More About Objects and Methods

Transcription

1 More About Objects and Methods Chapter 5 Chapter 5 1

2 Auto-Boxing and Unboxing and Wrapper Classes Many Java library methods work with class objects only Do not accept primitives Use wrapper classes instead! Java 5.0 will auto-box your primitives for you (and unbox them as necessary) Works on parameters as well Integer intobject = 5; int i = intobject; Chapter 5 2

3 Operation Overloading It is sometimes useful to have two methods of the same name but accept different parameters (very common in Math library) May not differ only by return type or variable name Constructors can be overloaded like other methods Valid: public String f(int x) { } public String f(double x) { } public String f(float f) { } Invalid: public int f(int x) { } public double f(int x) { } Chapter 5 3

4 Constructors Like methods, constructors can have parameters Used to initialize class variables, set up anything else necessary Different constructors may initialize object to different values No return type Java provides a default constructor for you if you do not declare any constructor yourself Chapter 5 4

5 Copy Constructors and Memory Management Copy constructors are useful in making a deep copy of an object Shallow copy: copy primitives, simple reference assignment with = for objects What problems may crop up with using simple assignments =? Deep copy: copy over all values and objects explicitly into new memory Chapter 5 5

6 public class A { Memory Management } private int x; public A() { x=0; } public A(int x) { set(x); } public A(A a) { x = a.get(); } public void set(int x) { this.x = x; } public int get() { return x; } A a1 = new A(); A a2 = new A(2); a1 = a2; a2.set(3); A a3 = new A(a1); a1.set(4); a1.get()? a2.get()? a3.get()? Chapter 5 6

7 static keyword Useful for methods which do not need to access class variables or methods Called using the class name Think: Math library Static methods cannot access non-static class methods or variables! (compilation error) Used for class variables which are shared over instances of the class (three people share a bowl of chips, one person eats some chips, all parties lose chips) class A { private static int a; } public void seta(int x) { a = x; } public int geta() { return a; } A a1 = new A(); A a2 = new A(); a1.seta(5); System.out.println( a: +a2.geta()); Chapter 5 7

8 static Memory Management public class A { private static int x; public A() { x=0; } public A(int x) { set(x); } public A(A a) { x = a.get(); } A a1 = new A(); A a2 = new A(2); a1 = a2; A.set(3); A a3 = new A(a1); a1.set(4); a3.set(1); } public static void set(int x) { this.x = x; } public static int get() { return x; } a1.get()? a2.get()? a3.get()? Chapter 5 8

9 null keyword Indicates the non-existence of an object (think address 0x ) Can be used to represent any object type A null object can NOT call any methods This will result in a runtime error called a NullPointerException Get in the habit of checking against the null object! Since a reference address is simply a number, use == or!= to compare Chapter 5 9

10 null Memory Management public class A { } private int x; public A() { x=0; } public A(int x) { set(x); } public A(A a) { x = a.get(); } public void set(int x) { this.x = x; } public int get() { return x; } A a1 = null; a1 = new A(); A a2 = new A(a1); A a3 = null; if (a3 == null) a3 = new A(); else a3.set(3); a1.set(4); a1.get()? a2.get()? a3.get()? Chapter 5 10

11 Case Study: Point.java Chapter 5 11

12 Packages: Outline Packages and Importing Package Names and Directories Name Clashes Chapter 5 12

13 Packages A package groups and names a collection of related classes. It can serve as a library of classes for any program. The collection of classes need not reside in the same directory as a program that uses them. The classes are grouped together in a directory and are given a package name. Chapter 5 13

14 Packages, cont. The classes in a package are placed in separate files. A file name is the same as the name of the class except that each that each file contains the following at the start of the file package Package_Name; example package general.utilities; Chapter 5 14

15 Directories Directories are called folders in some operating systems. To understand packages, you need to know about path names for directories, and you need to know how your operating system uses a path variable. These are operating system topics and their details depend on the operating system. Chapter 5 15

16 Importing A program or class definition can use all the classes in a package by placing a suitable import statement at the start of the file containing the program or class definition. import Package_Name; This is sufficient even if the program or class definition is not in the same directory as the classes in the package. Chapter 5 16

17 Package Names and Directories The package name must tell the compiler where to find the classes in the package. This is, it must provide the compiler with the path name for the directory containing the classes in the package. To find the directory, Java needs the name of the package the directories listed in the value of the class path variable. Chapter 5 17

18 Package Names and Directories, cont. The value of the class path variable tells Java where to begin its search for the package. The class path variable is part of the operating system, not part of Java. It contains path names and a list of directories, called the class path base directories. Chapter 5 18

19 Package Names and Directories, cont. The package name is a relative path name that assumes you start in a class path base directory and follow the path of subdirectories given by the package name. example class path base directory: \javastuff\libraries example package classes \javastuff\libraries\general\utilities Chapter 5 19

20 Package Names and Directories, cont. Example (required) package name general.utilities Chapter 5 20

21 Package Names and Directories, cont. The class path variable allows you to list more than one base directory, typically separating them with a semicolon. example \javastuff\libraries;f:\morejavastuff When you set or change the class path variable, include the current directory (where your program or other class is located) as one of the alternatives Chapter 5 21

22 Package Names and Directories, cont. Typically, the current directory is indicated by a dot example \javastuff\libraries;f:\morejavastuff;. Omitting the dot limits the locations you can use for packages and can interfere with programs that do not use packages. Chapter 5 22

23 Name Clashes Packages can help deal with name clashes which are situations in which two classes have the same name. Ambiguities can be resolved by using the package name. examples mystuff.coolclass object1; yourstuff.coolclass object2; Chapter 5 23

24 (optional) Graphics Supplement: Outline Adding Buttons Event-Driven Programming Programming Buttons Icons Changing Visibility Chapter 5 24

25 Adding Buttons A component in an applet that looks like a push button and can do something when it is clicked is referred to as a button. Buttons are added to applets the same way labels are added. But, unlike labels, actions can be associated with buttons. Chapter 5 25

26 Programming Tip When developing an applet, determine first what the applet will look like. Then determine the actions that will be performed by the applet (such as what happens when a button is clicked). Chapter 5 26

27 Creating Buttons example JButton sunnybutton = new JButton( Sunny ); (Until buttons are programmed, they do not perform any action besides depressing and returning to their undepressed state.) Chapter 5 27

28 Creating Buttons, cont. class PreliminaryButtonDemo Chapter 5 28

29 Creating Buttons, cont. Chapter 5 29

30 Event-Driven Programming Applets use events and listeners. An event is an object that represents some action such as clicking a mouse button. An object fires (or generates) an event. An object that can fire an event can have one or more listener objects, specified by the programmer. Chapter 5 30

31 Event-Driven Programming, cont. A listener object can have methods called event handlers, defined by the programmer, that specify what happens when events are sent to the listener. sent means that some method is invoked automatically with the event object as an argument. Chapter 5 31

32 Event-Driven Programming, cont. Chapter 5 32

33 Event-Driven Programming, cont. Events determine the order in which things happen. The next thing to happen is determined by the next event. Chapter 5 33

34 Event-Driven Programming, cont. class ButtonDemo Chapter 5 34

35 Event-Driven Programming, cont. Chapter 5 35

36 Programming Buttons The applet class definition needs to know two things: for each button, which objects are listeners (called registering the listener) the defined method(s) to be invoked when the event is sent to the listener Chapter 5 36

37 Programming Buttons, cont. registering the listener sunnybutton.addactionlistener(this); The class ButtonDemo itself is the listener class. Chapter 5 37

38 Programming Buttons, cont. Chapter 5 38

39 Listeners Different kinds of components require different kinds of listener classes. Buttons generate action events which are handled by action listeners. An action listener is an object of type ActionListener. ActionListener is not a class, but is instead an interface which we will discuss in Chapter 7. Chapter 5 39

40 Listeners, cont. Add the phrase implements ActionListener to the beginning of the class definition Define a method named actionperformed. Often it is convenient to put the method actionperformed in the applet it is intended to change. Because the applet itself is the action listener, the action event goes to the applet s object, where it is passed automatically to the method actionperformed. Chapter 5 40

41 Listeners, cont. Method actionperformed typically needs to know the source of the action event. e.getactioncommand() returns the string written on the button which can be used to determine the source. An import statement is needed to define an action listener class. import java.awt.event.*; Chapter 5 41

42 Applets and Constructors Applets normally have no constructors. Method init is used for any needed initialization. Chapter 5 42

43 Icons An icon typically is a small picture. Pictures can be produced in several formats for display on the screen (e.g. GIF and JPEG). These pictures can be used as the basis for an icon. A label or a button can include a string and/or an icon. Chapter 5 43

44 Icons, cont. Example ImageIcon dukeicon = new ImageIcon( duke_waving.gif ); nicelabel.seticon(dukeicon); Chapter 5 44

45 Icons, cont. class IconDemo Chapter 5 45

46 Icons, cont. Chapter 5 46

47 Icons, cont. Buttons can have icons. ImageIcon smileyfaceicon = new ImageIcon( smiley.gif ); sunnybutton.seticon(smileyfaceicon); Chapter 5 47

48 Icons, cont. Chapter 5 48

49 Changing Visibility Labels, buttons, and other components have a method setvisibility to make the component visible or invisible. If the argument is true, the component is visible. If the argument is false, the component is invisible (and no longer present). Chapter 5 49

50 Changing Visibility class VisibilityDemo Chapter 5 50

51 Summary You have learned more techniques for programming with classes and objects. You have learned about static methods and static variables. You have learned to define constructor methods. You have learned about packages and import statements. Chapter 5 51

52 Summary, cont. You have learned about the top-down design technique. You have learned techniques for testing methods (including the use of stub methods and driver programs). (optional) You have learned to add buttons and icons to applets. You have learned the basics of event driven programming. Chapter 5 52