Arrays in Java





  • An Array is a collection of similar data items stored in contiguously on memory.

  • Arrays are objects in Java that store multiple variables of the same type.

  • Arrays can hold either primitives or object references, but the array itself will always be an object on the heap, even if the array is declared to hold primitive elements.

  • Array Declaration:

  • Arrays are declared by stating the type of element the array will hold, which can be an object or a primitive, followed by square brackets to the left or right of the identifier.

  • Declaring an Array of Primitives :

    int  ar[ ]; //(work but not recommended)
                 
    int[ ]  ar; // (recommended way)
  • Array Construction:

  • Constructing an array means creating the array object on the heap.

  • To create an array object, Java needs to know how much space to allocate on the heap, so you must specify the size of the array at construction time.

  • Constructing One-Dimensional Arrays

  • int[ ] ar; // Declaration 
    ar = new int[4]; //construction
                  size is mandatory
  • When you construct an array,each index get initialized with default value of that type.

  • The given code creates a new object on the heap - an array object holding four elements - with each element containing an int with the default value 0.


  • length property :

    Every array has a length property that holds the size of array.


Types of Array :

There are two types of array.
     1. Single Dimensional Array
     2. Multidimensional Array
     2. Jagged Array

1. Single Dimensional Array :

Example :
simple example of java array, where we are going to declare, instantiate, initialize and traverse an array.

class ArrayDemo
{  
 public static void main(String []ar)
 {
//declaration and instantiation
   int a[]=new int[5];

   a[0]=10;//initialization
   a[1]=20;  
   a[2]=30;  
   a[3]=40;  
   a[4]=50;  
  
  //printing array
//length is the property of array
   for(int i=0;i< a.length;i++)
     System.out.println(a[i]);  
 }
}  
OUTPUT :
10
20
30
40
50          
Different ways to Declaration, Instantiation and Initialization of Java Array :
1. int ar [];
   ar = new int[5];
    OR
2. int ar [] = new int[5];
    OR
3. int ar [] = {10,20,30};
    OR
4. int ar [] = new int[]{10,20,30};

Example :

class ArrayDemo
{  
 public static void main(String []ar)
  {
    int a[] = {10,20,30};
  
//printing array  
   for(int i = 0 ; i< a.length ; i++)
    System.out.println(a[i]);  
  }
}          
OUTPUT :
10
20
30          


Passing Array to Method :

Example :

Find sum of array member an array using method, take the input from user
import java.util.Scanner;
 class ArrayDemo
{
 static void getSum(int ar[])
  {
   int sum = 0;
//using for-each loop
   for(int i : ar)
    {
     sum = sum + i;
    }
   System.out.println("SUM = "+sum);
  }
 public static void main(String []ar)
 {
  int i, size;
  Scanner sc= new Scanner(System.in);
  System.out.print("Enter size of Array :");
  size = sc.nextInt();
  int a [] = new int[size];
  System.out.print("Enter "+size+" Elements :");
  for (i = 0; i< a.length;i++)
   {
     a [i] = sc.nextInt();
   }
  getSum(a);
 }
}           
OUTPUT :
Enter size of Array :3
Enter 3 Elements :1
2
3
SUM = 6     

2. Multi Dimensional Array :

  • Multidimensional arrays, are simply arrays of arrays.

  • To declare a multidimensional array variable, specify each additional index using another set of square brackets.

  • Declaration :

    type[][] arrayRefVar;
       OR  
    type arrayRefVar[][];
       OR  
    type []arrayRefVar[];
  • Construction :

    arrayRefVar = new type[size][size];
    Example :
    int myArray [][];
    myArray = new int[3][3];
  • Access / Initialization :

     myArray[0][0] = 3;
     myArray[0][1] = 2;  
     myArray[1][2] = 5;  …etc.
    

    Example :

    class ArrayDemo
    {  
     public static void main(String []ar)
    {
      
    //declaring and initializing 2D array
    int arr[][]={
                 {1,2,3},
                 {4,5,6},
                 {5,7,8}
                };  
      
    //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 :
    1 2 3
    4 5 6
    5 7 8               

3. Jagged Array :

  • It is a new feature supported by Java.

  • In Jagged arrays, each row, in a two-dimensional array, may contain different lengths.

Declaration :                    

int [][] myArray;

Construction :

myArray = new int[3][];

Note :
Only the first bracket is given size. That is acceptable in Java, since the JVM needs to know only the size of the object assigned to the variable.

Initialization :

myArray[0] = new int[]{1,2,3};
myArray[1] = new int[]{4,6};

Next topic is strings




 










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