Arrays - Basics in Java

Arrays - Basics in Java

Let's learn about basics of arrays

Arrays :

What is an array?

  • Array is a datastructure which is a collection of datatypes, primitives basically similar elements in it.

What is the Syntax of array?

 datatype[] variable_name = new datatype[size];

 int[] rnos = new int[5]
 int[] rnos = {1,2,3,4,5}
  • Note:
  • int represents the datatype that is allowed to store

  • All the type of data in the array must be same

  • this int[] -> rnos reference variable is pointing towards an array object that contains type integer elements

  • int[] rnos -> means the rnos is getting defined in the stack means declaration of array.

    int[] nums; // declaration means the nums are defined in the stack

    nums = new int[5]; // initialisation means actually here object is being created in the heap

How this works internally :

  • declaration runs in compile time

  • initialization or object creation runs in runtime. This is known as Dynamic memory allocation. Memory gets allocated during runtime.

  • image

Continuity of Array :

  • Array is continous memory allocation in other languages.

  • Array objects are in heap.

  • Heap objects are not continous.

  • Dynamic memory allocation.

  • Hence array objects in java may not be continous, completely depends on JVM.

Array index:


  • index are numbers which says the position of array elements. It's like roll numbers to the array elements.

    int[] nums = {10,12,13,14,16,18}; sout(nums[0])-> prints 10 sout(nums[5]) -> prints 18

  • User: roll number 0 student10: yes sir!!! (lol I know this is funny)

    User : roll number 4 student16 : yes sir!!!

    (Hope you got the point)

  • Initially, all the integer values for an integer array by default all the values are 0.

  • for string array, by default the value is null.

  • internal working of arrays:


input using for loop

  for(int i = 0; i < arr.length; i++){
    arr[i] = in.nextInt();
// int i = 0 -> the input starting from 0 index of array
// i < arr.length -> condition for i is less than length of array
// i++ increament of i i.e move to next element in array
This is used if the input is very huge 1000's etc  

2D arrays in java

  int[][] nums = new int[size][];
  // double brackets for 2D array, size for rows is compulsory but size for columns is not necessary


  int[][] arr = {
  // above row 1,2,3 has different no.of columns thats why size is not compulsory for columns bracket


What are some important array methods?

   int[][] nums = new int[3][2]
   sout(nums.length); // ouputs no.of rows

   for(int row = 0; row < arr.length; row++){
    // for each colin every row
    for(int col; col < arr[row].length; col){
        nums[row][col] = in.nextInt();
  • Arrays are fixed size, what will you do when size is not known? ArrayList is the answer

  • ArrayList comes under OOPS collections concept.

What are some array methods?

This article on GFG covers array methods

Credits: Kunal Kushwaha Geeks for Geeks