Understanding Interactive Programs and Input/Output in Java

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Interactive programs in Java allow users to input data through the console, which can be captured and used in the program. This involves using the Scanner class to read user input, and understanding common Scanner methods to process different types of input. Importing Java class libraries is essential for using classes like Scanner effectively. A comprehensive overview of input/output mechanisms in Java is provided in this document.

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CHAPTER 5 GC 101 Input & Output 1

INTERACTIVE PROGRAMS 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. 2

INPUT AND SYSTEM.IN 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); 3

JAVA CLASS LIBRARIES, IMPORT 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 the very top of your program import packageName.*; Scanner is in a package named java.util import java.util.*; import java.util.Scanner; To use Scanner, you must place the above line at the top of your program (before the public class header). 4

SCANNER METHODS Method Description reads a token of user input as an int reads a token of user input as a double reads a token of user input as a String reads a line of user input as a String nextInt() nextDouble() next() nextLine() Each method waits until the user presses Enter. The value typed is returned. System.out.print("How old are you? "); // prompt int age = console.nextInt(); System.out.println("You'll be 40 in " + (40 - age) + " years."); 5 prompt: A message telling the user what input to type.

COMMON SCANNER METHODS Method Scanner input = new Scanner (System.in); nextDouble( ) double d = input.nextDouble( ); nextFloat( ) float f = input.nextFloat( ); nextInt( ) int i = input.nextInt( ); next() String str = input.next(); Example 6

EXAMPLE SCANNER USAGE import java.util.*;// so that I can use Scanner public class ReadSomeInput { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("How old are you? "); int age = console.nextInt(); System.out.println(age + "... That's quite old!"); } } Output (user input underlined): How old are you? 14 7 14... That's quite old!

ANOTHER SCANNER EXAMPLE import java.util.*;// so that I can use Scanner public class ScannerSum { public static void main(String[] args) { Scanner console = new Scanner(System.in); System.out.print("Please type three numbers: "); int num1 = console.nextInt(); int num2 = console.nextInt(); int num3 = console.nextInt(); int sum = num1 + num2 + num3; System.out.println("The sum is " + sum); } } Output (user input underlined): Please type three numbers: 8 6 13 The sum is 27 8 The Scanner can read multiple values from one line.

INPUT TOKENS token: A unit of user input, as read by the Scanner. Tokens are separated by whitespace (spaces, tabs, newlines). How many tokens appear on the following line of input? 23 John Smith 42.0 "Hello world" $2.50 " 19" When a token is not the type you ask for, it crashes. System.out.print("What is your age? "); int age = console.nextInt(); Output: What is your age? Timmy java.util.InputMismatchException at java.util.Scanner.next(Unknown Source) at java.util.Scanner.nextInt(Unknown Source) ... 9

import java.util.Scanner; public class TestInput { EXAMPLE public static void main(String[] args) { Scanner input ; int area ,length, width; input = new Scanner (System.in); // creating an instance System.out.println("enter the length "); length = input.nextInt(); //reading the length from the keyboard System.out.println("Enter the Width "); width = input.nextInt(); //reading the width from the keyboard area = length * width ; System.out.println("the length is "+ length); System.out.println("the width is "+ width); System.out.println("the area is "+ area); 10 } }

OUTPUT enter the length 2 Enter the Width 3 the length is 2 the width is 3 the area is 6 11

INPUT 1. import java.util.*; Required to use the class Scanner 2. public class Example2_16 3. { 4. static Scanner console = new Scanner(System.in); 5. public static void main(String[] args) 6. { 7. int feet; 8. int inches; 9. System.out.println("Enter two integers separated by spaces."); 10. feet = console.nextInt(); // reads int 11. inches = console.nextInt(); // reads int 12. System.out.println("Feet = " + feet); 13. System.out.println("Inches = " + inches); 14. } 15. } 12

INPUT Enter two integers separated by spaces. > 23 7 Feet = 23 Inches = 7 If the user enters a non integer number for example 24w5 or 3.4 console.nextInt() will cause a program termination. 13

import java.util.*; 1. public class Example2_17 2. { 3. INPUT 5. public static void main(String[] args) static Scanner console = new Scanner(System.in); 4. { 6. String firstName; 7. String lastName; 8. int age; 9. 10. double weight; 11. 12. System.out.println("Enter first name, last name, " 13. +"age, and weight separated by spaces."); 14. 15.firstName = console.next(); 16. lastName = console.next(); 17. age = console.nextInt(); 18. weight = console.nextDouble(); 19. 20. System.out.println("Name: " + firstName + " " + lastName); 21. System.out.println("Age: " + age); 14 22. System.out.println("Weight: " + weight); 23. } 24. }