Arrays

Lets take an Example where customer can have different contact numbers as residence number, mobile number, office number and create a class which can show us how they are declared.

class Customer{

                private int customerId;

                private long residenceNo;

                private long mobileNo;

                private long officeNo;

                public void setCustomerId(int id){

                                customerId = id;

                }

                public void setResidenceNo(long no1){

                                residenceNo=no1;

                }

                public void setMobileNo(long no2){

                                mobileNo=no2;

                }

                public void setOfficeNo(long no3){

                                officeNo=no3;

                }

}

Here we have three similar type of variable of type long is their a better way of storing this data?

Yes there is Arrays.

Arrays

• An Array is a collection of similar type of data in contiguous locations of memory having same name.
• Arrays can be used to store data as primitives type and reference type.
• Array are created dynamically in java.
• A new operator is used to dynamically allocation of memory to arrays
• Arrays are of two type:
• Single dimensional
• Multi dimensional

Single Dimensional Arrays

 Array with one dimension (one row) is single dimensional array.

Way of creating and initializing :

Method 1: 

int array[] = new int[2];(Declaration)

array[0] = 1; (Initialization)

array[1] = 2;

Method 2: 

int array_Var[] = {1,2,3};(Declaration and Initialization)

The symbol [] can be placed before of after the variable i.e

int [] array_Var = new int[];

int array_Var[] = new int[]; (This is used for more readability)

• Array variable is reference variable so stored in stack.
• Arrays elements are dynamic and stored in heap.
• The default value for array is the type of which it is declared in this case int so zero(0),if not explicitly initialized. 

Length property: Size f array can be defined using the length property of array and can be obtained using array_var.length which is 3 in this case.

int array_var[] = new int[5];

array_var[0] = 1;

array_var[0] = 2;

array_var[0] = 3;

Size will be array_var.length = 5

Multi Dimensional Arrays

Array can have multiple dimensions so are called Multi dimensional arrays 

Ways of declaring and initializing Multi dimensional arrays

Method 1:

char [][] multi_array = new char[2][2];(Declaration)

char[0][0] = 'a';

char[0][1] = 'b';

char[1][0] = 'c';

char[1][1] = 'd';

Method 2:

char[][] multi-array= {{'a','b'},{'c','d'}};

Length property: 

Number of row can be obtained by following statement

multi_array.length = 2

Number of columns in a row can be obtained by

array name[row_number].length

Like:

char[][] multi-array= {{'a','b'},{'c','d'}};

 multi_array[1].length = 2

char[][] multi-array= {{'a','b'},{'c'}};

 multi_array[1].length = 1

Now our Customer class will look like this 

class Customer{

   private int customerId;

   private long contactNos[] = new long[3];

   public void setCustomerId(int id){

     customerId = id;

   }

   public void setContactNo(long no1,long no2,long no3){

   contactNos[0]=no1;

   contactNos[1]=no2;

   contactNos[2]=no3;

  }

}

it is more efficient and with less line of code