Java Arrays

An array is a collection of similar data types. Array is a container object that hold values of homogeneous type. It is also known as static data structure because size of an array must be specified at the time of its declaration.
Array starts from zero index and goes to n-1 where n is length of the array.
In Java, array is treated as an object and stores into heap memory. It allows to store primitive values or reference values.

Features of Array

It is always indexed. Index begins from 0.
It is a collection of similar data types.
It occupies a contiguous memory location.
It allows to access elements randomly.

Single Dimensional Array

Single dimensional array use single index to store elements. You can get all the elements of array by just increment its index by one.

Array Declaration

Syntax :

datatype[] arrayName;;
or
datatype arrayName[];

Initialization Syntax

arrayName = new datatype[size]

new operator is used to initialize an array.

The arrayName is the name of array, new is a keyword used to allocate memory and size is length of array.

We can combine both declaration and initialization in a single statement.

Datatype[] arrayName = new datatype[size]

Example : Create An Array

Lets create a single dimensional array.

class Demo
{
public static void main(String[] args)
  {
    int[] arr = new int[5];
        for(int x : arr)
        {
                System.out.println(x);
        }
   }
}
0
0
0
0
0

In the above example, we created an array arr of int type and can store 5 elements. We iterate the array to access its elements and it prints five times zero to the console. It prints zero because we did not set values to array, so all the elements of the array initialized to 0 by default.

Set Array Elements

We can set array elements either at the time of initialization or by assigning direct to its index.

int[] arr = {10,20,30,40,50};

Here, we are assigning values at the time of array creation. It is useful when we want to store static data into the array.

arr[1] = 105

Here, we are assigning a value to array’s 1 index. It is useful when we want to store dynamic data into the array.

Array Example

Here, we are assigning values to array by using both the way discussed above.

class Demo
{
public static void main(String[] args)
  {
	int[] arr = {10,20,30,40,50}; 
        for(int x : arr)
        {
                System.out.println(x);
        }
        
     // assigning a value
        
        arr[1] = 105;
        System.out.println("element at first index: " +arr[1]);
   }
}
10
20
30
40
50
element at first index: 105

Accessing array element

we can access array elements by its index value. Either by using loop or direct index value. We can use loop like: for, for-each or while to traverse the array elements.

Example to access elements

class Demo
{
public static void main(String[] args)
  {
	int[] arr = {10,20,30,40,50}; 
        for(int i=0;i<arr.length;i++)
        {
                System.out.println(arr[i]);
        }
       
        System.out.println("element at first index: " +arr[1]);
   }
}
10
20
30
40
50
element at first index: 20

Here, we are traversing array elements using loop and accessed an element randomly.

Note:-To find the length of an array, we can use length property of array object like: array_name.length.

Multi-Dimensional Array

A multi-dimensional array is very much similar to a single dimensional array. It can have multiple rows and multiple columns unlike single dimensional array, which can have only one row index.

It represent data into tabular form in which data is stored into row and columns.

Multi-Dimensional Array Declaration

datatype[ ][ ] arrayName;

Initialization of Array

datatype[ ][ ] arrayName = new int[no_of_rows][no_of_columns];

The arrayName is the name of array, new is a keyword used to allocate memory and no_of_rows and no_of_columns both are used to set size of rows and columns elements.

Like single dimensional array, we can statically initialize multi-dimensional array as well.

int[ ][ ] arr = {{1,2,3,4,5},{6,7,8,9,10},{11,12,13,14,15}};

Example:

class Demo
{
public static void main(String[] args)
  {
	int arr[ ][ ] = {{1,2,3,4,5},{6,7,8,9,10},{11,12,13,14,15}};
        for(int i=0;i<3;i++)
        {
        	for (int j = 0; j < 5; j++) {
        		
        		System.out.print(arr[i][j]+" ");
			}
        	System.out.println();
                
        }
        
     // assigning a value
        System.out.println("element at first row and second column: " +arr[0][1]);
   }
}
Output
1 2 3 4 5
6 7 8 9 10
11 12 13 14 15
element at first row and second column: 2

element at first row and second column: 2

Jagged Array

Jagged array is an array that has different numbers of columns elements. In java, a jagged array means to have a multi-dimensional array with uneven size of columns in it.

Initialization of Jagged Array

Jagged array initialization is different little different. We have to set columns size for each row independently.

int[ ][ ] arr = new int[3][ ]; 
arr[0] = new int[3];
arr[1] = new int[4];
arr[2] = new int[5];

Example : Jagged Array

class Demo
{
public static void main(String[] args)
  {
	int arr[ ][ ] = {{1,2,3},{4,5},{6,7,8,9}};
        for(int i=0;i<3;i++)
        {
        	for (int j = 0; j < arr[i].length; j++) {
        		
        		System.out.print(arr[i][j]+" ");
			}
        	System.out.println();
                
        }
        
   }
}
 
 
1 2 3
4 5
6 7 8 9

Here, we can see number of rows are 3 and columns are different for each row. This type of array is called jagged array.

Interview Questions

1) What is ArrayStoreException in java? When you will get this exception?

ArrayStoreException is a run time exception which occurs when you try to store non-compatible element in an array object. The type of the elements must be compatible with the type of array object. For example, you can store only string elements in an array of strings. if you try to insert integer element in an array of strings, you will get ArrayStoreException at run time.

 
public class MainClass
{
    public static void main(String[] args)
    {
        Object[] stringArray = new String[5];   //No compile time error : String[] is auto-upcasted to Object[]
         
        stringArray[1] = "JAVA"; 	
         
        stringArray[2] = 100; 	//No compile time error, but this statement will throw java.lang.ArrayStoreException at run time
         
        //because we are inserting integer element into an array of strings
    }
}

2) Can you pass the negative number as an array size?

No. You can’t pass the negative integer as an array size. If you pass, there will be no compile time error but you will get NegativeArraySizeException at run time.

public class MainClass
{
    public static void main(String[] args)
    {
        int[] array = new int[-5]; 	//No compile time error
         
        //but you will get java.lang.NegativeArraySizeException at run time
    }
}

3) Can you change the size of the array once you define it? OR Can you insert or delete the elements after creating an array?

No. You can’t change the size of the array once you define it. You can not insert or delete the elements after creating an array. Only you can do is change the value of the elements.

4) What is an anonymous array? Give example?

Anonymous array is an array without reference. For example,

public class MainClass
{
    public static void main(String[] args)
    {
        //Creating anonymous arrays
  
        System.out.println(new int[]{1, 2, 3, 4, 5}.length);    //Output : 5
  
        System.out.println(new int[]{21, 14, 65, 24, 21}[1]);   //Output : 14
    }
}

5) What is the difference between int[] a and int a[] ?

Both are the legal methods to declare the arrays in java.

6) There are two array objects of int type. one is containing 100 elements and another one is containing 10 elements. Can you assign array of 100 elements to an array of 10 elements?

Yes, you can assign array of 100 elements to an array of 10 elements provided they should be of same type. While assigning, compiler checks only type of the array not the size.

public class MainClass
{
    public static void main(String[] args)
    {
        int[] a = new int[10];
         
        int[] b = new int[100];
         
        a = b;  	//Compiler checks only type, not the size
    }
}

8) What are the differences between Array and ArrayList in java?

Array	ArrayList
Arrays are of fixed length.	ArrayList is of variable length.
You can’t change the size of the array once you create it.	Size of the ArrayList grows and shrinks as you add or remove the elements.
Array does not support generics.	ArrayList supports generics.
You can use arrays to store both primitive types as well as reference types.	You can store only reference types in an ArrayList.

9) What are the different ways of copying an array into another array?

There are four methods available in java to copy an array.

1) Using for loop

2) Using Arrays.copyOf() method

3) Using System.arraycopy() method

4) Using clone() method

Programmes

Calculate the sum and average of array elements
Create java programme to add two matrix of size 2×3
Programme to check given element present on the array or not
Reverse array
- With reverse index
- By finding mid value
Programme to find maximum element value on array
Programme to find minimum element value on array
Write a programme to find array is sorted or not

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