Input and Output to a computer program

  • Input = entering data (usually with a keyboard) into a computer program

  • Output = printing out data (usually to a console) from a computer program

  • Java uses the following variables:

      • System.in to identify the standard input device (which is the keyboard)

      • System.out to identify the standard output device (which is the console)

  • The variables System.in and System.out are defined in the Java library's class named: 

       java.lang.System

How to read a Java method call

The method call    System.out.println(area) in the Java program: 

public class ComputeArea
{
   public static void main(String[] args)
   {
      double radius;
      double area;
  
      // (1) Assign a radius
      radius = 20;

      // (2) Compute area
      area = 3.14159 * radius * radius;

      // (3) Print result
      System.out.println(area);
   }                       
}

means:   execute the method call println(area) with the object System.out (= console)

Reading in numbers

  • Java variable System.in represents the keyboard

    • The keyboard can only read in text

  • Important fact:

    • Text and numbers uses different representation/code !

  • Java provides the Scanner class to convert text to numbers

  • Declaring a Scanner variable called input with: 

       Scanner input = new Scanner(System.in);

  • How to   read in real numbers from System.in: 

       radius = input.nextDouble(); // Read in a number and assign to radius

The original program

Here is the original ComputeArea program: 



public class ComputeArea
{
   public static void main(String[] args)
   {
      double radius;
      double area;
  

      // (1) Assign a radius
      radius = 20;

      // (2) Compute area
      area = 3.14159 * radius * radius;

      // (3) Print result
      System.out.println(area);
   }                       
}

DEMO: demo/02-elem-prog/02-scanner/ComputeArea.java

Change the program to read from System.in using a Scanner object

We change it so the program will read in the radius from a Scanner object named input: 



public class ComputeArea
{
   public static void main(String[] args)
   {
      double radius;
      double area;
      Scanner input = new Scanner(System.in); // (A) 

      // (1) Read in radius from System.in
      radius = input.nextDouble();            // (B)   (**) Compile -> error

      // (2) Compute area
      area = 3.14159 * radius * radius;

      // (3) Print result
      System.out.println(area);
   }                       
}

BlueJ says: Unknown type: Scanner

The final program

Fix (reason):   we must import the Scanner class from the Java library before we can use it in the program: 

import java.util.Scanner;       // (**) Fix error (BlueJ gave a HINT !)

public class ComputeArea
{
   public static void main(String[] args)
   {
      double radius;
      double area;
      Scanner input = new Scanner(System.in); // (A)

      // (1) Read in radius from System.in
      radius = input.nextDouble();            // (B)   (**) No error  

      // (2) Compute area
      area = 3.14159 * radius * radius;

      // (3) Print result
      System.out.println(area);
   }                       
}

Demo   (It's better to print a prompt message !!)

Exercise

Write a program that reads in 3 numbers and prints out the average of the numbers

Exercise

Write a program that reads in 3 numbers and prints out the average of the numbers 

import java.util.Scanner; // Scanner is in the java.util package

public class ComputeAverage 
{
  public static void main(String[] args) 
  {
    Scanner input = new Scanner(System.in);
    
    // Prompt the user to enter three numbers
    System.out.print("Enter three numbers: ");
    double number1 = input.nextDouble();
    double number2 = input.nextDouble();
    double number3 = input.nextDouble();

    // Compute average
    double average = (number1 + number2 + number3) / 3;

    System.out.println(average);
  } 
}

DEMO: demo/02-elem-prog/03-CompAvg/ComputeAverage.java