1. Input and Output

This chapter discusses input and output. Most programs input data, process it, and then output the results. This chapter discusses ways to input data from the keyboard and output results to the monitor. Later chapters will discuss input and output from disk files and other media.

Chapter Topics:

Input and Output Streams
The standard I/O streams
Scanner class
Converting character data to type int

Question 1:
When a computer program does an input operation, in which direction does the data flow?
From an outside device into the program?
From the program out to some device?

Source: Bradley Kjell, http://programmedlessons.org/Java9/chap12/ch12_01.html
This work is licensed under a Creative Commons Attribution-NonCommercial 4.0 License.

2. I/O

Answer:
From an outside device into the program.

I/O

oldArray

Input and output are so common that an abbreviation has been created: I/O (pronounced eye-oh). I/O operations are very complicated, both at the software level and at the electronic level. Part of the problem is that the way in which data is organized outside the computer is different from the way it is organized inside the computer. Lots of computation is needed to convert data between its various forms. Luckily, most of the complication has been hidden inside methods that your program can use.

These notes use methods of the class java.util.Scanner for doing input. Scanner is new to Java 5.0. Scanner is not part of the fundamental Java language, but is part of a package, java.util, that you can include in your program. A package is a collection of classes which may be used in your program. Think of a package as a tool box and the classes within it as tools. Different programs need different tools and include different packages.

It may seem odd to you that Java itself does not have I/O built into it. The reason for this is that there are very many types of I/O, and a language that has all of them built in would be large and cumbersome. A language used for professional software development such as Java (or C or C++ or any of several others) allows the programmer to pick the right I/O package for the job.

Question 2:
Your automobile has many computers that control the motor as it monitors data from dozens of sensors. Are these computers doing input and output?

3. I/O Streams

Answer:
Yes — data is input from sensors, computations are performed, and the results are output to devices that control the motor.
The average luxury car has dozens of computers running 100 million lines of code (as of 2018). Input and output is going on constantly.

I/O Streams

IOStreams

If your car's programs were written in Java, they would use a specialized I/O package that manages sensors and controllers. It would not use the I/O package that deals with keyboards and terminals.

In Java, a source of input data is called an input stream and the output data is called an output stream.

In the picture, each "O" represents a piece of data waiting in line to be input or leaving as output.

Data input is called reading data, and data output data is called writing data (or printing data if the output stream is connected to a monitor or a printer.)

The input stream is like a string of pearls which the program inputs one at a time, in order. The output stream is another string of pearls (not usually the same pearls as were read in). Often a program will read several data and then combine them somehow to produce one output value. For example, the input data might be a list of numbers, the output data might be their sum.

Question 3:
If the program is a text editor (like Notepad) what are the input and output data?

4. Standard Streams

Answer:
A stream of characters is input (from a file) and later a stream of characters is output (often to the same file).

Standard Streams

Some programs read in the entire input stream before they write the output stream. Other programs read the input stream item by item and write one output item for each input item. There are many other patterns of input and output. In general, a program may have several input streams flowing into it and several output streams flowing out of it.

There are three standard I/O streams:

System.in — the input stream.
System.out — the output stream for normal results.
System.err — the output stream for error messages.

Normally System.in is connected to the keyboard and the data are characters. System.out and System.err both are connected to the monitor, and also contain character data. These notes use System.err only occasionally.

Question 4:

What does the keyboard send to your program when you type the following:

5. Characters In, Characters Out

Answer:
The characters '1' , '2' , '3' , and '4' .

Characters In, Characters Out

charsINcharsOUT

The keyboard sends character data to the computer, even when the characters look like numbers. And the program sends characters to the monitor, even when it has calculated a numerical result. (Actually, characters are not sent directly to the monitor. They are sent to the graphics card which converts them into a video signal which is displayed on the monitor.)

If your program does arithmetic, the input characters must be converted into one of the primitive numeric data types. This is done using a Scanner object. Then a result is calculated using arithmetic with the numeric data. The result must then be converted into character data before it is sent to the monitor. This is done using a method of System.out.

Question 5:

Have you already used System.out?

6. Example I/O Program

Answer:
Yes,
System.out.println( "Some string" )
sends characters to the output stream.

Example I/O Program

EchoPicture

Here is a picture of a Java program doing character input and output. In this picture, the white box represents the entire program. The program has imported Scanner for use with input, and has imported System.out for use with output.

To use the methods of a Scanner an object must be constructed. (The diagram shows the object as a rectangle.)

The nextLine() method of Scanner reads one line of character data from the keyboard. The characters go into a String object. An assignment statement puts a reference to the object in the reference variable inData. To output the characters to the monitor, the program uses the println() method of System.out.

Question 6:

Do you suppose that a Scanner object has more methods than the one method in this picture?

7. Echo.java

Answer:
Yes — Scanner objects have many input methods.

`Echo.java`

import java.util.Scanner;

public class Echo
{
  public static void main (String[] args) 
  {
    String inData;
    Scanner scan = new Scanner( System.in );

    System.out.println("Enter the data:");
    inData = scan.nextLine();

    System.out.println("You entered:" + inData );
  }
}

Here is the Java program. It reads one line of characters from the keyboard and echoes them to the monitor.

First a Scanner object is created:

Scanner scan = new Scanner( System.in );

Then the user is prompted to type something in. Next a method of the Scanner is called:

inData = scan.nextLine();

This reads in one line of characters and creates a String object to contain them. A reference to this object is put into the reference variable inData. Next the characters from that String are sent to the monitor:

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

The line import java.util.Scanner; says to use the Scanner class from the package java.util. Here is a run of the program on a Windows system:

EchoRun

Question 7:

Could the user include digits like 123 in the input characters?

8. Digits are Characters

Answer:
Yes. They are characters just like any other.

`Digits are Characters`

As far as this program is concerned, a string of digits is a string of characters and is treated like any string of characters. The digits are NOT automatically converted into a numeric type. Here is another run of the program:

In a few pages you will see a program that reads in a string of digits and converts them into a numeric data type. Then arithmetic can be done with that data.

EchoDigits

Question 8:

Is data from the keyboard always only characters?

9. Details

Answer:
Yes. (Although frequently those characters are converted to a numeric type after they have been read in.)

`Details`

import java.util.Scanner;

public class Echo
{
  public static void main (String[] args)
  {
    String inData;
    Scanner scan = new Scanner( System.in );

    System.out.println("Enter the data:");
    inData = scan.nextLine();

    System.out.println("You entered:" + inData );
  }
}

Later on we will do something special to convert strings of digits into primitive numeric types. This program does not do that.

public class Echo

The program defines a class named Echo that contains a single method, its main() method.

public static void main ( String[] args )

All main() methods start this way. Every program should have a main() method.

String inData;

The program creates a reference variable inData which will soon refer to a String object.

Scanner scan = new Scanner( System.in );

This creates a Scanner object, referred to by the reference variable scan.

Question 9:

What data stream is this Scanner connected to?

10. Details (continued)

Answer:
The standard input stream — the keyboard.
(Scanner objects can be connected to other data streams.)

`Details (continued)`


import java.util.Scanner;

public class Echo
{
  public static void main (String[] args)
  {
    String inData;
    Scanner scan = new Scanner( System.in );

    System.out.println("Enter the data:");
    inData = scan.nextLine();

    System.out.println("You entered:" + inData );
  }
}

System.out.println("Enter the data:");

This calls the method println to print the characters "Enter the data:" to the monitor.

inData = scan.nextLine();

This uses the nextLine() method of the object referred to by scan to read a line of characters from the keyboard. A String object (referred to by inData) is created to contain the characters.

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

This first creates a String by concatenating "You entered:" to characters from inData, then calls println() to print that String to the monitor.

Question 10:

When the program runs, can the user edit the input string (using backspace and other keyboard keys) before hitting "enter"?

(Try this, if you have a running program.)

11. Digits as Input

Answer:
Yes.

`Digits as Input`

Collecting characters from the keyboard is done by the operating system. While this is going on, the Java program is suspended. The user can edit the line, and then hit "enter" to signal that it is complete. Only then does the Java program see the characters and resume execution.

Here is another run of the program:

Enter the data:
Columbus sailed in 1492.
You entered:Columbus sailed in 1492.

Notice that the characters '1', '4', '9', and '2' were read in and written out just as were the other characters.

Now consider yet another run:

Enter the data:
1492
You entered:1492

Nothing special here. The '1', '4', '9', and '2' are just characters. A String object is created, and those characters are stored in the object, just like any characters. If you want the user to enter numeric data, your program must convert from character data to a numeric type.

Question 11:

What primitive type should normally be used to hold integer data?

12. nextInt()

Answer:
int

`nextInt()`

charsConversion

The nextInt() method of a Scanner object reads in a string of digits (characters) and converts them into an int type.

The Scanner object reads the characters one by one until it has collected those that are used for one integer. Then it converts them into a 32-bit numeric value. Usually that value is stored in an int variable.

The picture shows a program that reads in character data and then converts it into an integer which is stored in num. Next the program does arithmetic with num and stores the result in square. Finally the result is sent to println which converts the numeric result into characters and prints them out.

The nextInt() method scans through the input stream character by character, gathering characters into a group that can be converted into numeric data. It ignores spaces and end-of-lines that may preceed the group.

A space or end-of-line character that follows a string of digits ends the group. A non-digit character cannot be part of a group (not even at the end.) For example,

and

are legitimate input, but

123X

is not.

Question 12:

Which of the following are legitimate input for nextInt() ?

13. EchoSquare.java

Answer:
 4              OK
 
    45          OK 
    
 456            OK 
 
 923X           BAD
 
     23   876   OK: the first group ends after the "3"
The last line is OK. If there is a second call to nextInt() it will pick up the "876" .

`EchoSquare.java`

Here is the program shown in the previous picture. It computes the square of an integer entered (as characters) by the user.

import java.util.Scanner;

public class EchoSquare
{
  public static void main (String[] args)
  { 
    Scanner scan = new Scanner( System.in );
    int num, square;      // declare two int variables 

    System.out.println("Enter an integer:");
    num = scan.nextInt();
    square = num * num ;  // compute the square 

    System.out.println("The square of " + num + " is " + square);
  }
}

Here is a picture of it running:

EchoSquarePicture

Please run this program and play with it. Beginning programmers are often confused about "character data" and "numeric data" and conversions between the two. Take the opportunity to make sure you know what is going on to avoid future confusion.

Question 13:

Do you think that the following input would work with this program?

twelve hundred

14. Converting to Integers

Answer:
No. The program only works with strings of digits that can be converted into integer data.

`Converting to Integers`

Here is a statement from the program:

num = scan.nextInt();

Assignment statements work in two steps:

Evaluate the expression on the right of the equal sign,
Put the value into the variable on the left.

In this particular assignment statement, the expression on the right scans a group of characters from the input stream and converts them into an int, if that is possible. Then the numeric result is stored into num.

If the group of characters cannot be converted, Java throws an Exception and stops your program. An Exception object contains information about what went wrong in a program. Industrial-strength programs may examine the exception and try to fix the problem. Our programs (for now) will just stop.

Question 14:

Which of the following inputs would be correct input for the program? Click to see if you are correct.

Enter an integer:  1492
Enter an integer:  Fourteen ninety two
Enter an integer:  14.92
Enter an integer:  -1492
Enter an integer:  1 4 9 2

15. InputMismatchException

Answer:
Enter an integer:  1492                  OK
Enter an integer:  Fourteen ninety two   WRONG
Enter an integer:  14.92                 WRONG
Enter an integer:  -1492                 OK
Enter an integer:  1 4 9 2               MAYBE , but only the '1' will be scanned

`InputMismatchException`

inputMismatchException

If the user enters one of the WRONG lines, the statement

num = scan.nextInt()

will not be able to scan the characters as an integer. It will throw an exception. You will see something like the above.

This exception was passed out of the running program to the Java system, which stopped the program and wrote the error messages.

Question 15:

At what line of your program did the problem occur? (Inspect the error message from the program.)

16. Another Example

Answer:
Line number 11.
Although your source code is translated from text into bytecodes, and it is the bytecodes that are executed by the Java virtual 
 machine, the file of bytecodes EchoSquare.class retains some line number information which is displayed
 when your program crashes.

`Another Example`

Here is another example. It asks the user for two integers which are then added together and the sum written out.

import java.util.Scanner;

public class AddTwo
{
  public static void main (String[] args) 
  { 
    Scanner scan = new Scanner( System.in );
    int first, second, sum ;      // declaration of int variables

    System.out.println("Enter first  integer:");
    first = scan.nextInt();       // read chars and convert to int

    System.out.println("Enter second integer:");
    second = scan.nextInt();      // read chars and convert to int

    sum = first + second;         // add the two ints, put result in sum

    System.out.println("The sum of " + first + " plus " + second +" is " + sum );
  }
}

Here is a sample run:

Enter first  integer:
12
Enter second integer:
-8
The sum of 12 plus -8 is 4

Question 16:

(A slightly hard question: ) explain what happened in the following run of the same program:

Enter first  integer:
12 -8
Enter second integer:
The sum of 12 plus -8 is 4

It looks like the user did not enter the second integer after the second prompt.

17. Integer Division Tester

Answer:
During the first call to nextInt(), the Scanner found the characters "12" , converted them to an int and stopped. It did what the program requested.
During the second call to nextInt(), the Scanner continued scanning from where it stopped. It found the characters "-8" , and converted them to an int.
The action of the Scanner does not depend on the prompt the program wrote out.

`Integer Division Tester`

The nextInt() method scans through the input stream, character by character, building a group that can be converted into numeric data. It scans over spaces and end-of-lines that may separate one group from another. Once it has found a group, it stops scanning.

In the above, the user entered two groups on one line. Each call to nextInt() scanned in one group.

It is tempting to think of user input as a sequence of separate "lines". But a Scanner sees a stream of characters with occasional line-separator characters. After it has scanned a group, it stops at wherever it is and waits until it is asked to scan again.

Here is a new program made by modifying the previous program.

The user enters two integers, dividend and divisor.
The program calculates and prints the quotient and the remainder.
The program calculates and prints quotient * divisor + remainder.

Run the program a few times. See what happens when negative integers are input.

import java.util.Scanner;
public class IntDivideTest
{
  public static void main (String[] args) 
  { 
    Scanner scan = new Scanner( System.in );
 
    int dividend, divisor ;                      // int versions of input
    int quotient, remainder ;                    // results of "/" and "%"

    System.out.println("Enter the dividend:");   // read the dividend
    dividend = scan.nextInt();          

    System.out.println("Enter the divisor:");    // read the divisor
    divisor  = scan.nextInt();          

    quotient = dividend / divisor ;              // perform int math
    remainder= dividend % divisor ;

    System.out.println( dividend + " / " + divisor + " is " + quotient );
    System.out.println( dividend + " % " + divisor + " is " + remainder );
    System.out.println( quotient + " * " + divisor + 
        " + " + remainder + " is " + (quotient*divisor+remainder) );
  }
}

Question 17:

Do these notes still have your undivided attention?

18. Integer Division Tester

Answer:
During the first call to nextInt(), the Scanner found the characters "12" , converted them to an int and stopped. It did what the program requested.
During the second call to nextInt(), the Scanner continued scanning from where it stopped. It found the characters "-8" , and converted them to an int.
The action of the Scanner does not depend on the prompt the program wrote out.

`Integer Division Tester`

The nextInt() method scans through the input stream, character by character, building a group that can be converted into numeric data. It scans over spaces and end-of-lines that may separate one group from another. Once it has found a group, it stops scanning.

In the above, the user entered two groups on one line. Each call to nextInt() scanned in one group.

It is tempting to think of user input as a sequence of separate "lines". But a Scanner sees a stream of characters with occasional line-separator characters. After it has scanned a group, it stops at where ever it is and waits until it is asked to scan again.

Here is a new program made by modifying the previous program.

The user enters two integers, dividend and divisor.
The program calculates and prints the quotient and the remainder.
The program calculates and prints quotient * divisor + remainder.

Run the program a few times. See what happens when negative integers are input.

import java.util.Scanner;
public class IntDivideTest
{
  public static void main (String[] args) 
  { 
    Scanner scan = new Scanner( System.in );
 
    int dividend, divisor ;                      // int versions of input
    int quotient, remainder ;                    // results of "/" and "%"

    System.out.println("Enter the dividend:");   // read the dividend
    dividend = scan.nextInt();          

    System.out.println("Enter the divisor:");    // read the divisor
    divisor  = scan.nextInt();          

    quotient = dividend / divisor ;              // perform int math
    remainder= dividend % divisor ;

    System.out.println( dividend + " / " + divisor + " is " + quotient );
    System.out.println( dividend + " % " + divisor + " is " + remainder );
    System.out.println( quotient + " * " + divisor + 
        " + " + remainder + " is " + (quotient*divisor+remainder) );
  }
}

Question 17:

Do these notes still have your undivided attention?

19. End of the Chapter

Answer:
Huh? What??

`End of the Chapter`

Perhaps it is time to quit. When you are awake, click on a subject that interests you to go to where it was discussed.

package I/O Packages
input stream I/O Streams
standard I/O streams standard I/O streams
program, Echo First I/O Example Program.
Scanner class The Scanner class.
nextLine() The nextLine() Method.
nextInt() The nextInt() Method.
string, conversion to int Converting a data String into an int.
InputMismatchException InputMismatchException
program, adding two ints Program to add two integers.

The next chapter discusses input of floating point numbers.

Site:	Saylor Academy
Course:	CS101: Introduction to Computer Science I
Book:	Input and Output

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Date:	Wednesday, 2 April 2025, 11:24 PM

Input and Output

Description

Table of contents

1. Input and Output

Question 1:

2. I/O

Answer:

I/O

I/O

Question 2:

3. I/O Streams

Answer:

I/O Streams

I/O Streams

Question 3:

4. Standard Streams

Answer:

Standard Streams

Standard Streams

Question 4:

5. Characters In, Characters Out

Answer:

Characters In, Characters Out

Characters In, Characters Out

Question 5:

6. Example I/O Program

Answer:

Example I/O Program

Example I/O Program

Question 6:

7. Echo.java

Answer:

Echo.java

Echo.java

Question 7:

8. Digits are Characters

Answer:

Digits are Characters

Digits are Characters

Digits are Characters

Question 8:

9. Details

Answer:

Details

`Echo.java`

`Echo.java`

`Digits are Characters`

`Digits are Characters`

`Details`