EEC 484 Computer Networks

1. EEC 484Computer Networks Java Tutorial #1 Wenbing Zhao Cleveland State University wenbing@ieee.org

2. Outline • Dissect a simple Java program • Object-oriented programming concepts • Exceptions handling • Packages • Materials taken from Sun’s Java Tutorial: • http://download.oracle.com/javase/tutorial/java/index.html • Another good Java tutorial: • http://www.cafeaulait.org/course/ EEC484 Computer Networks

3. public class Calculator { private int m_num1; / / first number private int m_num2; // second number private char m_op; // operator: add, sub, mul, div private int m_result; // result of the calculation public Calculator() { m_num1 = 0; m_num2 = 0; m_op = ' '; m_result = 0; } public void enter1stNumber(int num1) { m_num1 = num1; } public void enter2ndNumber(int num2) { m_num2 = num2; } public void enterOperator(char op) { m_op = op; } EEC484 Computer Networks

4. /* return the result of the calculation */ public int calculate() { switch(m_op) { case '+': return m_num1 + m_num2; case ' - ': return m_num1 - m_num2; case '*': return m_num1 * m_num2; case '/': return m_num1 / m_num2; default: return 0; } } EEC484 Computer Networks

5. public stat ic void main(String[] args) { Calculator c = new Calculator(); c.enter1stNumber(1); c.enterOperator('+'); c.enter2ndNumber(1); int result = c.calculate(); System.out.println("1+1="+result); } } EEC484 Computer Networks

6. How to learn a programming language quick and dirty? • Search for APIs and examples on the Web! • Tasks: • get input from standard input (stdin) • Make sure it is not empty • For number input, make sure it is an integer • For operator input, make sure it is a valid operator • Deal with exceptions EEC484 Computer Networks

7. Object Concept • Real-world objects: state and behavior • State of a bicycle: current gear, current pedal cadence, current speed • Behavior of a bicycle: changing gear, changing pedal cadence, applying brakes EEC484 Computer Networks

8. Software Object Fields (state) • A software object mimics a real-world object • Member variables (or fields): store the state of the object • Member functions (or Methods): expose the object behavior • Methods operate on an object's internal state Methods (behavior) EEC484 Computer Networks

9. Object Oriented Concept • Object-oriented communication: Different objects communicate (interact) with each other by calling methods defined in the objects • Data encapsulation: Hiding internal state and requiring all interaction to be performed through an object's methods EEC484 Computer Networks

10. public class Bicycle { private int speed = 0; private int gear = 1; public Bicycle() { gear = 1; speed = 0; } public void changeGear(int newValue) { gear = newValue; } public void spe edUp(int increment) { speed = speed + increment; } public void applyBrakes(int decrement) { speed = speed - decrement; } } Class • A class is the blueprint from which individual objects are created EEC484 Computer Networks

11. Access Level Modifiers • At the top level—public, or package-private (no explicit modifier) • At the member level—public, private, protected, or package-private (no explicit modifier) Access Levels EEC484 Computer Networks

12. Creating Objects Bicycle b = new Bicycle() • Declaration: associate a variable name with an object type • Instantiation: The new keyword is a Java operator that creates the object • Initialization: The new operator is followed by a call to a constructor, which initializes the new object EEC484 Computer Networks

13. Using Objects • Calling an object’s methods: • Referencing an object’s fields: Bicycle b = new Bicycle() b.speedup(10); objectReference.fieldName EEC484 Computer Networks

14. Returning a Value from a Method • A method returns to the code that invoked it when it • completes all the statements in the method, • reaches a return statement, or • throws an exception (covered later), whichever occurs first return returnValue; return; If no return value is needed EEC484 Computer Networks

15. Inheritance • Different kinds of objects often have a certain amount in common with each other • Mountain bikes, road bikes, all share the characteristics of bicycles (current speed, current pedal cadence, current gear) • Yet each also defines additional features that make them different • Road bikes have drop handlebars; some mountain bikes have an additional chain ring, giving them a lower gear ratio EEC484 Computer Networks

16. class MountainBike Bicycle { extends // new fields and methods defining a moun tain bike would go here } Inheritance • Classes can inherit commonly used state and behavior from other classes Bicycle is the superclass of MountainBike, RoadBike, and TandemBike • In the Java, each class is allowed to have one direct superclass, and each superclass has the potential for an unlimited number of subclasses EEC484 Computer Networks

17. Inheritance Super class Subclasses EEC484 Computer Networks

18. Inheritance • A class that is derived from another class is called a subclass (also a derived class, extended class, or child class) • The class from which the subclass is derived is called a superclass (also a base class or a parent class) • A subclass inherits all the members (fields, methods, and nested classes) from its superclass • Constructors are not members, so they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass EEC484 Computer Networks

19. public class MountainBike extends Bicycle { // the MountainBike subclass adds one field publi c int seatHeight; // the MountainBike subclass has one constructor public MountainBike(int startHeight) { super(); seatHeight = startHeight; } // the MountainBike subclass adds one method public void setHeight(int newVal ue) { seatHeight = newValue; } } Inheritance • MountainBike inherits all the fields and methods of Bicycle and adds the field seatHeight and a method to set it EEC484 Computer Networks

20. Inheritance • You can assign a subclass to a superclass variable • Bicycle b = new MountainBike(10); • But not the other way around. You have to cast a superclass object to a sublass object • MountainBike mb = (MountainBike)b; EEC484 Computer Networks

21. public abstract class GraphicObject { // declare fields // declare non - abstract methods abstract void draw(); } Abstract Class • An abstract class is a class that is declared abstract: it may or may not include abstract methods EEC484 Computer Networks

22. Abstract Class • An abstract method is a method that is declared without an implementation (without braces, and followed by a semicolon • If a class includes abstract methods, the class itself must be declared abstract • Abstract classes cannot be instantiated, but they can be subclassed EEC484 Computer Networks

23. class Circle extends GraphicObject { abstract class GraphicObject { void draw() { int x, y; ... ... } void moveTo(int newX, int newY ) { void resize() { ... ... } } abstract void draw(); } abstract void resize(); class Rectangle extends GraphicObject { } void draw() { ... } void resize() { ... } } Abstract Class: Example EEC484 Computer Networks

24. interface Bicycle { void changeCadence(int newValue); void changeGear(int newValue); void speedUp(int increment); void applyBrakes(int decrement); } Interface • Objects define their interaction with the outside world through the methods that they expose. • Methods form the object's interface with outside world • An interface is a group of related methods with empty bodies EEC484 Computer Networks

25. class ACMEBicycle Bicycle { implements // remainder of t his class implemented as before } Interface • To implement this interface, use the implements keyword in the class declaration: EEC484 Computer Networks

26. Interface • Interfaces form a contract between the class and the outside world • This contract is enforced at build time by the compiler • If your class claims to implement an interface, all methods defined by that interface must appear in its source code before the class will successfully compile EEC484 Computer Networks

27. Exception • An exception is an event, which occurs during the execution of a program, that disrupts the normal flow of the program's instructions EEC484 Computer Networks

28. Exception • Code that might throw certain exceptions must be enclosed by either of the following: • A try statement that catches the exception. The try must provide a handler for the exception • A method that specifies that it can throw the exception. The method must provide a throws clause that lists the exception public void writeList() throws IOException { … } EEC484 Computer Networks

29. try { } catch (FileNotFoundException e) { System.err.println("FileNotFoundException: " + e.getMessage()); throw new SampleException(e); } catch (IOException e) { System.err.println("Caught IOException: " + e.getMessage()); } finally { if (out != null) { System.out.println("Closing PrintWriter"); out.close(); } } Exception • Try-Catch-Finally EEC484 Computer Networks

30. public Object pop() { Object obj; if (size == 0) { throw new EmptyStackException(); } obj = objectAt(size - 1); setObjectAt(size - 1, null); size -- ; return obj; } How to Throw an Exception • All methods use the throw statement to throw an exception. The throw statement requires a single argument: a throwable object EEC484 Computer Networks

31. Throwable Class and Its Subclasses EEC484 Computer Networks

32. Package • A package is a namespace that organizes a set of related classes and interfaces • Think of packages as being similar to different folders • The Java platform provides an enormous class library (a set of packages) • This library is known as the "Application Programming Interface", or "API" for short • Java platform API specification: http://java.sun.com/reference/api/ • Example packages: java.lang (Fundamental classes), java.io (Classes for reading and writing) EEC484 Computer Networks

33. Benefits of Creating a Package • You and other programmers can easily determine that these types are related • You and other programmers know where to find types that can provide graphics-related functions • The names of your types won't conflict with the type names in other packages because the package creates a new namespace • You can allow types within the package to have unrestricted access to one another yet still restrict access for types outside the package EEC484 Computer Networks

34. Creating a Package • To create a package, you choose a name for the package and put a package statement with that name at the top of every source file that contains the types (classes, interfaces, enumerations, and annotation types) that you want to include in the package • The package statement (for example, package graphics;) must be the first line in the source file • There can be only one package statement in each source file EEC484 Computer Networks

35. in the Draggable.java file // package graphics; public interface Draggable { . . . } in the Graphic.java file // package graphics; public abstract class Graphic { . . . } in the Circle.java file // package graphics; public class Circle extends Gra phic implements Draggable { . . . } in the Rectangle.java file // package graphics; public class Rectangle extends Graphic implements Draggable { . . . } EEC484 Computer Networks

36. // in the Rectangle.java file package graphics; public class Rectangle() { . . . } Managing Source and Class Files • Put the source code for a class, interface, enumeration, or annotation type in a text file whose name is the simple name of the type and whose extension is .java. • For example: EEC484 Computer Networks

37. Managing Source and Class Files • Then, put the source file in a directory whose name reflects the name of the package: .....\graphics\Rectangle.java • The qualified name of the package member and the path name to the file are parallel class name graphics.Rectanglepathname to file graphics/Rectangle.java • Produce a jar file for your classes • Under the parent directory of graphics • jar –cf graphics.jar graphics EEC484 Computer Networks

38. Using a Package • To use a public package member from outside its package, you must do one of the following: • Refer to the member by its fully qualified name: System.out.println(“Hello World”); • Import the package member import graphics.Rectange; Rectangle r = new Rectangle(); • Import the member's entire package import graphics.*; Circle c = new Circle(); EEC484 Computer Networks

39. Accessing a Package (Compilation and Execution) • For compilation: • javac –classpath graphics.jar YourClass.java • For execution: • java –classpath graphics.jar YourClass EEC484 Computer Networks

40. Questions • Real-world objects contain ___ and ___. • A software object's state is stored in ___. • A software object's behavior is exposed through ___. • Hiding internal data from the outside world, and accessing it only through publicly-exposed methods is known as data ___. • A blueprint for a software object is called a ___. EEC484 Computer Networks

41. Questions • Common behavior can be defined in a ___ and inherited into a ___ using the ___ keyword. • A collection of methods with no implementation is called an ___. • A namespace that organizes classes and interfaces by functionality is called a ___. • The term API stands for ___? EEC484 Computer Networks

42. public interface SomethingIsWrong { void aMethod(int aValue){ System.out.println("Hi Mom"); } } public interface Marker { } Questions • What is wrong with the following interface? • Is the following interface valid? EEC484 Computer Networks

43. try { } finally { } Questions • Is the following code legal? EEC484 Computer Networks