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Interactive Input
• We have written programs that print console output, but it
is also possible to read input from the console.
• The user types input into the console. We capture the
input and use it in our program.
• Such a program is called an interactive program.
• Interactive programs can be challenging.
• Computers and users think in very different ways.
• Users misbehave.
Input using the Scanner class
• System.out
• An object with methods named println and print
• System.in
• not intended to be used directly
• We use a second object, from a class Scanner, to help us.
• Constructing a Scanner object to read console input:
Scanner name = new Scanner(System.in);
• Example:
Scanner console = new Scanner(System.in);
Packages and import Statements
• Java class libraries: Classes included with Java's JDK.
• organized into groups named packages
• To use a package, put an import declaration in your
program.
• Syntax:
//put this at top of your program
import packageName.*;
• Scanner is in a package named java.util
import java.util.*;
• To use Scanner, you must place the above line at the
top of your program (before the public class
header).
Scanner Methods
Methods from Scanner class:
Method
Description
nextInt()
reads a token of user’s input as an int
nextDouble()
reads a token of user input as double
next()
reads a token of user input as a String
nextLine()
reads a line of user input as a String
Each method waits until the user presses Enter.
The
value entered by the user is returned by method.
 token = value typed by user. Tokens are separated by
whitespace (blank spaces, tabs, newlines).
System.out.print("How old are you? "); // prompt
int age = console.nextInt();
System.out.println("You'll be 40 in " +
(40 - age) + " years.");
prompt:
A message telling the user what input to type.
Fun Stuff with User Input
import java.util.*; // so I can use Scanner
public class TwoTokens {
public static void main(String[] args) {
Scanner keyboard = new Scanner(System.in);
System.out.print(“Enter two integers: “); // prompt
int num1 = keyboard.nextInt(); // get first number
int num2 = keyboard.nextInt(); // get second number
int sum = num1 + num2; // sum ‘em
System.out.println(“Sum of “ + num1 + “ and “ + num2
+ “ is “
+ sum);
}
}
Sample Run
• Output (user’s input is underlined):
Enter two integers: 12 3
Sum of 12 and 3 is 15
Reading Strings
Scanner reader = new Scanner(System.in);
System.out.print(“What is your first name? “); // prompt
String firstName = reader.next();
System.out.print(“Where do you work? “);
String employer = reader.nextLine();
System.out.println(“Name: “ + firstName);
System.out.println(“Employer: “ + employer);
Sample Run:
What is your first name? Mary
Where do you work? The University of Texas at Austin
Name: Mary
Employer: The University of Texas at Austin