M255 Object Oriented Programming with Java

Transcription

1 Week 4 M255 Object Oriented Programming with Java This lecture note covers unit 4. Introduction: Prepared by: Dr. Bayan Abu Shawar, AOU-Jordan This unit will cover: Main structure of a java program and class; Writing new class with its attributes and methods using BlueJ editor; Creating new behaviour for objects through writing new methods or changing existing once; Introducing instance variables (Attributes) and their accessor methods (setters & getters); Introducing some of access modifiers: public and private; Exploring more complex methods, including those with return values and arguments; Introducing the principles of reuse and encapsulation, and data hiding; 1. Main Structure of a class in Java In object-oriented programming classes are used to group related variables and functions. A class is a collection of a fixed number of components (members). The general syntax of defining a class is: public class class_name //indicate beginning of a class private attribute1; private attribute2; public method1; public method2; //indicate ending of a class The attributes and methods (messages) are known as a class member list. If a member of a class is an instance variable (Attribute), it is declared just like any other variable (i.e. int position;) but without initializing in declaration parts. Access modifier: tells the Java compiler what other objects can send the corresponding class member. public, private: are called access modifiers, it is related to accessing the members of the class, who is allowed to access that member: 1

2 private: class members are accessed in within the same class. So members are invisible to other classes, and instances of other classes cannot directly access those members, even the subclass will not be able to access private member class from parent class directly. public: class members are accessible outside the class, or visible to all classes. Let us define a class called JRectangle that has following member list: Attributes: x, and y as float; Methods: the accessor methods for x and y; and a method named area that returns the area of a rectangle object. So to define the class with these attributes: public class JRectangle private float x, y;.. You define the attributes as any other variable you determine datat_type then attribute_name followed by semicolon. Usually the attributes in a class are defined as private which means only these attributes are accessible inside this class only. Note that: you could not give initial value to attribute upon creation so for example: int x=0; is not a valid statement, the initializing should be done through constructor, as you learned in Unit 3. Methods: A method is the piece of code that is executed when an object receives a particular message. A method can be thought of as the way the behavior in response to a message is provided, which is the concrete implementation of a message. Now, it s time to define methods where objects of this class can respond to. The general syntax for defining a method in Java is: public Return_data_type method_name(arguments) // begin of message code S1; s2; // end of message code 2

3 Each method is composed of mainly two parts: Method header and method body a. Method header (method declaration): The method header has three parts: Public Return_value_type Method_name( parameter list) 1. Accessor modifier: public or private. (methods are usually public) 2. Return-value-type: states the type of the value returned as a message answer from this method. This type is either to be void (nothing returned) or any other type such as: int, char, or a class name such as Frog, etc. 3. Method_name(arguments): method name is the same as message name, and should be followed by parentheses to distinguish it from variable name, sometimes there are arguments inside parentheses or non. b. Method body The method body is the set of declarations and Java statements that constitutes the method, each statement is ended by semicolon. The method body is enclosed by braces (curly brackets). The syntax in Java is: public Return_value_type method_name( parameter list) //method header Declarations and statements //method body Return back to JRectnagle class, the methods of this class will be: public void setx(float); //method prototype public void sety(float); public float getx(); public float gety(); public float area(); Usually a method is preceded by comments that describe in plain English what this message do. 3

4 The general structure of writing these methods inside the class is as follows: public class JRectangle private float x, y; // attributes public void setx(float a) x = a; public void sety(float b) y = b; public float getx() return x; public float gety() return y; public float area() return (x*y); In order to use the JRectangle class, you need to open another class, which will have main method. Then inside the main method write the statements as you used to do in OUWorkspace, so declare instances of this class and call the methods as below: public class JRectangle_test public static void main( String args[]) //begin of main() JRectangle rectangle = new JRectangle(); float z; rectangle.setx((float)5.5); rectangle.sety(4); z = rectangle.area(); System.out.println( The area is +z); //output statement //end of main() //end of class 4

5 Important Notices: The main program is a class by itself; you do not need to bother what is the meaning of main program header now. Each method has a method signature which is composed of method name, and type of arguments if found. So: o Method signature of: public void setx(float a) is setx(float); o Method signature of: public float area() is area() Two or more methods in the same class cannot have the same signature: public void dosomething(int m) public int dosomethong(int c) Both have the same signature dosomethong(int), so Java compiler would report an error. The argument in the message-send such as: rectangle.setx(5); which is 5, is known as the actual argument: that is the actual argument sent to receiver. To execute this message at run time, this message invokes the method setx(int a) that is found in the class, the argument that is found in the method header is known as formal argument (i.e. a). The actual and formal arguments should be the same in number of parameters, order, and data type. The type of actual argument should be compatible with type of formal ones. Return statements should be the last one in the method that returns an answer. Accessor methods: setters and getters methods. Accessor modifiers: public, and private. Current state of an object is determined by values of its attributes (instance variables). The object oriented language is used to achieve: Encapsulation: the ability to combine state and behaviour in a single unit object. Object encapsulates state and behaviour. Abstraction: separating the design details from the implementation Data hiding: an object becomes as a black box, with access to encapsulated data (Attributes) being possible only through a limited set of public method. Inheritance: create new classes from existing classes. Polymorphism: the ability to use the same expression to denote different operations 5

6 Exercise one: Launch BlueJ software, then: 1. Project New Project. Figure 1 will appear, enter project name and folder, let us name it as rectangle_1 Figure 1. saving new project in BlueJ 2. Press on new class from left hand pane. Figure 2 will appear where you need to insert the class name: JRectangle, be sure the option class is selected, then press OK. Figure 2. Create new class and give it a name 3. The JRectangle class will appear in BLueJ editor which is shaded in orange. Double click on it and write the class JRectnagle int it. 4. Compile and save the class 5. Click on New class again, name it as : JRectangle_test 6. Double click to open it then write the main method inside it. 6

7 7. Compile and save 8. Select JRectnagle_Test rightclick void main() OK 9. The result will appear in another window. 10. The BlueJ editor will look like as figure 3. Figure 3. BlueJ with the two classes we use Note that: even though, you did not write a constructor, an automatic constructor will be generated for any class if programmer did not do so. The initialization process this automatic constructor will do is based on attributes (instance variables) type, so any number will be initialized to 0 or 0.0 based if it is integer or real; the character and String will be initialised to, and consequently. To be sure: double click on JRectangle class, then on the right hand side of window there is view option implementation, open the arrow and select interface. You will see the documentation for this class where the constructor is generated. 7

8 Activity One: 1. Launch BlueJ, click Project Open project Unit4_Project_1. You will see three classes: Toad, Frog, and HoverFrog. 2. Double click on Frog class and see its implementation as in figure 4. Figure 4. Frog class implemtation 3. Notice the statement: public class Frog extends OUAnimatedObject The keyword extends means inherit, so Frog is a sub class of super class OUAnimatedObject. The OUAnimatedObject class is responsible of presenting Frog instance moving in Amphibian application. 8

9 4. You will notice that each method is preceded by comments, then method header, and method body. For example the Frog constructor is: * Constructor for objects of class Frog which initialises * colour to green and position to 1. public Frog() super(); this.colour = OUColour.GREEN; //colour = OUClolur.GREEN this.position = 1; super(): this statement invokes the constructor of upper class. this. : is known as a pseudo-variable, and it refers the receiver of the message who caused the execution of the method. And you cannot change the object to which this refers. colour and position: are two instance variables that are declared as private in Frog class so they can be accessed directly inside the class members, and they were having initial values. Any method that is written in Frog class is preceded by comments describing what this method do, then method heard and finally method body. o Comment: is enclosed by and which will be used later in documentation. It contains the description in plain English of what the method does. o Method header: tell the Java compiler important information about the method. o Method body: is the Java code that tells the compiler what should happen when the method is executed. 5. Add a new method to Frog class called: catchfly(),the new behaviour is that: an object receives catchfly() will perform jump(), then croak(), and then moves one position to the right. Solution: Before then end of the class add the following: /* * Performs jump, then croaks and finally moves one position to the right Public void catchfly() this.jump(); this.croak(); this.right(); 9

10 6. Save the class Frog and close it. What did you notice? You will notice that class Frog and HoverForg are shaded now, this means there is some modification and the class should be recompiled, this modification could be: adding new method as we did in step 6, or modifying an existing one. The Sub-class usually needs to recompile if any modification happened to super class. 7. Click on Frog class then press on compile button at left hand Pane, or from Tools menu. If no errors appear, then Frog class is ready to be used. 8. Open OUWorkspace from Tools to write Java statements in it, or make a new class name it as Test_Frog, which includes the main method, then write Java statements inside it. public class Test_Frog public static void main( String args[]) //begin of main()... //end of main() //end of class Write the following statements: Declare an object of Frog class called it Kermit. Send message left() to Kermit object Send message catchfly() to Kermit object Return position of kermit object When you finish, compile class, then close the file. Select Test_Frog class and from right click select void main then enter. Solution: public class Test_Frog public static void main( String args[]) //begin of main() Frog kermit = new Frog(); kermit.left(); kermit.catchfly(); System.out.println(kermit.getPosition()); //end of main() //end of class 10

11 OR simply in OUWorkspace you can directly write: Frog kermit = new Frog(); Kermit.left(); Kermit.catchFly(); kermit.getpositon(); Open Graphical display to see effect. 9. Add a new method to class Frog name it as yellow() which sets the colour of the receiver to yellow. * Sets the colour of the receiver to yellow public void yellow() this.setcolour(oucolour.yellow); 10. In the OUWorkspace, declare an object of type Frog name it frog1, and call yellow() message. See effect on graphical representation. Frog frog1 = new Frog(); Frog1.yellow(); Activity Two: Write a new method called: doubleleft(), for the class Toad. This method will decrement the value of instance variable position by 4. (use the same Unit4_Project_1). There are three ways to write this method as appear in forma, formb, and FormC, inside Toad class. Form A: * Move receiver left twice Public void doubleleft() this.setposition(this.getposition()- 4 ); 11

12 Form B: Form C: * Move receiver left twice Public void doubleleft() this.left(); this.left(); * Move receiver left twice Public void doubleleft() this.position = this.position-4; this.update( position ); The differences between these forms are in the reusability issue as follows: When you add a new method to a class, you can reduce some of the complexity of the new method, and also make new method easier to maintain, by reusing methods that have already been developed and tested. All previous forms will give the right answer now. But if Toad class designers change their mind, and decreed the toads move left 3 stones at a time instead of 2, so forma will not work correctly, you need to replace 4 by 6. If you have more than one method whose result depends on move left written in the same way as forma, you need to change it all and recompile. However, formb, will behave correctly; It will always produce the same effect as two moves left. So designer could change the meaning of left() as they want, but FormB will always produce the correct behaviour. In this case only body of left() method is changed but any other method that reuse left() will not require any changes at all. The difference between formc, and others is that in fromc you used the instance variable (attribute) position directly, where froma, and formb uses accessor methods. Actually the left() of Toad classmethod body is based on accessor methods as follows: public void left() this.setposition(this.getposition()-2); 12

13 Direct access to instance variables can take less processing time than using accessor methods. However, o Using accessor methods is more economical in terms of coding: if you change the behaviour of any setters or getters, the modifications done only once, then other methods that reuse the setters and getters will not effect o So accessor method aid reuse. Another benefit of using form B or accessor methods, as you know, the OUWorkspace is connected with a graphical display that shows the graphical effects on Amphibian window, in order to refresh the Amphibian window in case any change happened to current state of an object, the method update() is used for this purpose: to alert Amphibian window to refresh. update() is used in case of formc, where in formb it will be automatically called within left() method that invokes setposition() which in turn invokes update() in its body, and in forma, it will be called from setter method setposition(), no need to write it in specific. The same in case of setcolour(), setposition(), and setheight(), all setters invoke update() in their methods body. If you forgot to add update() in case of formc, the position will change but no effect will be shown in Amphibian window. As a result the best form of the above that will save time and effort in case of any change happen will be formb. Always try to reuse existing methods, especially the methods that based on setters and getters ones. Add formb to class Toad, then recompile the class. Using OUWorkspace: declare an object of toad class name it toad1, and send it left() message then send it doubleleft()message and see Amphibian effects. Toad toad1=new Toad(); toad1.left(); toad1.doubleleft(); In general, as a programmer you can reuse some of the existing methods, or the Java libraries: such as String class we used before. 13

14 Notice the import statements at the beginning of Frog class: this is an example of reusing some Java libraries.(i.e import ou.*; ) This is a special library that has all the projects that you will deal with in this module, in addition to Amphibian graphical representation. Activity Three: Open project: Unit4_Project_2. 1. Declare another instance variable (Attribute) flycount which will be used to record number of times the message catchfly() has been sent to a Frog object. Since it will count number of times, it should be integer. 2. Initialise this flycount to be zero. (This should be done in constructor). 3. Declare setters and getters of attribute flycount. 4. Update method catchfly() so this flycount attribute will be incremented inside catchfly(). Solution: 1. Search in Frog class for private attribute, add: private int flycount; 2. Go to Frog() constructor an add: flycount = 0; so Frog() constructor becomes: public Frog() super(); this.colour = OUColour.GREEN; this.position = 1; flycount = 0; 3. * set flycount of receiver to argument value a * public void setflycount(int a) flycount = a ; * return value of flycount public int getflycount() return this.flycount; 14

15 4. public void catchfly() Activity Four this.jump(); this.croak(); this.right(); this.setflycount(this.getflycount()+1); Open project: Uni4_Project_3 from block 1 1. Add new method positionreport() that returns position of a Frog object as report not as integer. public String positionreport() return "The position is " + this.getposition(); 2. Define a new object of Frog class name it kermit, then call getposition(), and name it as positionreport(). String frogposition; Frog kermit = new Frog(); frogposition = kermit.positionreport(); 3. Define a similar method heightreport() in HoverFrog class that returns a report about height. *Returns the String "The height is " concatenated with *the value of the height of the receiver. public String heightreport() return "The height is " + this.getheight(); 4. Define an object named Wezzy of HoverFrog class, and call up(), then call heighreport(). String frogheight; HoverFrog wizzy = new HoverFrog(); wizzy.up(); frogheight = wizzy.heightreport(); 15

16 Activity Five: Open project Unit4_Project_4. Save it as Unit4_Project_4_Act15. Note that Frog class has twomethods samecolouras(): * Sets the colour of the receiver to the argument's colour. public void samecolouras (Frog afrog) this.setcolour(afrog.getcolour()); * Sets the colour of the receiver to the argument's colour. public void samecolouras (Toad atoad) this.setcolour(atoad.getcolour()); 1. Execute the following statements: Frog kermit = new Frog(); Frog gribbit = new Frog(); Toad toad1 = new Toad(); kermit.setcolour(oucolour.red); gribbit.samecolouras(kermit); gribbit.samecolouras(toad1); HoverFrog wizzy = new HoverFrog(); gribbit.samecolouras(wizzy); 2. Open Frog class, and delete samecolouras(toad toad1) from it. The try the statements in (1) again. You should have found that the message gribbit.samecolouras(toad1); no longer works and you get the error message Semantic error: Message samecolouras(toad) not understood by class 'Frog' However, the message gribbit.samecolouras(wizzy); should still work correctly. This is because wizzy is a HoverFrog object which could be assigned to a parent class one. So in case of message-send: gribbit.samecolouras(wizzy) It will invoke method: samecolouras(frog afrog) 16

17 3. Write a new method setpositionandcolour() in Frog class which takes two arguments, colour, and integer. *Sets the position of the receiver to the argument * aposition and the colour of the receiver to the *argument acolour. public void setpositionandcolour(int aposition, OUColour acolour) this.setposition(aposition); this.setcolour(acolour); 4. Try to execute: (a) frog1.setpositionandcolour(5, OUColour.BLUE); (b) frog3.setpositionandcolour(oucolour.magenta, 4); (a) Will be executed correctly, but (b) will produce an error as type mismatch. Documenting Methods In the previous methods we developed, and the way the methods are written in provided classes, we found that comments has special format that it should be enclosed within and. This comment in such format is used by a program called javadoc. The javadoc program picks up information from specially formatted comments and other parts of the class code such constructor and method header to create an HTML file, which described the class in a standard way. This documentation is useful for human readers to know what is the class and its methods, and interface of these methods. You can vie documentation of whole project by: Tools Project documentation And if you want to see it in a class level, each class alone: 1. Double click on class 2. When it is opened, select interface from view menu on the top right side. Important Notice: Try the activities in section 7 of unit 4 by yourself. Also read glossary, summary, and try all SAQs, and exercises in this unit. 17

18 Class Work: Question 1: The following is the method home() in the class Frog. *Resets the receiver to its "home" position of 1. public void home() this.setposition(1); Write down: (a) the comment; (b) the method header; (c) the method body. ANSWER... (a) The comment is: *Resets the receiver to its "home" position of 1. (b) The method header is: public void home() (c) The method body is: this.setposition(1); Question 2: What is the difference between a message and a method? ANSWER... A method is the piece of code (in the case of M255, Java code) that is executed when an object receives a particular message. In other words, when you send a message to an object, it is the corresponding method code that is executed at run-time. Question 3: What do you need to do to add a new message to the protocol of a class? ANSWER... Adding a message to a protocol requires writing a new method for the class. The editor is used to do this. 18

19 Question 4: How can an object send a message to itself? ANSWER... By using the special reference variable this as the receiver of the message. Question 5: What is the purpose of the each statement in the following constructor body? The constructor is as follows. *Constructor for objects of class Frog which initialises colour *to green and position to 1. public Frog() super(); this.colour = OUColour.GREEN; this.position = 1; Solution... super(): call constructor of super class. this.colour = OUColour.GREEN; This statement sets the instance variable colour to the initial colour OUColour.GREEN. this.position = 1; This statement sets the instance variable position to the initial home position, 1. The effect of these two statements is to initialise the two instance variables, colour and position. Initialising instance variables is commonly done by the constructor. 19

20 Question 5: Explain what happen when the message left() from Frog class is sent to Kermit object which is of Frog class. Solution... The message-send kermit.left() causes the code of method left(), which is shown below, to be executed. public void left() this.setposition(this.getposition() 1); This entails three steps: The body of the method getposition() is executed first. The code for the method is given below. public void getposition() return this.position; Notice the use of this to reference the object that was sent the getposition() message, which enables the value of the instance variable position stored for that object to be accessed. The value of position is then returned as the message answer. The next step is to take 1 away from the value that has just been returned, and use this value as the argument to this.setposition(). The final step is to execute the method setposition(). The code for the method is given below. public void setposition(int aposition) this.position = aposition; Again this is used, so that the instance variable position for kermit will be set to the value just calculated in the previous step. Question 6: Write down the method header for a method that has the name mymethod, is accessible to any object, has no arguments, and returns an integer value. ANSWER... public int mymethod() 20

21 Question 7: Write a new method called distancefromhome() for the Frog class, which will return the value of the position of the receiver minus 1 (1 being the home position). Solution... *Return the position of the receiver minus 1 (the home position). public int distancefromhome() return (this.getposition() 1); Question 8: What do you think will happen when you execute the following statements in the OUWorkspace? Frog frog1 = new Frog(); frog1.setposition(oucolour.red); Solution... OUColour.RED is not an integer, so an error message will be displayed in the Display Pane of the OUWorkspace. In fact, the actual error message would be: Semantic error: Message setposition(ou.oucolour) not understood by class 'Frog'. Question 9: Write down the receivers and the arguments of the following message-sends. (a) frog1.samecolouras(frog3); (b) frog4.setposition(3); ANSWER... (a) frog1 is the receiver and frog3 is the argument. (b) frog4 is the receiver and 3 is the argument. Question 10: Write a method increaseposition() in Frog class, such that it increases the value of position attribute by amount defined in method argument step. Solution *Increases the value of the position of the receiver by the value of the argument step. public void increaseposition(int step) this.setposition(this.getposition() + step); 21

22 Question 11: Describe what happens when the following message is sent to the Frog object referenced by frog2, whose instance variable position has the value 6. frog2.increaseposition(4); Solution... On receiving the message increaseposition(), the object referenced by frog2 causes the method increaseposition() to execute. The method s formal argument, step, is set to the value of the message s actual argument, which is 4. The message getposition( ) produces an answer that is the value of the current position of the receiver, frog2, i.e. the integer 6. The integer 6 is then added to the actual argument, 4, by means of the arithmetic operator +, and results in the integer 10. The integer 10 becomes the actual argument for the setposition() message sent to the receiver this, so the instance variable position of the object referenced by frog2 is set to 10. Question 12: What is the signature of: public void setpositionandcolour(int aposition, OUColour acolour) Solution: The signature is: setpositionandcolour(int, OUColour) Question 13: How is encapsulation implemented in Java? ANSWER... In Java you can define classes that are templates for the creation of objects. These objects then encapsulate both state and behaviour. Question 14: How do you implement data hiding in Java? ANSWER... Data hiding is implemented by declaring instance variables as private. Question 15: Write the header of a class Circle that inherits class Shape. Solution: public class Circle extends Shape 22

23 Home Work: Assume you have the following classes: Class: Square float x Class: Rectangle float x,y Class: Triangle float x,y area(), setx(), getx() area(), sety(), setx(), gety(), getx() area(), setx(), getx(), Sety(), gety() 1. Find common attributes and behavior, and insert them in a super class named: Shape. Draw the new class diagram representing inheritance relationship. Note that the area() has different behaviour in each class as follows: Square area = x * x Triangle area = 0.5 * x * y Rectangle area = x * y 2. Represent the super class Shape with its sub-classes Rectangle, Square, and Triangle in Java language. 3. Define three objects as follows: asquare of Square class arectangle of Rectangle class atriangle of Triangle class Then invoke setters for each object, and area of each. Use System.out.println() to print area of each shape. Write those statements inside the following main method: public class Test_Shape public static void main( String args[]) // end of main // end of class 23