1. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Advanced Java Programming CSE 7345/5345/ NTU 531 Session 5 Welcome Back!!!

2. Liang, Oreilly, Herbert Schildt, Joseph O’Neil claurent@engr.smu.edu Chantale Laurent-Rice Welcome Back!!! trice75447@aol.com Office Hours: by appt 3:30pm-4:30pm SIC 353

3. Liang, Oreilly, Herbert Schildt, Joseph O’Neil In-class Warm-up Write a method to reverse a string without using the reverse method in the StringBuffer class. The method signature is as follows: public static String reverse(String s) { // Implement it }

4. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Answer public class InClassNotSoWarm { public static void main(String[] args) { System.out.println(reverse("acb")); } public static String reverse(String s) { // Implement it StringBuffer strBuf = new StringBuffer(); for (int i = s.length() - 1; i >= 0; i--) { strBuf.append(s.charAt(i)); } return strBuf.toString(); }

5. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Chapter 8 Class Inheritance and Interfaces Objectives  Understand the concept of class inheritance and the relationship between superclasses and subclasses.  Create new classes from existing classes.  Learn to use the super keyword.  Learn to use three modifiers: final, protected, and abstract.  Create abstract classes.  Understand polymorphism and object casting.  Understand the concept of interfaces.  Become familiar with inner classes.

6. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Key Concepts The class derived from the superclass is called the subclass. Sometimes a superclass is referred to as a parent class or a base class, and a subclass is referred to as a child class, an extended class, or a derived class.

7. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Key Concepts Subclasses usually have more functionality than their superclasses. The keywords super and this are used to reference the superclass and the subclass, respectively.

8. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Key Concepts The final modifier is used to prevent changes to a class, a method, or a variable. A final class cannot be extended. A final method cannot be overridden. A final variable is a constant.

9. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Key Concepts The abstract modifier is used to design generic superclasses. An abstract class cannot be instantiated. An abstract method contains only the method description without implementation. Its implementation is provided by subclasses.

10. Liang, Oreilly, Herbert Schildt, Joseph O’Neil final One of the common uses of final is to create named constants. For example, the following application illustrates this use of final. It creates a variable x whose value cannot be changed.

11. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Example: Final final class V1 { } class V2 extends V1 { } class FinalClass { public static void main(String args[]) { V1 obj = new V1(); } //Will not compile because cannot inherit from final V1 //class V2 extends V1 // cannot inherit from final V1 //class V2 extends V1 because final cannot be extended

12. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Example: final class L { static final int x = 5; } class FinalVariable { public static void main(String[] args) { System.out.println(L.x); }

13. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Inheritance Using inheritance, you can derive one class, called the derived class or subclass, from another, called the base class or superclass. The idea here is that you add what you want to the new class to give it more customized functionality than the original class.

14. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Inheritance Inheritance is the most crucial concepts in object-oriented programming, and it has a very direct effect on how you design and write your Java classes. Inheritance is a powerful mechanism that means when you write a class you only have to specify how that class is different from some other class; Inheritance will give you automatic access to the information contained in that other class.

15. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Inheritance Inheritance is a concept in object-oriented programming in a strict hierarchy. Each class in the hierarchy has superclasses (classes above it in the hierarchy) and any number of subclasses ( classes below it in the hierarchy). Subclasses inherit attributes and behavior from their superclasses.

16. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Inheritance Class A Class B Class CClass DClass E Class A is the superclass of B Class B is the subclass of A Class B is the superclass of C, D, E Class C, D and E are subclasses of B

17. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Subclasses Inheritance allows one class to reuse the functionality provided by its superclasses. The extends clause in a class declaration establishes an inheritance relationship between two classes. Syntax: class clsname2 extends clsname1 { //class body } If the extends clause is omitted from the declaration of a class, the Java compiler assumes that Object is its superclass.

18. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Example: Inheritance class X { } class X1 extends X { } class X2 extends X { } class X11 extends X1 { } class X12 extends X2 { } class X21 extends X2 { } class X22 extends X2 { }

19. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Example con’t class InheritanceHierarchy { public static void main(String[] args) { X x; System.out.println(“Instantiating X “); x = new X(); System.out.println(“Instantiating X1 “); x = new X1(); System.out.println(“Instantiating X110 “); x = new X11(); System.out.println(“Instantiating X12 “); x = new X12(); System.out.println(“Instantiating X2 “); x = new X2(); }

20. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Answer

21. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Inheritance When a class doesn’t do exactly what you want, you can build a new class based on it. Your class inherits the original class’s methods and instance variables, to which you can add your own code and data.

22. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Inheritance Inheriting classes is a great way to reuse existing code. The term code reuse does not refer to a text editor’s cut and paste commands. Developing reusable code means writing and debugging classes, and then building new classes from them.

23. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Show the output of running the class Test in the following code lines: interface A { void print();} class C {} class B extends C implements A { public void print() { } } public class Test { public static void main(String[] args) { B b = new B(); if (b instanceof A) System.out.println("b is an instance of A"); if (b instanceof C) System.out.println("b is an instance of C"); } a.Nothing. b.b is an instance of A. c.b is an instance of C. d.b is an instance of A followed by b is an instance of C.

24. Liang, Oreilly, Herbert Schildt, Joseph O’Neil toString() method Previous slide answer is D. (b is an instance of A b is an instance of C) Press any key to continue... The toString() method returns a string equivalent of the current object. Its signature String toString() It is common for classes to override this method so useful information may be provided via the print() and println() method

25. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Inheritance Inheritance and variables The class inherits the state and behavior defined by all of its superclasses. State is determined by variables; Behavior is determined by methods. Therefore, an object has one copy of every instance variable defined not only by its class but also by every superclass of its class.

26. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Inheritance and Variables A static or instance variable in a subclass may have the same name as a superclass variable. In that case, the variable hides the superclass variable. These two variables may have the same type or different types.

27. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Inheritance and variables The following application demonstrates a class inheritance hierarchy. Class W extends Object and has one instance variable of type float. Class X extends W and has one instance variable of type StringBuffer. Class Y extends X and has one instance variable of type String. Class Z extends Y and has one instance variable of type Integer.

28. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Inheritance and Variable con’t An object of class Z has the instance variables that are defined in all of the superclasses. The main() method instantiates class Z with the new operator, initializes the instance variables, and displays their values.

29. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Example: Inheritance and variables class W { float f; } class X extends W { StringBuffer sb; } class Y extends X { String s; } class Z extends Y { Integer i; }

30. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Example: Inheritance and Variables class Wxyz { public static void main(String[] args) { Z z = new Z(); z.f = 4.567f; z.sb = new StringBuffer(“abcde”); z.s = “ Learning this Inheritance Stuff”; z.i = new Integer(41); System.out.println(“z.f = “ + z.f); System.out.println(“z.sb = “ + z.sb); System.out.println(“z.s = “ + z.s); System.out.println(“z.i = “ + z.i); }

31. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Answer

32. Liang, Oreilly, Herbert Schildt, Joseph O’Neil What is the output of running the class C. public class C { public static void main(String[] args) { Object[] o = {new A(), new B()}; System.out.print(o[0]); System.out.print(o[1]); } class A extends B { public String toString() { return "A"; } class B { public String toString() { return "B"; } a. AB b. BA c. AA d. BB e. None of above

33. Liang, Oreilly, Herbert Schildt, Joseph O’Neil What is the output of running class C? class A { public A() { System.out.println( "The default constructor of A is invoked"); } class B extends A { public B(String s) { System.out.println(s); } public class C { public static void main(String[] args) { B b = new B("The constructor of B is invoked"); } a. none b. "The constructor of B is invoked" c."The default constructor of A is invoked" "The constructor of B is invoked" d."The default constructor of A is invoked"

34. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Variable Hiding Previous slide answer is C. Class E declares an instance variable named x of type int. Class F is a subclass of E and also declares an instance variable named x of type String The main() method first creates an objects of class F. A reference to this object is assigned to a local variable named f of type F.

35. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Variable Hiding f.x is, therefore, of type String. Next, an object of class E is created. A reference to this object is assigned to a local variable named e of type E. The expression e.x is, therefore, of type int.

36. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Variable Hiding Note: The declaration of x in class F hides the declaration of x in class E.

37. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Example: Variable Hiding class E { int x; } class F extends E { String x; }

38. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Example con’t class Ef { public static void main(String args[]) { F f = new F(); f.x = “This is a string”; System.out.println(“f.x = “ + f.x); E e = new E(); e.x = 45; System.out.println(“e.x = “ + e.x); }

39. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Answer f.x = This is a string e.x = 45 Press any key to continue...

40. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Method Overriding class A1 { void hello() { System.out.println("hello from A1"); } class B1 extends A1 { void hello() { System.out.println("hello from A1"); } class C1 extends B1 { void hello() { System.out.println("Hello from C1"); } public class MethodOverriding { public static void main(String[] args) { C1 obj = new C1(); obj.hello(); }

41. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Another Warm-up Create a class named IntClass as follows: public class IntClass { private int number; public IntClass(int number) { // Implement it } public boolean isPrime() { // Implement it } public static boolean isPrime(int num) { // Implement it } public boolean isEven() { // Implement it } public boolean equals(int anotherNum) { // Implement it } public String toString() { // Implement it }

42. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Answer public class NotToKool { public static void main(String[] args) { IntClass n1 = new IntClass(5); System.out.println("n1 is even? " + n1.isEven()); System.out.println("n1 is prime? " + n1.isPrime()); System.out.println("5 is prime? " + IntClass.isPrime(15)); } class IntClass { private int number; public IntClass(int number) { this.number = number; } public boolean isPrime() { return isPrime(number); }

43. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Answer con’t public static boolean isPrime(int num) { if ((num ==1) || (num ==2)) { return true; } for (int i=2; i<=num/2; i++) { if (num%i == 0) return false; } return true; }

44. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Answer con’t public boolean isEven() { return number%2 == 0; } public boolean equals(int anotherNum) { return number == anotherNum; } public String toString() { return number + " "; }

45. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Chapt 10 Applets How applets and Applications Are Different? Java applications are standalone Java programs that can be run by using just the Java interpreter. Java applets, however, are from inside a WWW browser.

46. Liang, Oreilly, Herbert Schildt, Joseph O’Neil 10 Applets How Applets and applications are different? Java applications are standalone Java programs that can be run by using just the Java interpreter Java applets, however, are from inside a WWW browser, they have access to the structure the browser provides: an existing window, an event-handling and graphics context, and the surrounding user interface.

47. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Example: Java Applet import java.applet.Applet; import java.awt.Graphics; public class FirstApplet extends Applet { public void paint(Graphics g) { g.drawString(“This is my first applet!”, 20, 100); } Compile this code and then enter the following command appletviewer First Applet.java

48. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Creating Applets To create an applet, you create a subclass of the class Applet The Applet class, part of the java.applet package, provides much of the behavior your applet needs to work inside a Java-enabled browser. Applets also take strong advantage of Java’s Abstract Windowing Toolkit (AWT), which provides behavior for creating graphical user interface (GUI)-based applets and applications:drawing to the screen; creating windows, menu bars, buttons, check boxes, and other UI elements; and managing user input such as mouse clicks and keypresses.

49. Liang, Oreilly, Herbert Schildt, Joseph O’Neil The keyword “super” It is possible to access a hidden variable by using the super keyword, as follows: super.VarName Here, varName is the name of the variable in the superclass. This syntax may be use to read or write the hidden variable.

50. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Creating applets To create an applet, you create a subclass of the class Applet. The applet class, part of the java.applet package provides much of the behavior your applet needs to work inside a java-enabled browser.

51. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Creating applets con’t Applets also take strong advantage of Java’s Abstract Windowing Toolkit and applications: drawing to the screen: creating windows, menu bars, buttons, check boxes, and other UI elements; and managing user input such as mouse clicks and keypresses. The AWT classes are part of the java.awt.package.

52. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Major applet activities To create a basic Java application, your class has to have one method, main() method, with a specific signature. Then, when your application runs, main() is found and executed, and from main() you can set up the behavior that your program needs to run.

53. Liang, Oreilly, Herbert Schildt, Joseph O’Neil Creating applets con’t Applets are similar but more complicated - and in facts, applets don’t need a main() method at all. Applets have many different activities that correspond to various major events in the life cycle of the applet. For example, initialization, painting, and mouse events. Each activities has a corresponding method, so when an event occurs, the browser or other Java-enabled tool calls those specific methods.

54. Liang, Oreilly, Herbert Schildt, Joseph O’Neil 5 important Applets methods Initialization- occurs when the applet is first loaded or reloaded, similar to the main() method. public void init(){... } Starting- start the applet (can happen many different times during an applet’s lifetime. public void start(){... }

55. Liang, Oreilly, Herbert Schildt, Joseph O’Neil 5 important Applets methods Painting- is the way the applet actually draws something on the screen, be it text, a line, a colored background, or an image. –public void paint(Graphics g){…..}

56. Liang, Oreilly, Herbert Schildt, Joseph O’Neil 5 important applets methods Stopping- goes hand in hand with starting. Stopping occurs when the reader leaves the page that contains a currently running applet, or you can stop the applet yourself by calling stop(). public void stop(){….} Destroying- enables the applet to clean up after itself just before it is freed or the browser exits. –public void destroy(){…. }