1. Scanner class for input Instantiate a new scanner object Scanner in = new Scanner(System.in); Getting input using scanner – int i = scanner.nextInt() – double d = scanner.nextDouble(); – String s = scanner.nextLine(); Differences from IO.read methods – There is no popup window – You just enter by typing at the bottom – You need to prompt the user what to enter

2. Command line Input Using command line from Jgrasp – Click on build, then on run arguments – Enter the arguments that you want separated by spaces – Run the program and the program will use those arguments Using the real command line – In Windows, click on start, then on run, then type cmd and click OK. You will be able to type Windows commands – Type: java – Your program will run and use those command line arguments

3. Command Line Example 1.In Jgrasp, click on build, and then on run arguments, and enter 1 2 3 4 5 into the text box at the top 2.Enter the following program public class CommandLine { public static void main(String[] args) { int sum = 0; for (int i=0; i<args.length; i++) { sum += Integer.parseInt(args[i]); } System.out.println("Sum = " + sum); } } 3.Compile and run, the output should be: sum = 15 Note: parseInt is a static method that converts a string to an integer

4. Multiple Classes 1.Some applications can have hundreds of classes that work together to solve a problem 2.Each class is entered as a separate.java source file. 3.Compiling the main program will also compile all the classes it needs. Design Challenge: How do we define an appropriate number of classes to create a structure for an application. This is a skill you learn in other Computer Science classes Example: Creating a phone book (Following slides) – Class for a person – Class for a list of persons – Main class So far our applications had only a single class

5. The Person class public class Person {private String name, number; // Instance Variables public Person(String name, String number) // Constructor {this.name = name; this.number = number; } public String toString() // Return nicely formatted string {String spaces = “ ”; String s = (name + spaces).substring(0,spaces.length()); String t = (number + spaces).substring(0,spaces.length()); return s + “ ” + t; }// End of toString method }// End of Person class Note: We don’t want outside class methods accessing the instance variables, hence private Note: The toString() method is handy as you will see Note: The strings s and t mean that all Person outputs will be formatted the same way

6. Class for list of Persons public class PersonList {private Person[] persons; private int howMany; public PersonList(int size) { persons=new Person[size]; howMany=0; } public boolean addPerson(String person, String number) {if (howMany == persons.length) return false; else persons[howMany++] = new Person(person, number); return true; } public String toString() { String str = “Phone Book Listing\n”; for (int i=0; i<howMany; i++) str += persons[i].toString() + “\n”; return str;}

7. Main class public class Main {private static PersonList personList; public static void main(String[] args) {String phone, name; Scanner in = new Scanner(System.in); personList = new PersonList(10); do { System.out.print(“Enter name: ” ); name = in.nextString(); System.out.print(“Enter number: ”); phone = in.nextString(); } while (personList.addPerson(name, phone)); System.out.println(personList); } Input a group of persons, and then print out the list

8. Final Thoughts Using multiple classes makes the application more general – The Person class can be used by other programs – The details of the Person class is internal. Users only have to know how to call its methods – We did this from day one without knowing it Examples: System.out.println(), Math.round(), and IO.readString() The private modifier is important – Normally make instance variables visible only where they are declared – This makes for easier modifications to an application – It enables the principle of limiting scope The purpose of a constructor is to initialize an object General Rule: Use the static modifier only when necessary