Java Arrays Tutorial: Declare, Create, Initialize

What is an Array?

An array is a very common type of data structure wherein all elements must be of the same data type. Once defined, the size of an array is fixed and cannot increase to accommodate more elements.The first element of an array starts with index zero.

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 Array3) 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

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

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] + " ");
}

}

Output:

1