Arrays in Java : Introduction

• Arrays in java are considered as objects. Arrays are the collection of homogeneous (same type) elements referred to by a single name.
• Arrays elements can be accessed randomly using index. In Java array index starts from 0.
• Arrays in java are of two types:
  • One- Dimensional Arrays
  • Multi- Dimensional Arrays

1. Arrays in Java : One Dimensional Array

   • 1-D array is the collection of homogeneous elements. We can represent 1-D array as a single row of elements.
   
   Arrays In Java : One-Dimension Array
   • The above diagram represents an integer array arr of size 10 and index going from 0 to 9.
   • If size of array is ‘n’ the index goes from 0 to n-1.
   
   Declaring Array

   type [ ] array_name ;  or type array_name[ ] ;

   
   
   • Type is the data type of the array elements. It will tell what type of data array will hold.
   • [ ] tells the compiler that the variable is an array variable. For example : int [ ]  arr ;
   • Declaring an array does not mean that array exist in memory. Declaring an array will set arr to to null.
   • We have to allocate memory to array using new keyword.
   
   Syntax

   array_name = new type[size] ;

   
   
   • "size" tells the number of elements array can hold. Initial allocation of memory to an array initializes all array elements to 0.
   • We can assign an array element with a new value by using array_name and index.
   For example
   arr = new int[5];   // will allocate memory to an array of size 5
   arr[0] = 10;   // will assign the first element of array to 10
   arr[3] = 40;   //  will assign the fourth element of array to 40
   
   
   

   Note : 1) In Java array are dynamically allocated, i.e. memory to an array is allocated at run time. 2) We can traverse an array using a loop ( mainly for-each loop). 3) We can also declare and allocate memory to an array with a single statement by combining both declaration and allocation statement. For example :  int[ ] arr = new int [5] ;

   
   Example
   • Let us take an example to demonstrate arrays in java :
   
   
   Arrays In Java : Example
   
   Arrays In Java : Output
   Initializing an Array While Declaring it
   • Array can be initialized while they are declared. We don’t need new keyword to allocate memory. It is allocated automatically.
   
   Syntax

   type[ ] array_name = { List of elements separated by comma};

   
   Example

   char[ ] alpha = {‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’};

   • The above statement will declare and initialize a character array alpha at the same time. There is no need of new keyword then. Comma (,) separates the different array elements.
   • Let us take an example to find mean of heights of 10 persons.
   
   
   1-D Array : Example
   
   1-D Array : Output
   

2. Arrays In Java : Multi- Dimensional Arrays

   • Array can be multidimensional.
   • Conceptually multidimensional arrays are represented as having multiple rows and columns.
   • Multi- dimensional array can be 2-dimensional, 3-dimensional up-to n-dimensional.
   • Multi-dimensional arrays are simply array within an array. If we talk about 2-D array then they are array within array i.e. array(array) .
   • If we talk about 3-D array they are array within array within array i.e. array(array(array)) .
   • The simplest multidimensional array is 2-D array. Conceptually they can be represented as having more than one row ( matrix like structure).
   
   
   Multidimensional Array : Structure
   
   • Above figure is the conceptual representation of 2-D array having 3 rows and 3 columns.
   • If we write arr[1][2], then it represents element of 2^nd row and 3^rd Here 1 is the row index and 2 is the column index.
   
   Declaring Multi-Dimensional Array
   • To declare 2-D array we specify two brackets [ ] [ ]. n dimension means n brackets. For example:
     • int[ ] [ ] arr ;// declaring 2-D array
     • int [ ] [ ] [ ] arr1 ;   // declaring 3-D array
   
   Allocation Memory to Multi- Dimensional Array
   • It is done using new keyword, as we did in 1-D array. But here also we specify n [ ] brackets for n dimension. For example:
     • arr = new int[4][5] ;  // 2-D array of 4 rows and 5 columns
   
   Allocating While Declaring
   •  An example could be , char[ ] [ ] mat = new char[3][2] ; // Declares and allocates memory
   
   

   Note : We can use loop to traverse multidimensional array.

   
   Example
   • Let us take an example to demonstrate 2-D arrays.
   
   Multi-Dimensional Array : Example
   
   Multi-Dimensional Array : Output
   
   Initializing 2-D Array While Declaring
   • We can also declare and initialize 2-D array at the same time as we did in 1-D array.
   • In case of 2-D array each set of rows will have their own set of curly braces. For example
   • int[ ][ ] arr ={ {1,2,3}, {4,5,6}, {7,8,9} } ;
     • The above statement will create a 2-D array of size 3 X 3.
     • Let us take an example where we will add two matrices of size 2 X 3.
   
   
   Multidimensional Array : Example
   
   Multidimensional Array : Output