Java Tutorial: Introduction to Arrays

• An array is a collection of similar types of data having contiguous memory allocation.
• The indexing of the array starts from 0., i.e 1st element will be stored at the 0th index, 2nd element at 1st index, 3rd at 2nd index, and so on.
• The size of the array can not be increased at run time therefore we can store only a fixed size of elements in array.
• Use Case: Storing marks of 5 students

Accessing Array Elements :

• Array elements can be accessed as follows,

/* marks[0] = 100       //Note that index starts from 0
marks[1] = 70
.                                  
.
marks[4] = 98 */

• So in a nut shell, this is how array works:

int[] marks; //Declaration!
marks = new int[5];   //Memory allocation!

int[] marks = new int[5]; //Declaration + Memory allocation!

int[] marks = {100,70,80,71,98} // Declare + Initialize!

Note : Array indices start from 0 and go till (n-1) where n is the size of the array.

Array length :

• Unlike C/C++, we don't need to use the sizeof() operator to get the length of arrays in Java because arrays are objects in Java therefore we can use the length property.

  marks.length //Gives 5 if marks is a reference to an array with 5 elements
  

Displaying an Array :

• An array can be displayed using a for loop:

for (int i=0; i<marks.length; i++)
{
            Sout(marks[i]);    //Array Traversal
}

Quick Quiz: Write a Java program to print the elements of an array in reverse order.

public class cwh_26_arrays {
    public static void main(String[] args) {
    int [] marks = {98, 45, 79, 99, 80};
    for (int i=marks.length-1; i>=0; i--) {
        System.out.println(marks[i]);
        }
    }
}

Code as Described in the Video

package com.company;

public class cwh_26_arrays {
    public static void main(String[] args) {
        /* Classroom of 500 students - You have to store marks of these 500 students
        You have 2 options:
        1. Create 500 variables
        2. Use Arrays (recommended)
        */
        // There are three main ways to create an array in Java
        // 1. Declaration and memory allocation
        // int [] marks = new int[5];

        // 2. Declaration and then memory allocation
        // int [] marks;
        // marks = new int[5];
        // Initialization
        // marks[0] = 100;
        // marks[1] = 60;
        // marks[2] = 70;
        // marks[3] = 90;
        // marks[4] = 86;

        // 3. Declaration, memory allocation and initialization together
        int [] marks = {98, 45, 79, 99, 80};

        // marks[5] = 96; - throws an error
        System.out.println(marks[4]);
    }
}

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