Applications in Java

1. Applications in Java Towson University 2013. *Ref: http://chortle.ccsu.edu/java5/index.html

2. The Guessing Game

3. The Guessing Game • Summary: • The game involves a ‘game’ object and three ‘player’ objects. • The game generates a random number between 0 and 9, and the three player objects try to guess it. • Classes: GuessGame.class, Player.class, GameLauncher.class • The Logic: • The GameLauncher class is where the application starts; it has the main() method. • In the main() method, a GuessGame object is created, and its startGame() method is called. • The GuessGame object’s startGame() method is where the entire game plays out. It creates three players, then “thinks” of a random number. • It then asks each player to guess, checks the results, and either prints out information about the winning player(s) or ask them to guess again.

4. Classes GameLauncher main(String[] args) GuessGame p1 p2 p3 startGame() Player number guess() The number this player guesses Instance variables for the three players Make a GuessGame object and tells it to startGame Method for making a guess

5. GuessGame class public class GuessGame { Player p1; Player p2; Player p3; public void startGame() { p1 = new Player(); p2 = new Player(); p3 = new Player(); int guessp1 = 0; int guessp2 = 0; int guessp3 = 0; boolean p1isRight = false; boolean p2isRight = false; boolean p3isRight = false; int targetNumber = (int) (Math.random() * 10); System.out.println("I'm thinking of a number between 0 and 9..."); while (true) { System.out.println("Number to guess is " + targetNumber); p1.guess(); p2.guess(); p3.guess(); guessp1 = p1.number; System.out.println("Player one guessed " + guessp1); guessp2 = p2.number; System.out.println("Player two guessed " + guessp2); guessp3 = p3.number; System.out.println("Player three guessed " + guessp3); if (guessp1 == targetNumber) { p1isRight = true; } if (guessp2 == targetNumber) { p2isRight = true; } if (guessp3 == targetNumber) { p3isRight = true; } if (p1isRight || p2isRight || p3isRight) { System.out.println("We have a winner!"); System.out.println("Player one got it right? " + p1isRight); System.out.println("Player two got it right? " + p2isRight); System.out.println("Player three got it right? " + p3isRight); System.out.println("Game is over."); break; } else { System.out.println("Players will have to try again."); } // end of if/esle } // end of while loop } // end of StartGame() } // end of class GuessGame

6. Player and GameLauncher public class Player { int number = 0; public void guess() { number = (int) (Math.random()*10); System.out.println("I'm guessing " + number); } // end of guess() } // end of class Player • public class GameLauncher { • public static void main(String[] args) { • GuessGame game = new GuessGame(); • game.startGame(); • } // end of main() • } // end of class GameLauncher

7. Numbers and Statics

8. Math class • Math class doesn’t have any instance variables. • If you try to make an instance of class Math: Math mathObject = new Math();  You’ll get error! • Methods in the Math class don’t use any instance variable values. (known as a kind of utility method) • Because the methods are ‘static’, you don’t need to have an instance of Math. • All you need is the Math class. int x = Math.round(42.2); int y = Math.min(56, 12); int z = Math.abs(-343);

9. Regular (non-static) vs. static methods • The keyword static lets a method run without any instance of the class. • A static method means “behavior not dependent on an instance variable, so no instance/object is required. Just the class.” • Static method • public static int min(int a, int b) { • // returns the lesser of a and b • } // end of min() • Regular (non-static) method • public class Song { • String title; • public Song(String t) { • title = t; • } // end of Song() • public void play() { • SongPlayer player = new SoundPlayer(); • player.playSound(title); • } // end of paly() • } // end of class Song Song title play() Math min() max() abs() random() … Math.min(42,36); s1 = new Song(); s2 = new Song(); s1.play(); s2.play();

10. Calling a method • Static mehtod  using a class name Math.min(88, 77); Math.max(88, 77); Math.round(88.77); Math.abs(-88); Math.random()*10; • Non-static method  using a reference variable name Song s1 = new Song(); s1.play();

11. Using non-static variables from inside a static method • The compile thinks, “I don’t know which object’s instance variable you’re talking about!” • public class Duck { • private int size; • public static void main(String[] args) { • System.out.println("Size of duck is " + size); • } // end of main() • public void setSize(int s) { • size = s; • } // end of setSize() • public int getSize() { • return size; • } // end of getSize() • } // end of class Duck 1 error found: Error: non-static variable size cannot be referenced from a static context

12. Using non-static methods from inside a static method • The compile thinks, “I don’t know which object’s instance variable you’re talking about!” • public class Duck { • private int size; • public static void main(String[] args) { • System.out.println("Size of duck is " + getSize()); • } // end of main() • public void setSize(int s) { • size = s; • } // end of setSize() • public int getSize() { • return size; • } // end of getSize() • } // end of class Duck 1 error found: Error: non-static method getSize() cannot be referenced from a static context

13. Static variable • Value is the same for All instances of the class • class Duck2 { • int duckCount; • public Duck2() { • duckCount++; • } // end of Duck2() • } // end of class Duck2 The constructor Duck2() always set duckCount to 1 each time a Duck2 was made after resetting to 0. • public class Duck3 { • private int size; • private static int duckCount = 0; • public Duck3() { • duckCount++; • } // end of Duck3() • public void setSize(int s) { • size = s; • } • public int getSize() { • return size; • } • } // end of class Duck3 duckCount keep incrementing each time the Duck3() constructor runs, because duckCount is static and won’t be reset to 0.

14. Static variable vs. instance variable • A Duck object doesn’t keep its own copy of duckCount (static). • Duck objects all share a single copy of it. • Each Duck object has its own size variable, but only one copy of the duckCount in the class Duck size static duckCount getSize() setSize() Size : 22 duckCount : 4 Size : 20 duckCount : 4 Duck object Duck object Size : 8 duckCount : 4 Size : 12 duckCount : 4 Duck object Duck object

15. static final variables: constants • A variable marked final means that – once initialized – it can never change. • The value of the static final variable will stay the same as long as the class is loaded. • Math.PI public static final double PI=3.141592653689793; • Naming convention:  Constant variable names should be in all caps! • Any code can access it. • No need for an instance of class Math • PI doesn’t change

16. Wrapping a primitive

17. Wrapper class for every primitive type • Wrapper class for every primitive type is in the java.lang package • public class Num { • public void doNums() { • ArrayList<Integer> listOfNumbers = new ArrayList<Integer>(); • listOfNumbers.add(3); • int num = listOfNumbers.get(0); • } // end of doNums() • } // end of class Num Compiler does all the wrapping for you from number 3 (int) to Integer. Compiler automatically unwraps the Integer object to int so you can assign the int value directly to a primitive.