Reading and Writing Console I/O

Reading and Writing Console I/O

Reading Console Input

• In this section, we will understand how to read the input from console?

• In Java, System is a class defined in java.lang and in, out and err are the variables of the class System.

• Hence System.out is an object used for standard output stream and System.in and System.err are the objects for standard input stream and error.

Method 1: Reading input using Buffered Reader, Inputstream Reader, system. in

• When we want to read some character from the console we should make use of system.in.

• The character stream class BufferedReader is used for this purpose. In fact we should the variable System.in to BufferedReader object. Along with it we should also mention the abstract class of BufferedReader class which is InputStreamReader.

• Hence the syntax is,

BufferedReader object_name =new

BufferedReader(new InputStreamReader(System.in));

For example the object named obj can be created as

BufferedReader obj =new BufferedReader(new InputStreamReader(System.in));

• Then, using this object we invoke read() method. For e.g. obj.read().

• But this is not the only thing needed for reading the input from console; we have to mention the exception IOException for this purpose. Hence if we are going to use read() method in our program then function main can be written like this -

public static void main(String args[]) throws IOException

• Let us understand all these complicated issues with the help of some simple Java program

Java program [ReadChar.java]

/*

This is a java program which is for reading the input from console

*/

import java.io.*;

class ReadChar

{

public static void main(String args[])

throws IOException

{

//declaring obj for read() method

BufferedReader obj=new BufferedReader(new InputStreamReader(System.in));

int count=0;

char ch;

{

System.out.println("\n Enter five characters");

while(count<5)

{

ch=(char)obj.read();//reading single character System.out.println(ch);//outputting it

count++;//count to keep track of 5 characters

}

}

}

Output

Enter five characters

hello

h

e

l

l

o

Program Explanation

• This program allows you take the input from console.

• As given in above program, read() method is used for reading the input from the console.

•  The method read() returns the integer value, hence it is typecast to char because we are reading characters from the console.

• And last but not least we should write,

import java.io.*;

in our java program, because these I/O operations are supported by the java package java.io. Let us discuss one more example of reading input from console.

Ex. 4.2.1: Write a Java program to read entire line through keyboard.

Sol. Java Program [ReadString.java]

/*

This is a java program which is for reading the input from console

*/

import java.io.*;

class ReadString

{

public static void main(String args[])

throws IOException

{

BufferedReader obj=new BufferedReader(new InputStreamReader(System.in));

String s;

s=obj.readLine(); //for reading the string

while(!s.equals("end"))

{

System.out.println(s);

s=obj.readLine();

}

}

}

Output

hello how are you

hello how are you

end

Note that to read the string input we have used readLine() function. Otherwise this program is almost same as previous one.

Ex. 4.2.2 Write a Java Program by which user can enter his/her lucky number using keyboard. This number should also be displayed on the console.

Sol.:

import java.io.*;

public class ReadNumber

{

public static void main(String[] args) throws IOException

{

String s;

int num;

BufferedReader reader;//create BufferedReader object

reader=new BufferedReader(new InputStreamReader(System.in));

System.out.print("Enter your Lucky number:");

s=reader.readLine();

num=Integer.parseInt(s); //string value is converted to integer value

System.out.println("Your Lucky number is "+num);

}

}

Output

Method 2: Reading Console input using Scanner and System.in

• The Scanner is a class defined in java.util package. With the help of System.in we can read the console input by creating the instance for the class Scanner.

• The steps to read the console input are as follows

o Step 1: Import java.util package

o Step 2: Create instance for Scanner class as follows

Scanner scanner = new Scanner(System.in);

o Step 3: Invoke the nextXX() method to read the input of specific data type

• Example 1: Reading String

import java.util.*; //Step 1

public class Example

{

public static void main(String[] args)

{

Scanner scanner = new Scanner(System.in); //Step 2

System.out.print("What is your Name? ");

String name = scanner.next(); //Step 3

System.out.println("My Name is: "+name);

}

}

Output

What is your Name? Ankita

My Name is: Ankita

• Example 2: Reading integer

import java.util.*;

public class Example

{

public static void main(String[] args)

{

Scanner scanner = new Scanner(System.in);

System.out.print("Enter your Lucky number ");

int num= scanner.nextInt();

System.out.println("My Lucky number is: "+num);

}

}

Output

Enter your Lucky number 5

My Lucky number is: 5

• Example 3: Reading float

import java.util.*;

public class Example

{

public static void main(String[] args)

{

Scanner scanner = new Scanner(System.in);

System.out.print("Enter your marks ");

float num= scanner.nextFloat();

System.out.println("My marks are: "+num);

}

}

• Example 4: Reading double

import java.util.*;

public class Example

{

public static void main(String[] args)

{

Scanner scanner = new Scanner(System.in);

System.out.print("Enter double number ");

double num = scanner.nextDouble();

System.out.println("You have entered: "+num);

}

}

Output

Enter double number 212.444

You have entered: 212.444

• Example 5: Reading boolean

import java.util.*;

public class Example

{

public static void main(String[] args)

{

Scanner scanner = new Scanner(System.in);

System.out.print("Enter boolean value as true or false ");

boolean b = scanner.nextBoolean();

System.out.println("You have entered: "+b);

}

}

Output

Enter boolean value as true or false false

You have entered: false

Ex. 4.2.3: Write a Java program for calculating the bill amount. User enters the price of the item and its quantity.

Sol. :

import java.util.*;

public class Example

{

public static void main(String[] args)

{

Scanner scanner = new Scanner(System.in);

System.out.print("Enter the price: ");

float price = scanner.nextFloat();

System.out.print("Enter the quantity: ");

int quantity= scanner.nextInt();

float amount;

amount=price* quantity;

System.out.println("\nPrice: "+price);

System.out.println("Quantity: "+quantity);

System.out.println("------------------------");

System.out.println("Total Bill in Rs: "+amount);

System.out.println("-----------------------");

}

}

Output

Enter the price: 20.50

Enter the quantity: 10

Price: 20.5

Quantity: 10

-----------------------------

Total Bill in Rs: 205.0

------------------------------

Writing Console Output

The simple method used for writing the output on the console is write(). Here is the syntax of using write() method -

System.out.write(int b)

The bytes specified by b has to be written on the console. But typically print() or println() is used to write the output on the console. And these methods belong to PrintWriter class.