Chapter 4 Writing Classes. © 2004 Pearson Addison-Wesley. All rights reserved4-2 Writing Classes We've been using predefined classes. Now we will learn.

Chapter 4 Writing Classes

© 2004 Pearson Addison-Wesley. All rights reserved4-2 Writing Classes We've been using predefined classes. Now we will learn to write our own classes to define objects Chapter 4 focuses on:  class definitions  instance data  encapsulation and Java modifiers  method declaration and parameter passing  constructors  graphical objects  events and listeners  buttons and text fields

© 2004 Pearson Addison-Wesley. All rights reserved4-4 Writing Classes The programs we’ve written in previous examples have used classes defined in the Java standard class library Now we will begin to design programs that rely on classes that we write ourselves The class that contains the main method is just the starting point of a program True object-oriented programming is based on defining classes that represent objects with well- defined characteristics and functionality

© 2004 Pearson Addison-Wesley. All rights reserved4-5 Classes and Objects Recall from our overview of objects in Chapter 1 that an object has state and behavior Consider a “counter” like those operated by museum attendants  It’s state can be defined as the number counted so far  It’s primary behavior is that it can be incremented We can represent a counter in software by designing a class called Counter that models this state and behavior  The class serves as the blueprint for a counter object We can then instantiate as many counter objects as we need for any particular program

© 2004 Pearson Addison-Wesley. All rights reserved4-6 Classes A class can contain data declarations and method declarations int size, weight; char category; Method declarations Data declarations “Instance data,” or “Instance variables”

© 2004 Pearson Addison-Wesley. All rights reserved4-7 Classes The values of the data define the state of an object created from the class The functionality of the methods define the behaviors of the object For our Counter class, we might declare an integer that represents the current value of the count One of the methods increments that count by 1

© 2004 Pearson Addison-Wesley. All rights reserved4-8 Classes Let’s first consider how we might use the Counter class inside the main method of another class: Counter c = new Counter(); c.count = 0; c.increment(); System.out.println(c.count); Design Counter so that this gives 1 as output Warning: We won’t start by writing this class “correctly”; let’s start simply, and work up to what’s right gradually So…how do we write Counter so that it can do this?

© 2004 Pearson Addison-Wesley. All rights reserved4-9 Class Definition: First Try Before writing down the class definition, here’s a picture: Method declaration (only one for now) int count; void increment() Data declarations “Instance data,” or “Instance variables”

© 2004 Pearson Addison-Wesley. All rights reserved4-10 Now let’s write down the class definition: public class Counter { public int count; public void increment() { count = count + 1; } } Instance variable declaration Method header Class Definition: First Try When we invoke the increment() method, it changes the state of the Counter accordingly Let’s code this and run it.

© 2004 Pearson Addison-Wesley. All rights reserved4-11 Information Hiding There is a problem with doing things like this: c.count = 0; For example, suppose count is 2 and the user of the class tries to do this: c.count = 7; which makes no sense for a counter object! For this reason, we want to prohibit users of the class from directly accessing count This is an important concept in OOP called information hiding or encapsulation We hide the state of an object, and change it only via methods

© 2004 Pearson Addison-Wesley. All rights reserved4-12 Class Definition: Second Try State information is hidden inside objects by making the variables private, not public Private instance data cannot be directly accessed outside the class That is, if we made count private, trying to get the value as in c.count is syntactically illegal So let’s make count private! public class Counter { private int count; //(the rest is as before) } Information hiding

© 2004 Pearson Addison-Wesley. All rights reserved4-13 Class Definition: Access Method Well, darn it, there’s still another problem: How do we “see” the value of count ? Answer: Write a method that returns it! Data declarations “Instance data,” or “Instance variables” Method declarations int count; void increment() int getCount() Method to return the value in count

© 2004 Pearson Addison-Wesley. All rights reserved4-14 Class Definition: Access Method Well, darn it, there’s still another problem: How do we “see” the value of count ? Answer: Write a method that returns it! That method is quite simple to write: public int getCount () { return count; } And you can easily invoke it: Counter c = new Counter(); System.out.println(“count = “ + c.getCount()); c.increment(); System.out.println(“count = “ + c.getCount());

© 2004 Pearson Addison-Wesley. All rights reserved4-15 Class Definition: Mutator Method No, we’re not done yet A counter ought to be capable of being reset to 0 But since count is private, we can’t set it to 0 directly So (again) we write a method to set it to 0 Add the following method to the Counter class definition: public void reset () { count = 0; } Easy to invoke this too: c.reset();

© 2004 Pearson Addison-Wesley. All rights reserved4-16 Class Definition: Constructors Yes, we’re almost done When a counter is created, the count should be 0 As it happens, it is already 0, because int -type instance variables are automatically initialized to 0 But we would like to guarantee that with a constructor that we write ourselves For other classes, constructors will be more essential For the Counter class, it should look like this: public Counter () { count = 0; } See Counter.java and CountDemo.java Counter.java CountDemo.java

© 2004 Pearson Addison-Wesley. All rights reserved4-17 Another Example Let’s now consider the slightly more sophisticated class defined in the text Consider a six-sided die (singular of dice)  It’s state can be defined as which face is showing  It’s primary behavior is that it can be rolled We can represent a die in software by designing a class called Die that models this state and behavior  The class serves as the blueprint for a die object We can then instantiate as many die objects as we need for any particular program

© 2004 Pearson Addison-Wesley. All rights reserved4-18 Classes The picture is similar to before, but we now have different instance variables and methods: Data declarations Method declarations (not all of them) int faceValue; final int MAX; int roll() int getFaceValue() String toString()

© 2004 Pearson Addison-Wesley. All rights reserved4-19 Classes The values of the data define the state of an object created from the class The functionality of the methods define the behaviors of the object For our Die class, we might declare an integer that represents the current value showing on the face One of the methods “rolls” the die by setting that value to a random number between one and six

© 2004 Pearson Addison-Wesley. All rights reserved4-20 Classes We’ll want to design the Die class with other data and methods to make it a versatile and reusable resource Any given program will not necessarily use all aspects of a given class See RollingDice.java (page 157)RollingDice.java See Die.java (page 158)Die.java

© 2004 Pearson Addison-Wesley. All rights reserved4-21 The Die Class The Die class contains two data values  a constant MAX that represents the maximum face value  an integer faceValue that represents the current face value The roll method uses the random method of the Math class to determine a new face value There are also methods to explicitly set and retrieve the current face value at any time

© 2004 Pearson Addison-Wesley. All rights reserved4-22 The toString Method All classes that represent objects should define a toString() method The toString() method returns a character string that represents the object in some way It is called automatically when an object is concatenated to a string or when it is passed to the println() method We could have defined one for Counter : public String toString () { return Integer.toString(count); }

© 2004 Pearson Addison-Wesley. All rights reserved4-23 Constructors As mentioned previously, a constructor is a special method that is used to set up an object when it is initially created A constructor has the same name as the class, and NO return type It is the only type of method like this The Die constructor is used to set the initial face value of each new die object to one We examine constructors in more detail later in this chapter

© 2004 Pearson Addison-Wesley. All rights reserved4-24 Data Scope The scope of data is the area in a program in which that data can be referenced (used) Data declared at the class level can be referenced by all methods in that class Data declared within a method can be used only in that method Data declared within a method is called local data In the Die class, the variable result is declared inside the toString() method -- it is local to that method and cannot be referenced anywhere else

© 2004 Pearson Addison-Wesley. All rights reserved4-25 Instance Data The faceValue variable in the Die class is called instance data because each instance (object) that is created has its own version of it A class declares the type of the data, but it does not reserve any memory space for it Every time a Die object is created, a new faceValue variable is created as well The objects of a class share the method definitions, but each object has its own data space That's the only way two objects can have different states

© 2004 Pearson Addison-Wesley. All rights reserved4-26 Instance Data We can depict the two Die objects from the RollingDice program as follows: die1 5 faceValue die2 2 faceValue Each object maintains its own faceValue variable, and thus its own state Invoking roll() for die1 (as in die1.roll()) has no effect on die2

© 2004 Pearson Addison-Wesley. All rights reserved4-27 A Convenient Notation: UML UML stands for the Unified Modeling Language UML diagrams show relationships among classes and objects A UML class diagram consists of one or more classes, each with sections for the class name, attributes (data), and operations (methods) Lines between classes represent associations A dotted arrow shows that one class uses the other (calls its methods)

© 2004 Pearson Addison-Wesley. All rights reserved4-28 UML Class Diagrams A UML class diagram for the CountDemo program: CountDemo main (args : String[]) : void Counter count : int increment() : void reset () : void getCount() : int

© 2004 Pearson Addison-Wesley. All rights reserved4-29 UML Class Diagrams A UML class diagram for the RollingDice program: RollingDice main (args : String[]) : void Die faceValue : int roll() : int setFaceValue (int value) : void getFaceValue() : int toString() : String

© 2004 Pearson Addison-Wesley. All rights reserved4-30 UML Class Diagrams UML is “unified” because the notation applies to any object-oriented programming language You don’t even have to know a class definition to use the notation, and that’s part of the point StringMutation main (args : String[]) : void toUppercase() : String replace (char, char) : String substring(int, int) : String length() : int String UML allows us to think about object/class relationships abstractly

© 2004 Pearson Addison-Wesley. All rights reserved4-32 Encapsulation We can take one of two views of an object:  internal - the details of the variables and methods of the class that defines it  external - the services that an object provides and how the object interacts with other objects From the external view, an object is an encapsulated entity, providing a set of specific services, whose structure we can’t “see” These services comprise the interface to the object, that dictates how other objects interact with it

© 2004 Pearson Addison-Wesley. All rights reserved4-33 Encapsulation One object (called the client) may use another object for the services it provides (example: CountDemo is a client of a Counter object) The client of an object may request its services (call its methods), but it should not have to be aware of how those services are accomplished Any changes to the object's state (its variables) should be made by that object's methods We should make it difficult, if not impossible, for a client to access an object’s variables directly Often “should” becomes “must”; e.g., CountDemo could violate the rules of a Counter by incorrectly setting the instance variable count, as we saw earlier That is, an object must be self-governing

© 2004 Pearson Addison-Wesley. All rights reserved4-34 Encapsulation An encapsulated object can be thought of as a black box -- its inner workings are hidden from the client The client invokes the interface methods of the object, which manages the instance data Methods Data Client

© 2004 Pearson Addison-Wesley. All rights reserved4-35 Visibility Modifiers In Java, we accomplish encapsulation through the appropriate use of visibility modifiers A modifier is a Java reserved word that specifies particular characteristics of a method or data We've used the final modifier to define constants Java has three visibility modifiers: public, protected, and private The protected modifier involves inheritance, which we will discuss later

© 2004 Pearson Addison-Wesley. All rights reserved4-36 Visibility Modifiers Members of a class that are declared with public visibility can be referenced anywhere Members of a class that are declared with private visibility can be referenced only within that class Members declared without a visibility modifier have default visibility and can be referenced by any class in the same package An overview of all Java modifiers is presented in Appendix E

© 2004 Pearson Addison-Wesley. All rights reserved4-37 Visibility Modifiers Public variables violate encapsulation because they allow the client to “reach in” and modify the values directly Therefore instance variables should not be declared with public visibility It is acceptable to give a constant public visibility, which allows it to be used outside of the class Public constants do not violate encapsulation because, although the client can access it, its value cannot be changed

© 2004 Pearson Addison-Wesley. All rights reserved4-38 Visibility Modifiers Methods that provide the object's services are declared with public visibility so that they can be invoked by clients Public methods are also called service methods A method created simply to assist a service method is called a support or helper method Since a support method is not intended to be called by a client, it should not be declared with public visibility Note that most classes are declared as public since they are intended to be used elsewhere

© 2004 Pearson Addison-Wesley. All rights reserved4-39 Support Method: Example It’s a bit contrived, but suppose in Die we want to design a separate method to change the state of the object when we roll the die, and invoke it in the method roll() It might be done like this: public int roll () { //invoke support method: rollTheDie(); return faceValue; } //declare support method: private void rollTheDie () { faceValue = (int)(Math.random()*MAX) + 1; }

© 2004 Pearson Addison-Wesley. All rights reserved4-40 Visibility Modifiers publicprivate Variables Methods Provide services to clients Support other methods in the class Enforce encapsulation Violate encapsulation

© 2004 Pearson Addison-Wesley. All rights reserved4-41 Accessors and Mutators Because instance data is private, a class usually provides services to access and modify data values An accessor method returns the current value of a variable A mutator method changes the value of a variable The names of accessor and mutator methods take the form getX and setX, respectively, where X is the name of the value They are sometimes called “getters” and “setters”

© 2004 Pearson Addison-Wesley. All rights reserved4-42 Mutator Restrictions The use of mutators gives the class designer the ability to restrict a client’s options to modify an object’s state A mutator is often designed so that the values of variables can be set only within particular limits For example, the setFaceValue mutator of the Die class should have restricted the value to the valid range (1 to MAX ) We’ll see in Chapter 5 how such restrictions can be implemented

© 2004 Pearson Addison-Wesley. All rights reserved4-44 Method Declarations Let’s now examine method declarations in more detail A method declaration specifies the code that will be executed when the method is invoked (called) When a method is invoked, the flow of control jumps to the method and executes its code When complete, the flow returns to the place where the method was called and continues The invocation may or may not return a value, depending on how the method is defined

© 2004 Pearson Addison-Wesley. All rights reserved4-45 rollTheDie(); rollTheDie()roll() Method Control Flow If the called method is in the same class, only the method name is needed. Recalling slide 39:

© 2004 Pearson Addison-Wesley. All rights reserved4-46 roll() rollTheDie() rollTheDie(); die1.roll(); main Method Control Flow The called method is often part of another class or object. Again recalling slide 39:

© 2004 Pearson Addison-Wesley. All rights reserved4-47 Method Header A method declaration begins with a method header char calc (int num1, int num2, String message) method name return type parameter list The parameter list specifies the type and name of each parameter The name of a parameter in the method declaration is called a formal parameter

© 2004 Pearson Addison-Wesley. All rights reserved4-48 Method Body The method header is followed by the method body char calc (int num1, int num2, String message) { int sum = num1 + num2; char result = message.charAt (sum); return result; } The return expression must be consistent with the return type sum and result are local data They are created each time the method is called, and are destroyed when it finishes executing

© 2004 Pearson Addison-Wesley. All rights reserved4-49 The return Statement The return type of a method indicates the type of value that the method sends back to the calling location A method that does not return a value has a void return type A return statement specifies the value that will be returned return expression; Its expression must conform to the return type

© 2004 Pearson Addison-Wesley. All rights reserved4-50 Parameters When a method is called, the actual parameters or arguments in the invocation are copied into the formal parameters in the method header char calc (int num1, int num2, String message) { int sum = num1 + num2; char result = message.charAt (sum); return result; } ch = obj.calc (25, count, "Hello");

© 2004 Pearson Addison-Wesley. All rights reserved4-51 Local Data As we’ve seen, local variables can be declared inside a method Formal parameters of a method are created as automatic local variables when the method is invoked When the method finishes, all local variables are destroyed (including the formal parameters) Keep in mind that instance variables, declared at the class level, exist as long as the object exists Do not confuse instance variables (created when an object is created) with local variables and parameters (created when a method is invoked)

© 2004 Pearson Addison-Wesley. All rights reserved4-52 Bank Account Example Let’s look at another example that demonstrates the implementation details of classes and methods We’ll represent a bank account by a class named Account It s state can include the account number, the current balance, and the name of the owner An account’s behaviors (or services) include deposits and withdrawals, and adding interest

© 2004 Pearson Addison-Wesley. All rights reserved4-53 Driver Programs A driver program drives the use of other, more interesting parts of a program Driver programs are often used to test other parts of the software The Transactions class contains a main method that drives the use of the Account class, exercising its services See Transactions.java (page 172)Transactions.java See Account.java (page 173)Account.java

© 2004 Pearson Addison-Wesley. All rights reserved4-55 Bank Account Example There are some improvements that can be made to the Account class Formal getters and setters could have been defined for all data The design of some methods could also be more robust, such as verifying that the amount parameter to the withdraw method is positive

© 2004 Pearson Addison-Wesley. All rights reserved4-56 Constructors Revisited Note that a constructor has no return type specified in the method header, not even void A common error is to put a return type on a constructor, which makes it a “regular” method that happens to have the same name as the class The programmer does not have to define a constructor for a class Each class has a default constructor that accepts no parameters, and that initializes variables to “obvious” default values (e.g., 0 for int )

© 2004 Pearson Addison-Wesley. All rights reserved4-57 Problems with Existing Examples In a Die object, the setFaceValue() method can set faceValue to anything, even a value not between 1 and MAX In an Account object, the withdraw() method can cause a negative balance We need some way of preventing clients from violating the internal laws of these objects (a Die should only have values from 1 to MAX ; an Account should have a non-negative balance) We will find out how to do this in Chapter 5 Meanwhile, there is quite a bit we can do just with objects and graphics.

© 2004 Pearson Addison-Wesley. All rights reserved4-59 Graphical Objects Some objects contain information that determines how the object should be represented visually Most GUI components are graphical objects We can have some effect on how components get drawn We did this in Chapter 2 when we defined the paint method of an applet Let's look at some other examples of graphical objects

© 2004 Pearson Addison-Wesley. All rights reserved4-60 Smiling Face Example The SmilingFace program draws a face by defining the paintComponent method of a panel See SmilingFace.java (page 177)SmilingFace.java See SmilingFacePanel.java (page 178)SmilingFacePanel.java The main method of the SmilingFace class instantiates a SmilingFacePanel and displays it The SmilingFacePanel class is derived from the JPanel class using inheritance

© 2004 Pearson Addison-Wesley. All rights reserved4-61 Smiling Face Example Every Swing component has a paintComponent method The paintComponent method accepts a Graphics object that represents the graphics context for the panel We define the paintComponent method to draw the face with appropriate calls to the Graphics methods Note the difference between drawing on a panel and adding other GUI components to a panel

© 2004 Pearson Addison-Wesley. All rights reserved4-62 Splat Example The Splat example is structured a bit differently It draws a set of colored circles on a panel, but each circle is represented as a separate object that maintains its own graphical information The paintComponent method of the panel "asks" each circle to draw itself See Splat.java (page 180)Splat.java See SplatPanel.java (page 181)SplatPanel.java See Circle.java (page 182)Circle.java

© 2004 Pearson Addison-Wesley. All rights reserved4-64 Graphical User Interfaces A Graphical User Interface (GUI) in Java is created with at least three kinds of objects:  components  events  listeners We've previously discussed components, which are objects that represent screen elements  labels, panels, buttons, text fields, menus, etc. Some components are containers that hold and organize other components  frames, panels, applets, dialog boxes

© 2004 Pearson Addison-Wesley. All rights reserved4-65 Events An event is an object that represents some activity to which we may want to respond For example, we may want our program to perform some action when the following occurs:  the mouse is moved  the mouse is dragged  a mouse button is clicked  a graphical button is clicked  a keyboard key is pressed Events often correspond to user actions, but not always

© 2004 Pearson Addison-Wesley. All rights reserved4-66 Events and Listeners The Java standard class library contains several classes that represent typical events Components, such as a graphical button, generate (or fire) an event when it occurs A listener object "waits" for an event to occur and responds when the system (not the programmer!) invokes an appropriate method of that object We (the programmer) design the method bodies of listener objects to take the actions we want to take when an event occurs

© 2004 Pearson Addison-Wesley. All rights reserved4-67 Events and Listeners Component A component object may generate an event Listener A corresponding listener object is designed to respond to the event Event When the event occurs, the component calls the appropriate method of the listener, passing an object that describes the event Listener is “attached” to (in Java terminology, “registered with”) the component

© 2004 Pearson Addison-Wesley. All rights reserved4-68 GUI Development Generally we use components and events that are predefined by classes in the Java class library Therefore, to create a Java program that uses a GUI we must:  instantiate and set up the necessary components (establish the “look” of the GUI)  implement listener classes for any events we care about, and register instantiations of those classes with the components (establish the “feel” of the GUI) Let's now explore some new components and see how this all comes together

© 2004 Pearson Addison-Wesley. All rights reserved4-70 Buttons A push button (or just plain “button”) is a component that allows the user to initiate an action by pressing a graphical button using the mouse A button is defined by the JButton class It generates an action event The PushCounter example displays a button that increments a counter each time it is pushed See PushCounter.java (page 186)PushCounter.java See PushCounterPanel.java (page 187)PushCounterPanel.java

© 2004 Pearson Addison-Wesley. All rights reserved4-71 Push Counter Example The components of the GUI are the button, a label to display the counter, a panel to organize the components, and the main frame The PushCounterPanel class represents the panel used to display the button and label The PushCounterPanel class is derived from JPanel using inheritance The constructor of PushCounterPanel sets up the elements of the GUI and initializes the counter to zero

© 2004 Pearson Addison-Wesley. All rights reserved4-72 Push Counter Example The ButtonListener class is the listener for the action event generated by the button It is implemented as an inner class, which means it is defined within the body of another class That facilitates the communication between the listener and the GUI components - inner class objects can access the instance variables Inner classes should only be used in situations where there is an intimate relationship between the two classes and the inner class is not needed in any other context

© 2004 Pearson Addison-Wesley. All rights reserved4-73 Push Counter Example Listener classes are written by implementing a listener interface The ButtonListener class implements the ActionListener interface An interface is a list of methods that the implementing class must define The only method in the ActionListener interface is the actionPerformed method The Java class library contains interfaces for many types of events See Chapter 6 for more about interfaces

© 2004 Pearson Addison-Wesley. All rights reserved4-74 Push Counter Example The PushCounterPanel constructor:  instantiates the ButtonListener object  registers the listener with the button by the call to addActionListener When the user presses the button, the button component creates an ActionEvent object and calls the actionPerformed method of the listener Pushing the button invokes actionPerformed, not the programmer! The actionPerformed method increments the counter and resets the text of the label

© 2004 Pearson Addison-Wesley. All rights reserved4-75 Text Fields Let's look at another GUI example that uses another type of component A text field allows the user to enter one line of input If the cursor is in the text field, the text field component generates an action event when the enter key is pressed See Fahrenheit.java (page 190)Fahrenheit.java See FahrenheitPanel.java (page 191)FahrenheitPanel.java

© 2004 Pearson Addison-Wesley. All rights reserved4-76 Fahrenheit Example Like the PushCounter example, the GUI is set up in a separate panel class The TempListener inner class defines the listener for the action event generated by the text field The FahrenheitPanel constructor instantiates the listener and adds it to the text field When the user types a temperature and presses enter, the text field generates the action event and calls the actionPerformed method of the listener The actionPerformed method computes the conversion and updates the result label

© 2004 Pearson Addison-Wesley. All rights reserved4-77 Adding Machine Example Let’s do something more useful See Adder.java and AdderPanel.javaAdder.java AdderPanel.java In AdderPanel there are two buttons (one to add, one to reset) and one text field The buttons are both placed in a sub-panel, and so is the text field In this example we explicitly use layout managers (BorderLayout and FlowLayout) - see Chapter 6 Each button needs its own listener, hence there is an inner class for each button

© 2004 Pearson Addison-Wesley. All rights reserved4-78 Summary Chapter 4 focused on:  class definitions  instance data  encapsulation and Java modifiers  method declaration and parameter passing  constructors  graphical objects  events and listeners  buttons and text fields

Chapter 4 Writing Classes. © 2004 Pearson Addison-Wesley. All rights reserved4-2 Writing Classes We've been using predefined classes. Now we will learn.

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Chapter 4 Writing Classes. © 2004 Pearson Addison-Wesley. All rights reserved4-2 Writing Classes We've been using predefined classes. Now we will learn.

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Presentation on theme: "Chapter 4 Writing Classes. © 2004 Pearson Addison-Wesley. All rights reserved4-2 Writing Classes We've been using predefined classes. Now we will learn."— Presentation transcript:

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