Welcome to the 3rd Meeting of Java Discovering It’s Powers

1. Welcome to the 3rd Meeting ofJava Discovering It’s Powers Meeting Time: September Tuesday 6:30pm to 9:30pm SF Downtown Center, UC Berkeley Extension

2. Tonight’s Agenda List • API, and Java Methods • Strings • Arrays • Object Oriented Programming / Inheritance • Abstract class and interface • Discuss of Programming Assignment two

3. What material was covered in Meeting 3 ? • Idenitifiers, Tokens • Valid Java syntax and variable naming, what was white space, blocks {}, and semicolons. • Different styles of commenting • Primitives and default memory allocation for each primitive type • Looked at coding convention of capitalizing classes, and making methods to be verbs.

4. Java API • Not a lot of programmers write code from scratch. • The Java Development kid included prewritten classes and packages(collection of related classes). • Prominent Package with the Java class library include • java.lang //include System, Object, and Class • java.awt //abstract windowing toolkit • java.applet //applet, internet client application • java.net //networking application • java.io //file input,output • java.util //collections, hashtables, and Random • We shall explore different packages as we go through the class

5. == verse equal() • For example, String s1 = new String(“JDK1.4”); String s2 = new String(“JDK1.4”); if (s1 == s2) // true or false ? if (s1.equals(s2) ) //true or false ?

6. Object, equals() • In java.lang class • Has a method public boolean equals(Object obj) { } • And most of the classes like Date, String, File, and the class you define can have this method to override what happens in the a object to object comparision. This is a deep comparision verses a sallow comparasion.

7. Object, toString() • Example. Date now = new Date(); System.out.println(now); is translated into Date now = new Date(); System.out.println(now.toString() ); Where toString() has the following signature String toString() { do something} • The object class has the String toString() and is a derived method for all classes. The method just return class name and reference address

8. Abstract Classes • Number class in the java.lang package represents the abstract concept of numbers • class that you define represents an abstract concept and, as such, should not be instantiated • I.e. color, shape, food (as we eat carrots, chocolate, so we eat real representation of food)

9. Defining an abstract class • Definition of an abstract class uses the key work abstract abstract class Number { . . . } abstract class GraphicObject { int x, y; . . . void moveTo(int newX, int newY) { . . . } abstract void draw(); } • class Circle extends GraphicObject { • void draw() { . . . } • } • class Rectangle extends GraphicObject { • void draw() { . . . } • }

10. Never instantiate an abstract class • So we would never say Number a = new Number(); //Number is an abstract class • But we could say Number a; a = new BigDecimal(“99999999999999999”); or a= new Integer(“5”);

11. Interfaces • An interface defines a protocol of behavior that can be implemented • An interface is a named collection of method definitions (without implementations) • An interface can also declare constants. • A variation on the idea of abstract classes.

12. Difference between an Abstract Class and an interface • An interface cannot implement any methods, whereas an abstract class can • A class can implement many interfaces but can have only one superclass. • An interface is not part of the class hierarchy. Unrelated classes can implement the same interface

13. How to define an interface • Use the key word interface • All variables inside interface are “static final”, by default. • All methods are abstract

14. Interface that extend other interfaces • SuperInterfaces are parent interface which InterfaceName inherits from, and there could be more than one

15. In Java, there are many predefine interfaces • Runnable is an interface; Public interface Runnable { void run(); • Thread class is an implementation of Runnable interface Public class Thread extend Object implements Runnable { void run() { do something in the thread when it is awake} }

16. Can Implement many interfaces Example: public class MyApplet extends Applet implements Runnable, MouseListener { ….. }

17. Simple interface example interface SayHello { void printMessage(); } class SayHelloImpl implements SayHello { void printMessage() { System.out.println(“Hello”); } }

18. What are interfaced good for ? • Useful for • Delcaring methods that one or more classes are expected to implment • Determining an object’s programming interface without revealing the actual body of the class • Capturing similarities between unrelated classes without forcing a class relationship • Describing “function-like” object that can be passed as parameters to methods invoked on other objects.

19. Packages • Java provides a way group relayed classes. • For assignment 2, a package called flashcard could include classes like Card, Deck, and Session. • Package names are hierarchical and follow the dot convention. • Example package ucbx.financeDept; public class Employee { }

20. Defining a Package • Package statement must be being of the source file and only one package statement is allow per source file • All files with the file package name will end up in the same package • As to using the packages in other files • Example import ucbx.financeDept.*; public class Manager extends Employee { Employee [] subordinates; Manager() { do something…}; }

21. Packages and Directory structure • Packages are store in a directory tree containing the package name. package ucbx.financedept; public class Employee { } now suppose you run javac -d . Employee.java • What is the directory path for Employee.class ? • The roots of package trees to search for class files are in the CLASSPATH • The “-d” option in compiler uses the next argument as the root

22. ClassPATH and new packages created • Suppose you needed to use the Employee.class file in other class files such as the Manager.class • Manager.class has a import statement import ucbx.financeDept; • When you compile you Manager.class, there is a compile error that tells you that Employee.class “ClassNoFound”. • What is the problem ??

23. What was the error ? • Need to add directory of where you created packages to the classpath. • javac -d /home/anton/packages Employee.java • Need to add your package directory to the classpath. • Set CLASSPATH = CLASSPATH;/home/anton/packages

24. Assignment 2 :This assignment will give you practice with classes, packages, and interfaces. The goal of this assignment is to emulate a deck of flashcards. These flashcards contain a word and a definition. All the classes written will be in a package called flashcards. Here are the following classes you will write:Card: A class which represents an individual card. It will be useful to override the toString method here to provide ease of displaying a Card.Deck: A class which represents a collection of cards. The Deck class will have responsibility such as shuffling itself, flashing a card on the top of the deck(display a card and after a constant wait period, it displays the definition), adding a Card to the Deck, deleting a Card, go to the next card, go to the previous card, check if there are Cards in the Deck.

25. Session: A driver class to instaniate Deck class and present the user with a menu of the following choices: 1. Add a Card2. Delete a Card3. Show the current Card4. Move to the next Card5. Move to the previous Card6. Shuffle Deck7. Exit ProgramEnter Option[1-7]:1The user should be presented with this menu until he/she hits option 7.

26. Details about the option: • Add a Card takes in the two user input, one is the word, and another is the definition • Delete a Card takes in no input and deletes the current card • Show the current Card will display the current card and after a delay time, the definition of the card is outputted to the console. • Move to the next Card will display the next card in the deck and will set that card to be the current card. • Move to the previous Card will display the previous card in the deck and will set that card to be the current card. • Shuffle Deck will randomize the position of the Cards. • Exit Program will exit out of the program.

27. Notes about this assignment • You will need to use java.util collection classes such as Set or Vector class to create a dynamic array which can grow as you add new cards. • It will be a long assignment. • Wise to create some class diagrams to help design the implementation of program.