Array in java

Array

Array is a data structure which is collection of same type of values. We can access individual value using an integer index. The complete set of values is called array and the individual value in the array is called an element.

Declaration of an array

Syntax

            int[]         marks;

       float[]       average;

       int           total[];

       float         num[];

The [] indicates an arry in array declaration. To declare an array first we take data type like int, float char etc followed by [] then name of variable as shown above. [] can be used either with data type or variable name both are supported by java.

Creation of array

After declaration an array should be created in the memory then only we can use it.

Syntax

            int[]  marks;        // Declaration of array

       marks = new int[3]; // Creation of array

Array is created in memory using new statement. First array should be declared then it should be created. For creation of array first we should write the name of variable which is already declared then” =” symbol, after that new and data type followed by [] with number of elements of the array as shown in second line above. Total number of elements in above example is 4 which is called size of array, means we can store 4 values in the array.

Initialization of array

Putting values in array is called initialization of the array.

Syntax

            marks[0] = 32;

       marks[1] = 48;

       marks[2] = 16;

       marks[3] = 19;

The size of array is 4, the address of first element is marks[0] and value stored in this element is 32. Similarly the address of second location of the array is marks[1] and value stored is 48 and so on. The address of last (4^th) element of the array is marks[3].

There is one more way to initialize the array as shown below.  

Syntax

            int marks[] = {32,48,16,19};

This expression creates an array and initializes it with the values given in curly braces. In this method of array creation new is not used.  It sets the size of array equal to the number of values in braces.

Array Length

At the time of creation of an array size of array is stored in a variable called length which can be accessed from the name of the array variable.

Example

            int marks[] = {32,48,16,19};

       int arrayLength;

       arrayLength = marks.length;

We can get length of the array as shown above.  The  “ marks.length” expression returns size of array or length of array. We can use “Arrays.sort(marks)” method to sort the array.

Example:

public class ArrayExample{

       public static void main(String[] args)

        {

             

                      

                     int marks[] = {32,48,16,19};

                     int arrayLength;

                    

                     arrayLength = marks.length;

                     System.out.println("Given list is: ");                

                     for(int i =0; i < arrayLength; i++)

                                  {

                                        

                                         System.out.println(marks[i]);

                                  }

                     Arrays.sort(marks);

                     System.out.println("Sorted array : ");                

                     for(int i =0; i < arrayLength; i++)

                                  {

                                        

                                         System.out.println(marks[i]);

                                  }            

       }

        

}

The output of this example is:

Given list is:

32

48

16

19

Sorted array :

16

19

32

48

Similar to “Arrays.sort(marks)” and marks.length there are some other methods also provided by java.

Multi Dimensional Array

Tow dimensional arrays are used when table of values is to be stored. For example we can make a table of marks obtained by students in different subjects as given below:

            Table Name: Marks Table

	Subject 1	Subject 2	Subject 3	Subject 4
Student 1	56	92	85	45
Student 2	76	46	88	65
Student 3	68	443	76	87

This table is consists of three rows and four columns. Students represented in row and subjects represented in columns. When we need a data structure to store data of table form we use two dimensional arrays. In single dimensional array each dimension of array is indexed from 0 to maximum size minus one like marks[0]….marks[5]. Similarly in two dimensional arrays we use the row number and column number to represent index of array.

marks[row number][column number]   

It is necessary to write row number in first braces [], and column in second braces [] after variable name. 

Here in this example first row is Student 1, second row is Student 2, and third row is Student 3. Similarly first column is Subject 1, Second column is Subject 2, third column is Subject 3 and forth column is Subject 4.

To save values of above table in an array we will need to use tow dimensional array.

Creation of two dimensional array:

            int marks[][];

       marks = new int[3][4];     // Three rows and four columns.

Or

            int marks[][] = new int[3][4];

Like one dimensional array we can also initialize two dimensional array at the time of declaration.

           

       int marks[][] = {0,0,0,0,1,1,1,1,2,2,2,2};          

The initialization is done row by row, in this example all 0s represents value of all the columns of first row from column 1 to 4 serially, and similarly value 1 is for row 1 and so on. We can also initialize tha array as:

            int marks[][] = {{0,0,0,0},{1,1,1,1},{2,2,2,2}};

                                                OR

            int marks[][] = {{0,0,0,0},

                            {1,1,1,1},

                            {2,2,2,2}};

We can save the value in array direct at some location using row number and column number.

Like:   

marks[2][3] = 88; // second row third column.

              marks[3][3] = 76; //third row third column.

Example

public class FirstJavaProgram {

              public static void main(String[] args)

              {

                     int marks[][] ={{56,92,85,45},

                                      {76,46,88,65},

                                      {68,43,76,87},

                                     };

                     int i,j=0;

             

                     for(i =0; i<=2; i++)

                     {

                     for(j= 0; j<=3; j++ )

                           {

System.out.println("Row: "+ i + ",Column: " + j + " = " + marks[i][j]);

                           }

                     }

       }

}

The result of this program is:

       Row: 0,Column: 0 = 56

Row: 0,Column: 1 = 92

Row: 0,Column: 2 = 85

Row: 0,Column: 3 = 45

Row: 1,Column: 0 = 76

Row: 1,Column: 1 = 46

Row: 1,Column: 2 = 88

Row: 1,Column: 3 = 65

Row: 2,Column: 0 = 68

Row: 2,Column: 1 = 43

Row: 2,Column: 2 = 76

Row: 2,Column: 3 = 87

Strings       

Java strings are sequence of characters. String in java is a predefined class in java library. Java provides two string classes String and StringBuffer.

We can define an initialize a string as:

            String e = "";                    // Empty string

       String firstName = "Amitabh";     // String with value

       String lastName = "Bacchan";

              OR

       String cityName;

       cityName = new String("Mumbai"); 

             

              OR

String stateName = new String("Maharastra");

String Array

We can use strings in arrays also. We can define array of strings such as:

            String cityArray[] = new String[3];

             

              cityArray[0] = "New Delhi";

              cityArray[1] = "Mumbai";

              cityArray[2] = "Jaipur";

Concatenation

Java string can be concatenated using + sign.

String fullName =  firstName + lastName;

String capitalOfIndia = "New " + "Delhi";

String Methods

Sub String: We can copy a part of string into another string.

                String subStr = firstName.substring(0, 4);

This statement will copy first 4 characters of firstName string starting from 0^th to 3^rd character in subStr.

Length of String: We can get length of string using Length method of string class

            int lengthOfString = firstName.length();

String Comparison: Equals method of String class allows us to compare two strings. This expression returns true if strings are same otherwise false.

boolean stringComp1 = firstName.equals(lastName);

boolean stringComp2 = firstName.equals("Amit");

Example:        

public class FirstJavaProgram {

       public static void main(String[] args)

        {

             

              String cityArray[] = new String[3];

             

              cityArray[0] = "New Delhi";

              cityArray[1] = "Mumbai";

              cityArray[2] = "Jaipur";

             

              for(int i =0; i< cityArray.length; i++)

              {

                     System.out.println(cityArray[i]);

              }

             

              String e = "";

              String firstName = "Amitabh";

              String lastName = "Bacchan";

              String address = new String("Jalsha, Juhu");

              String cityName;

              cityName = new String("Mumbai");

                          

              System.out.println("First Name: " + firstName);

              System.out.println("First Name: " + lastName);

             

              String fullName =  firstName + lastName; // Concatenation of strings

              System.out.println("Full Name after concatenation: " + fullName);

              System.out.println("Address: " + address);

              System.out.println("Address: " + cityName);

             

              String capitalOfIndia = "New " + "Delhi"; // Concatenation of strings

              System.out.println("Capital of India: " + capitalOfIndia);

             

      String subStr = firstName.substring(0, 4); // copy first 4 characters

              System.out.println("Sub string " + subStr);

             

              int lengthOfString = firstName.length();

              System.out.println("Length of String " + lengthOfString);

             

              boolean stringComp = firstName.equals("Amit");

              System.out.println("String Comparision: " + stringComp);

       }

}

The output of the program is:

            String arra example:

New Delhi

Mumbai

Jaipur

First Name: Amitabh

First Name: Bacchan

Full Name after concatenation: AmitabhBacchan

Address: Jalsha, Juhu

Address: Mumbai

Capital of India: New Delhi

Sub string: Amit

Length of String: 7

String Comparision: false

StringBuffer

StringBuffer is another class which supports strings. String creates string of fixed length whereas StringBuffer creates string of flexible length. The length of StringBuffer can be changed as per the length of the content we want to save in StringBuffer. We can define and initialize string using StringBuffer as:

StringBuffer capitalofIndia = new StringBuffer("New Delhi capital of India");

StringBuffer mainly support following methods:

charAt: Returns the character at specified position. Example:

       System.out.println("Character at position 4:  " + capitalofIndia.charAt(4));

This statement returns the character at position 4 in the string. First position starts from 0 and then 1,2,3 etc. In above string at position 4 character is “D”.

setCharAt: Set the character at specified position. Example:

capitalofIndia.setCharAt(3, '-');       

       System.out.println("String after adding - at position 3: " + capitalofIndia);

The above statement adds character “-“ at position 3 in the string and result will be “New-Delhi”.

indexOf: This method returns the position of first character of the given substring. For example we want to check the position of substring “capital” in main string.

int pos = capitalofIndia.indexOf("capital");

       System.out.println("Position of substring capital in the string: " + pos);                          

The position of substring capital in the string is 10.

length:  It returns length of the string. Example:

System.out.println("Length of the string: " + capitalofIndia.length());

The length of string stored in “capitalofIndia”  is 26.

insert: We can insert a substring with in an existing string like character we have seen above. Example:

capitalofIndia.insert(10, "is ");

       System.out.println("After inserting is in the string: " + capitalofIndia); 

A string “is” is inserted at position 10 in the original string.

append: We can append a new string at the end of existing string. Example

capitalofIndia.append("and Mumbai is financial capital of India");

       System.out.println("After Append: " + capitalofIndia);

Example:

public class FirstJavaProgram {

              public static void main(String[] args)

              {

             

StringBuffer capitalofIndia = new StringBuffer("New Delhi capital of India");

             

                     System.out.println("Original String:  " + capitalofIndia);

             

                     // Display the character at position 5 in the string

             

System.out.println("Character at position 4:  " + capitalofIndia.charAt(4));

             

                     capitalofIndia.setCharAt(3, '-');

System.out.println("String after adding - at position 3: " + capitalofIndia);

             

                     int pos = capitalofIndia.indexOf("capital");

System.out.println("Position of substring capital in the string: " + pos);

             

System.out.println("Length of the string: " + capitalofIndia.length());

             

                     capitalofIndia.insert(10, "is ");

System.out.println("After inserting is in the string: " + capitalofIndia);

             

capitalofIndia.append(" and Mumbai is financial capital of India");

                     System.out.println("After Append: " + capitalofIndia);

       }

}

The output of this program is:

Original String:  New Delhi capital of India

Character at position 4:  D

String after adding - at position 3: New-Delhi capital of India

Position of substring capital in the string: 10

Length of the string: 26

After inserting is in the string: New-Delhi is capital of India

After Append: New-Delhi is capital of India and Mumbai is financial capital of India