Arrays in Java

In Java, Array is a group of like-typed variables referred to by a common name. Arrays in Java work differently than they do in C/C++. Following are some important points about Java arrays. 

Arrays in Java

• In Java, all arrays are dynamically allocated. (discussed below)
• Arrays may be stored in contiguous memory [consecutive memory locations].
• Since arrays are objects in Java, we can find their length using the object property length. This is different from C/C++, where we find length using sizeof.
• A Java array variable can also be declared like other variables with [] after the data type.
• The variables in the array are ordered, and each has an index beginning with 0.
• Java array can also be used as a static field, a local variable, or a method parameter.

An array can contain primitives (int, char, etc.) and object (or non-primitive) references of a class depending on the definition of the array. In the case of primitive data types, the actual values might be stored in contiguous memory locations(JVM does not guarantee this behavior). In the case of class objects, the actual objects are stored in a heap segment. 

Note: This storage of arrays helps us randomly access the elements of an array [Support Random Access].

Creating, Initializing, and Accessing an Arrays

One-Dimensional Arrays

The general form of a one-dimensional array declaration is 

-- type var-name[];
-- 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[]; 
int[] intArray; 
// similar to int we can declare 
// byte , short, boolean, long, float
// double, char
// an array of references to objects of
// the class MyClass (a class created by user)
MyClass myClassArray[]; 
// array of Object
Object[]  ao,        
// array of Collection                               
// of unknown type
Collection[] ca;   

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

Instantiating an Array in Java

When an array is declared, only a reference of an array is created. To create or give memory to the 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 determines the number of elements in the array, and var-name is the name of the array variable that is linked to the array. To use new to allocate an array, you must specify the type and number of elements to allocate.

Example: 

//declaring array
int intArray[];
// allocating memory to array
intArray = new int[20];  
// combining both statements in one
int[] intArray = new int[20]; 

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). Do refer to 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 to hold the array, using new, and assign it to the array variable. Thus, in Java, all arrays are dynamically allocated.

Array Literal in Java

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

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

• 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]);

Implementation:

// Java program to illustrate creating an array
// of integers,  puts some values in the array,
// and prints each value to standard output.

class GFG {
    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]);
    }
}

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

Complexity of the above method:

Time Complexity: O(n)
Auxiliary Space: O(1)

You can also access java arrays using for each loops. 

Arrays of Objects in Java

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

Student[] arr = new Student[5]; //student is a user-defined class

Syntax:

-- data type[] arrName;
-- datatype arrName[];
-- datatype [] arrName;

Example of Arrays of Objects

Example 1:

Below is the implementation of the above mentioned topic:

import java.io.*;

class GFG {
    public static void main (String[] args) {
       
      int [] arr=new int [4];
      // 4 is the size of arr
      
      System.out.println("Array Size:"+arr.length);
    }
}

Array Size:4

The student Array contains five memory spaces each of the size of student class in which the address of five 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. 

Example 2:

Below is the implementation of the above mentioned topic:

// 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 Students
        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);
    }
}

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

Complexity of the above method:

Time Complexity: O(n)
Auxiliary Space : O(1)

Example 3

An array of objects is also created like : 

// Java program to illustrate creating
//  an array of objects
  
class Student
{
   
    public String name;
    Student(String name)
    {
        this.name = name;
    }
      @Override
    public String toString(){
        return name;
    }
}
  
// Elements of the array are objects of a class Student.
public class GFG
{
    public static void main (String[] args)
    {
           // declares an Array and initializing  the elements of the array
        Student[] myStudents = new Student[]{new Student("Dharma"),new Student("sanvi"),new Student("Rupa"),new Student("Ajay")};
  
        // accessing the elements of the specified array
        for(Student m:myStudents){    
            System.out.println(m);
        }
    }
}

Dharma
sanvi
Rupa
Ajay

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

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

Below code shows what happens if we try to access elements outside the array size:

// Code for showing error "ArrayIndexOutOfBoundsException"

public class GFG {
    public static void main(String[] args)
    {
        int[] arr = new int[4];
        arr[0] = 10;
        arr[1] = 20;
        arr[2] = 30;
        arr[3] = 40;

        System.out.println(
            "Trying to access element outside the size of array");
        System.out.println(arr[5]);
    }
}

Output

Trying to access element outside the size of array
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: Index 5 out of bounds for length 4
at GFG.main(GFG.java:13)

Example (Iterating the array):

public 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]);
    }
}

10
20

Complexity of the above method:

Time Complexity: O(n),here n is size of array.
Auxiliary Space: O(1), since no extra space required.

Multidimensional Arrays in Java

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

Syntax of Java Multidimensional Array

There are 2 methods to declare Java Multidimensional Arrays as mentioned below:

-- datatype [][] arrayrefvariable;
-- datatype arrayrefvariable[][];

Example:

// Java Program to demonstrate
// Java Multidimensional Array
import java.io.*;

// Driver class
class GFG {
    public static void main(String[] args)
    {
        // Syntax
        int[][] arr = new int[3][3];
        // 3 row and 3 column

          // Number of Rows
        System.out.println("Number of Rows:"+
                           arr.length);
      
          // Number of Columns
        System.out.println("Number of Columns:"+
                           arr[0].length);
    }
}

Number of Rows:3
Number of Columns:3

Multidimensional Array Declaration

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

Example of Muilti Dimensional Array in Java

Example 1:

Below is the implementation of the above method:

// Java Program to Multidimensional Array

// Driver Class
public class multiDimensional {
      // main function
    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();
        }
    }
}

2 7 9 
3 6 1 
7 4 2 

Passing Arrays to Methods

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

// Java program to demonstrate
// passing of array to method

public 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);
    }
}

sum of array values : 15

Complexity of the above method:

Time Complexity: O(n)
Auxiliary Space : O(1)

Returning Arrays from Methods

As usual, a method can also return an array. For example, the 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 };
    }
}

1 2 3 

Complexity of the above method:

Time Complexity: O(n) 
Auxiliary Space : O(1)

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());
    }
}

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

Explanation of the above method:

1. The string “[I” is the run-time type signature for the class object “array with component type int.”
2. The only direct superclass of an array type is java.lang.Object.
3. The string “[B” is the run-time type signature for the class object “array with component type byte.”
4. The string “[S” is the run-time type signature for the class object “array with component type short.”
5. 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.

Java Array Members

Now, as you know that arrays are objects of a class, and a direct superclass of arrays is a class Object. The members of an array type are all of the following: 

• The public final field length contains the number of components of the array. Length may be positive or zero.
• All the members are inherited from class Object; the only method of Object that is not inherited is its clone method.
• The public method clone() overrides the clone method in class Object and throws no checked exceptions.

Arrays Types and Their Allowed Element Types

Cloning of Single-Dimensional Array in Java

When you clone a single-dimensional array, such as Object[], a “deep copy” is performed with the new array containing copies of the original array’s elements as opposed to references.

Below is the implementation of the above method:

// Java program to demonstrate
// cloning of one-dimensional arrays

class Test {
    public static void main(String args[])
    {
        int intArray[] = { 1, 2, 3 };

        int cloneArray[] = intArray.clone();

        // will print false as deep copy is created
        // for one-dimensional array
        System.out.println(intArray == cloneArray);

        for (int i = 0; i < cloneArray.length; i++) {
            System.out.print(cloneArray[i] + " ");
        }
    }
}

false
1 2 3 

Cloning Multidimensional Array in Java

A clone of a multi-dimensional array (like Object[][]) is a “shallow copy,” however, which is to say that it creates only a single new array with each element array a reference to an original element array, but subarrays are shared. 

// Java program to demonstrate
// cloning of multi-dimensional arrays

class Test {
    public static void main(String args[])
    {
        int intArray[][] = { { 1, 2, 3 }, { 4, 5 } };

        int cloneArray[][] = intArray.clone();

        // will print false
        System.out.println(intArray == cloneArray);

        // will print true as shallow copy is created
        // i.e. sub-arrays are shared
        System.out.println(intArray[0] == cloneArray[0]);
        System.out.println(intArray[1] == cloneArray[1]);
    }
}

false
true
true

Frequently Asked Questions in Java Arrays

1. Can we specify the size of array as long?

No we can’t specify the size of array as long but we can specify it as int or short.

2. Which is the direct superclass of an array in Java?

An Object is direct superclass of an array in Java.

3. Which Interfaces are implemented by Arrays in Java?

Every array type implements the interfaces Cloneable and java.io.Serializable. 

4. Can we alter the size of Array?

The size of the array cannot be altered(once initialized).  However, an array reference can be made to point to another array.

Related Articles to the topic

• Jagged Array in Java
• For-each loop in Java
• Arrays class in Java