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Scanner class can be used to read the data in the form of tokens, where each token could be a primitive byte, short, int, float, double or even an object of String class. We may read these tokens of data from various places such as -
• Using Scanner class to read user input from the keyboard.
• Using Scanner class to read a File on the disk.
• Using Scanner class to read from a String.







Constructors of Scanner class




Constructor	Description
Scanner(File f) throws FileNotFoundException	This constructor allows Scanner class to read from a file at the disk.
Scanner(String s)	Allows a Scanner class to read from a String.
Scanner(InputStream is)	Allows a Scanner class to read user input from user keyboard.
Scanner(Readable fr)	Allows a Scanner class to read a Readable object as a source for input.





Methods of Scanner class

Methods	Description
boolean hasNext()	This method returns true if the next token of value is available to read.
boolean hasNextInt()	This method returns true if the next token is of type float.
boolean hasNextFloat()	This method returns true if the next token is of type float.
boolean hasNextDouble()	This method returns true if the next token is of type boolean.
boolean hasNextShort()	This method returns true if the next token is of type short.
boolean hasNextByte()	This method returns true if the next token is of type byte.
int nextInt()	This method returns the next token as an int type of value.
byte nextByte()	This method returns the next token as an byte type of value.
short nextShort()	This method returns the next token as an short type of value.
float nextFloat()	This method returns the next token as a float type of value.
double nextDouble()	This method returns the next token as an double type of value.
String next()	This method returns the next token as an String type of value.








Using Scanner class to read user input from the keyboard.


We can read the user inputs from the keyboard using an object System.in of type InputStream.



//program when scanner class reads through the keyboard.


import java. util.*;

class A
{
public static void main(String... ar)
{
Scanner s = new Scanner(System.in);

while(s.hasNext())			//checks if there is a token value available to be read
{
if(s.hasNextInt())			//checks if next token value is of type int
System.out.println("Int is " + s.nextInt());


else if(s.hasNextDouble())		//checks if next token value is of type double
System.out.println("Double is "+s.nextDouble());

else if(s.hasNextBoolean())		//checks if next token value is of type boolean
System.out.println("Boolean is "+s.nextBoolean());

else		
{
if(s.next().equals("exit")) 		//checks if user has entered a String value, exit
break;					//if user has entered exit, control breaks out of while loop
else
System.out.println("String "+ s.next());	//prints the token value of type String.
}

} 
}
}





Output




19
Int is 19
21
Int is 21
23.4
Double is 23.4
true
Boolean is true
false
Boolean is false
Hello
Bonjour!
String Bonjour!
exit





Program Analysis


In the preceding code, we are reading LIVE user inputs from the keyboard at the command prompt and as soon as user enters the first token, an int value of 19 -
• hasNext() finds a token of value to be read by Scanner class and it returns true.
• hasNextInt() finds this token of type, int and it returns true.
• nextInt() reads this token value, 19 and prints it in the output.


Hence, each token of value entered by the user is first checked of its type and then matched with a hasNextXXX() method and then nextXXX() method is executed. This program keeps waiting for the user to type and enter any value until user types exit to exit the program.




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Using Scanner class to read a File on the disk.


In this program, we are going to create a file on the disk and enter the data in it. Next, we will read this file using constructor of the Scanner class which takes a File type in its argument.



import java.util.*;
import java.io.*;


class A
{
public static void main(String... ar)
{

File file = new File("D://File.txt");

try
{
FileWriter f= new FileWriter(file);		        //Creating a file
f.write("20 hello true 15.2 Michael  false 34");	//Writing to the file
f.close();



Scanner s = new Scanner(file);				//Reading the file using Scanner class.	

while(s.hasNext())
{
if(s.hasNextInt())
System.out.println("Int is "+ s.nextInt());

else if(s.hasNextFloat())
System.out.println("Int is "+ s.nextFloat());

else if(s.hasNextBoolean())
System.out.println("Int is "+ s.nextBoolean());

else 
System.out.println("String is "+ s.next());

}							//while loop ends

}							//try block ends
catch(Exception e)
{
System.out.println("Exception : "+ e);
}							//catch block ends

}
}





Output -




Int is 20
String is hello
Int is true
Int is 15.2
String is Michael
Int is false
Int is 34


A try-catch block was required in this program, as we are creating a new File and then we are also calling the constructor of Scanner class that may return a FileNotFoundException.






Using Scanner class to read from a String.


Now, we are going to learn how to read individual tokens of values from a String, using Scanner class.

import java.util.*;

class A
{
public static void main(String...ar)
{

String str=  "1 truth is 99.9 true rest false";
Scanner s = new Scanner(str);

while(s.hasNext())
{
if(s.hasNextInt())
{
System.out.println("Int is "+ s.nextInt());
}
else if(s.hasNextFloat())
{
System.out.println("Float is "+ s.nextFloat());
}
else if(s.hasNextBoolean())
{
System.out.println("Boolean is "+s.nextBoolean());
}
else
{
System.out.println("String is "+ s.next());
}

} //while loop ends here.

}
}





Output -




D:\Java>java A
Int is 1
String is truth
String is is
Float is 99.9
Boolean is true
String is rest
Boolean is false








InputMismatchException while reading through Scanner class.

//Exception while reading through the Scanner class.

import java. util.*;

class A
{
public static void main(String... ar)
{
Scanner s = new Scanner(System.in); 		//reading user inputs from the keyboard.

while(s.hasNext())
{
if(s.hasNextInt())
{
System.out.println("Int is " + s.nextBoolean()); //trying to convert an int value to a boolean value
}

else if(s.hasNext())
System.out.println("String "+ s.next());
}

}
}

Output -

12
Exception in thread "main" java.util.InputMismatchException
        at java.util.Scanner.throwFor(Unknown Source)
        at java.util.Scanner.next(Unknown Source)
        at java.util.Scanner.nextBoolean(Unknown Source)
        at A.main(Scanner6.java:16)

Program Analysis

In the preceding code, we are reading LIVE user inputs from the keyboard and as soon as user entered an int value of 12.

• s.hasNext() found a token of value to be read by Scanner class and it returned true.
• s.hasNextInt() found this token of value of type, int and it returend true.
• s.nextBoolean() has tried to convert this token value of int type, 12 to boolean, hence an InputMismatchException is thrown.

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