Java Arrays Tutorial: Declare, Create, Initialize [Example]

In simple words, it’s a programming construct which helps to replace this

x0=0;
x1=1;
x2=2;
x3=3;
x4=4;
x5=5;

with this …

x[0]=0;
x[1]=1;
x[2]=2;
x[3]=3;
x[4]=4;
x[5]=5;

In this tutorial, you will learn-

• What is an array?
• Array Variables
• First Array Program
• Java Array: Pass by reference
• Multidimensional arrays

how this helps is that a variable can reference the index (the number in the bracket[]) for easy looping.

for(count=0; count<5; count++) {
     System.out.println(x[count]);
   }

Array Variables

Using an array in your program is a 3 step process –

1) Declaring your Array

2) Constructing your Array

3) Initialize your Array

1) Declaring your Array

Syntax

<elementType>[] <arrayName>;

or

 <elementType> <arrayName>[];

Example:

int intArray[];
 // Defines that intArray is an ARRAY variable which will store integer values
int []intArray;

2) Constructing an Array

 arrayname = new dataType[]

Example:

intArray = new int[10]; // Defines that intArray will store 10 integer values

Declaration and Construction combined

int intArray[] = new int[10];

3) Initialize an Array

intArray[0]=1; // Assigns an integer value 1 to the first element 0 of the array

intArray[1]=2; // Assigns an integer value 2 to the second element 1 of the array

Declaring and initialize an Array

[]  = {};

Example:

 int intArray[] = {1, 2, 3, 4};
// Initilializes an integer array of length 4 where the first element is 1 , second element is 2 and so on.

First Array Program

Step 1) Copy the following code into an editor.

class ArrayDemo{
     public static void main(String args[]){
        int array[] = new int[7];
        for (int count=0;count<7;count++){
           array[count]=count+1;
       }
       for (int count=0;count<7;count++){
           System.out.println("array["+count+"] = "+array[count]);
       }
      //System.out.println("Length of Array  =  "+array.length);
      // array[8] =10;
      }
}

Step 2) Save , Compile & Run the code. Observe the Output

Expected Output:

array[0] = 1
array[1] = 2
array[2] = 3
array[3] = 4
array[4] = 5
array[5] = 6
array[6] = 7

Step 3) If x is a reference to an array, x.length will give you the length of the array.

Uncomment line #10. Save, Compile & Run the code.Observe the Output

Length of Array  =  7

Step 4) Unlike C, Java checks the boundary of an array while accessing an element in it. Java will not allow the programmer to exceed its boundary.

Uncomment line #11. Save, Compile & Run the code.Observe the Output

Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 8
        at ArrayDemo.main(ArrayDemo.java:11)
Command exited with non-zero status 1

Step 5) ArrayIndexOutOfBoundsException is thrown. In case of C, the same code would have shown some garbage value.

Java Array: Pass by reference

Arrays are passed to functions by reference, or as a pointer to
the original. This means anything you do to the Array inside the
function affects the original.

Example: To understand Array are passed by reference

Step 1) Copy the following code into an editor

class ArrayDemo {
   public static void passByReference(String a[]){
     a[0] = "Changed";
   }
 
   public static void main(String args[]){
      String []b={"Apple","Mango","Orange"};
      System.out.println("Before Function Call    "+b[0]);
      ArrayDemo.passByReference(b);
      System.out.println("After Function Call    "+b[0]);
   }
}

Step 2) Save, Compile & Run the code. Observe the Output

Expected Output:

Before Function Call    Apple
After Function Call    Changed

Multidimensional arrays

Multidimensional arrays are actually arrays of arrays.

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

Ex: int twoD[ ][ ] = new int[4][5] ;

When you allocate memory for a multidimensional array, you need only specify the memory for the first (leftmost) dimension.

You can allocate the remaining dimensions separately.

In Java, array length of each array in a multidimensional array is under your control.

Example

public class Guru99 {
public static void main(String[] args) {

// Create 2-dimensional array.
  int[][] twoD = new int[4][4];

  // Assign three elements in it.
  twoD[0][0] = 1;
  twoD[1][1] = 2;
  twoD[3][2] = 3;
  System.out.print(twoD[0][0] + " ");
}

}

Expected Output:

1

This code is editable. Click Run to Compile + Execute