Console Input in Java

Console input in Java is cumbersome. It uses the java.io package, which must be imported into the program. For console input, the keyboard is called System.in. But it is not as simple to use as console output, where the computer monitor is called System.out. Where we can just put something like

System.out.println (name + "'s age is " + age);

in a program, we cannot do the same with input.

Java like most languages buffers input. That means that keystrokes are stored in a temporary location in memory called a buffer. Any backspace or delete characters are used to clean up the input string. For example, suppose that you make a mistake while typing in some data and then decide to change what you have typed by using the backspace key. The entire string, along with the backspace character will be stored in a buffer. Then before the string is sent to the program, it is cleaned up with all deletions removed.

H

e

l

l

o

<sp>

A

o

<bs>

l

i

c

e

<enter>

<sp> stands for the space bar, <bs> stands for the backspace key, and <enter> stands for the enter key. Here, the backspace immediately follows the o, so both the o and the backspace will be removed. Then the cleaned up string will be sent for use by the program. It would look as follows:

H

e

l

l

o

<sp>

A

l

i

c

e

<enter>

If Java did not have buffered input, all input would have to be typed very carefully, since errors could not be corrected.

Input and output in Java (and most other languages) arrive and leave in streams. These are strings of characters, like "Hello Alice" above. A stream of characters is read character by character, by a program, but methods associated with the BufferedReader can read an entire line at a time. The one we will use most often is readLine. Notice that this is similar to println, which we used with output to the console screen.

The characters are taken from the input stream by the InputStreamReader. As mentioned above, the keyboard where the characters are produced is called System.in. So we have to get a new instance of the InputStreamReader with System.in as the parameter. Since what we really want to work with is the BufferedReader, we do not have to give a specific name to the InputStreamReader. So when we get a buffer, stdin, for input, we use the following statement:

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

The buffer is often named stdin, which is short for standard input.

After we have a reader, we can read a string of data using stdin.readLine (). If we want the result stored in a String variable called name, we can put

name = stdin.readLine ();

in our program.

But if what we want is the person’s age, an integer, there is an additional problem. The data might be entered incorrectly. The user might make a mistake and type a letter instead of a number. Or the input might contain a decimal point, making the result a decimal number and so not an integer. Because of this, Java is set up to try to make sense of the input. The input comes in as a string of characters.

2

1

<enter>

Java tries to interpret this as an integer using a method called parseInt. This method is in a class called Integer. If the characters do represent an integer, the method sends the integer to the program. In the example above, this is the number 21. But suppose that by mistake you type in 21a instead.

2

1

a

<enter>

This cannot be interpreted as an integer. Since Java has no idea what you meant, it throws an exception. That means that it simply tells the program that a mistake has occurred, and then lets the program decide what to do. The program has to catch the exception and do something about it. The something is often just the display of an error message to warn the user that there is a problem.

The kind of exception created here is called a NumberFormatException. It is thrown any time that a numerical input is expected, but not received. This error is an instance of a more general class of errors called IOExceptions. When using a BufferedReader, you always have to catch an IOException.

Because so much has to be done in order to read some data, we usually do the reading in a separate method. This method includes the try-catch block, the declaration of the BufferedReader, and all the reading. And since it is confusing to have to enter data without knowing what is wanted, each read statement is usually preceded by an output statement that prompts the user for what kind of data is expected.

A sample method is shown below.

// A method to read data from the keyboard. It uses a buffered reader.

public void readData ()

{

try

{

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

System.out.print ("Name: "); // Prompt for the person’s name.

name = stdin.readLine (); // Read in the name as a String.

System.out.print ("Age: "); // Prompt for the person’s age.

age = Integer.parseInt (stdin.readLine ()); // Interpret the input as an integer.

}

catch (NumberFormatException e){ System.out.println ("Invalid numerical input.");}

catch (IOException e) { System.out.println ("Invalid input.");}

} // method readData

If this is a method in your program, and the data entered is 21a, the console screen will appear as follows:

A method like the one above can be copied directly into a program and used. More lines can be added for additional data. For example, if you want the person’s birth year, you could add

System.out.print ("Year of Birth: ");

year = Integer.parseInt (stdin.readLine ());

Note that you can use print instead of println so that the data entry will be on the same line as the prompt.

An entire program that uses this method follows. Type it in and see how it works.

import java.io.*;

/* An application with a class that reads information about a person from the keyboard and then displays the data on the screen. */

public class People

{

public static void main (String [] args)

{

Person person = new Person ();

person.readData ();

person.displayAge ();

} // main method

} // class People

// Class that reads in a person’s name and age and then displays the result.

class Person

{

private String name;

private int age;

// A method to read data from the keyboard using a buffered reader.

public void readData ()

{

try

{

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

System.out.print ("Name: ");

name = stdin.readLine ();

System.out.print ("Age: ");

age = Integer.parseInt (stdin.readLine ());

}

catch (NumberFormatException e){ System.out.println ("Invalid numerical input.");}

catch (IOException e) { System.out.println ("I/O error.");}

} // method readData

// Method that displays the name and age of a person.

public void displayAge ()

{

System.out.println (name + "'s age is " + age);

} // method displayAge

} // class Person

We can use the age information entered to draw some conclusions about the person. The following program does just that.

import java.io.*;

// An application with a class that provides information about a person.

class People

{ public static void main (String [] args)

{ Person person = new Person ();

person.readData ();

person.displayAge ();

person.checkAge ();

} // main method

} // class People

/* Class that reads information about a person's name and age and then uses the age to provide additional information about the person. */

class Person

{

private String name;

private int age;

// A method to reads data from the keyboard. It uses a buffered reader.

public void readData ()

{ try

{ BufferedReader stdin = new BufferedReader (new InputStreamReader (System.in));

System.out.print ("Name: ");

name = stdin.readLine ();

System.out.print ("Age: ");

age = Integer.parseInt (stdin.readLine ());

}

catch (NumberFormatException e){ System.out.println ("Invalid numerical input.");}

catch (IOException e) { System.out.println ("I/O error.");}

} // method readData

// Method that displays the name and age of a person.

public void displayAge ()

{ System.out.println (name + "'s age is " + age);

} // method displayAge

public void checkAge ()

{ if (age >= 21)

System.out.println (name + " is old enough to drink.");

else

if (age >= 18)

System.out.println (name + " is old enough to vote.");

else

System.out.println (name + " has a way to go yet.");

} // method checkAge

} // class Person