dimensional Array.jpeg

class multiDimensional
{
    public static void main(String args[])
    {
        // declaring and initializing 2D array
        int arr[][] = { {2,7,9},{3,6,1},{7,4,2} };
  
        // printing 2D array
        for (int i=0; i< 3 ; i++)
        {
            for (int j=0; j < 3 ; j++)
                System.out.print(arr[i][j] + " ");
  
            System.out.println();
        }
    }
}

Output:

2 7 9 
3 6 1 
7 4 2

Passing Arrays to Methods

Like variables, we can also pass arrays to methods.For example, below program pass array to method sum for calculating sum of array’s values.

// Java program to demonstrate 
// passing of array to method
  
class Test
{    
    // Driver method
    public static void main(String args[]) 
    {
        int arr[] = {3, 1, 2, 5, 4};
          
        // passing array to method m1
        sum(arr);
      
    }
  
    public static void sum(int[] arr) 
    {
        // getting sum of array values
        int sum = 0;
          
        for (int i = 0; i < arr.length; i++)
            sum+=arr[i];
          
        System.out.println("sum of array values : " + sum);
    }
}

Output :

sum of array values : 15

Returning Arrays from Methods

As usual, a method can also return an array. For example, below program returns an array from method m1.

// Java program to demonstrate 
// return of array from method
  
class Test
{    
    // Driver method
    public static void main(String args[]) 
    {
        int arr[] = m1();
          
        for (int i = 0; i < arr.length; i++)
            System.out.print(arr[i]+" ");
      
    }
  
    public static int[] m1() 
    {
        // returning  array
        return new int[]{1,2,3};
    }
}

Output:

1 2 3

Class Objects for Arrays

Every array has an associated Class object, shared with all other arrays with the same component type.

// Java program to demonstrate 
// Class Objects for Arrays
  
class Test
{ 
    public static void main(String args[]) 
    {
        int intArray[] = new int[3];
        byte byteArray[] = new byte[3];
        short shortsArray[] = new short[3];
          
        // array of Strings
        String[] strArray = new String[3];
          
        System.out.println(intArray.getClass());
        System.out.println(intArray.getClass().getSuperclass());
        System.out.println(byteArray.getClass());
        System.out.println(shortsArray.getClass());
        System.out.println(strArray.getClass());
    }
}

Output:

class [I
class java.lang.Object
class [B
class [S
class [Ljava.lang.String;

Explanation :

The string “[I” is the run-time type signature for the class object “array with component type int“.
The only direct superclass of any array type is java.lang.Object.
The string “[B” is the run-time type signature for the class object “array with component type byte“.
The string “[S” is the run-time type signature for the class object “array with component type short“.
The string “[L” is the run-time type signature for the class object “array with component type of a Class”. The Class name is then followed.

← Function & Scope ArrayList In Java →

Data Structures & Algorithms

Arrays – Introduction to Arrays

Introduction to Arrays

What is an Array ?

1.One-Dimensional Arrays

Instantiating an Array in Java

Example:

Note :

Array Literal

Accessing Java Array Elements using for Loop

Output:

Arrays of Objects

Output:

What happens if we try to access element outside the array size?

Runtime error

Output:

2. Multidimensional Arrays

Output:

Passing Arrays to Methods

Output :

Returning Arrays from Methods

Output:

Class Objects for Arrays

Output:

Explanation :