ELEMENTARY PROGRAMMING - Introduction to Java Programming: Comprehensive Version - page 37

2.1

✓ ✓

Identify and fix the errors in the following code:

Check

Point

1 public class Test {
2 public void main(string[] args) {

3 double i = 50.0 ;

4 double k = i + 50.0 ;

5 double j = k + 1 ;

6
7 System.out.println( "j is " + j + " and
8

k is " + k);

9 }
10 }

2.3 Reading Input from the Console
Reading input from the console enables the program to accept input from the user.

Key
Point

In Listing 2.1, the radius is fixed in the source code. To use a different radius, you have to
modify the source code and recompile it. Obviously, this is not convenient, so instead you can
use the Scanner class for console input.
Java uses System.out to refer to the standard output device and System.in to the
standard input device. By default, the output device is the display monitor and the input
device is the keyboard. To perform console output, you simply use the println method to
display a primitive value or a string to the console. Console input is not directly supported
in Java, but you can use the Scanner class to create an object to read input from System.in ,
as follows:

VideoNote

Obtain input

Scanner input = new Scanner(System.in);

The syntax new Scanner(System.in) creates an object of the Scanner type. The syntax
Scanner input declares that input is a variable whose type is Scanner . The whole line
Scanner input = new Scanner(System.in) creates a Scanner object and assigns its
reference to the variable input . An object may invoke its methods. To invoke a method on
an object is to ask the object to perform a task. You can invoke the nextDouble() method to
read a double value as follows:

double radius = input.nextDouble();

This statement reads a number from the keyboard and assigns the number to radius .
Listing 2.2 rewrites Listing 2.1 to prompt the user to enter a radius.

L ISTING 2.2

ComputeAreaWithConsoleInput.java
1 import java.util.Scanner; // Scanner is in the java.util package
2
3 public class ComputeAreaWithConsoleInput {
4

import class

public static void main(String[] args) {

5 // Create a Scanner object

6 Scanner input = new Scanner(System.in); create a Scanner

7
8 // Prompt the user to enter a radius
9 System.out.print( "Enter a number for radius: " );
10 double radius = input.nextDouble(); read a double

11
12 // Compute area

13 double area = radius * radius * 3.14159;

14
15 // Display results