Create a Java project named week9

Transcription

1 Objectives of today s lab: Through this lab, students will explore a hierarchical model for object-oriented design and examine the capabilities of the Java language provides for inheritance and polymorphism. Inheritance is a Java (OO) language feature by which a new type can be declared that extends the capabilities of an existing type. is a language feature that allows the same function call to be associated with different definitions during the same program's execution, by delaying the binding of the call to run-time. [More on this in the next lab] Create a Java project named week9 Create a package named basicoop2 in the project week9

2 Exercise 1: Understanding the basic concept of Inheritance When you inherit from an existing class, you can reuse methods and fields of parent class, and further you can add new methods and fields also to your derived class. Task 1.1. Write the following program, save, compile, and run it. /* file name should be TestInheritance.java in the basicoop2 package*/ class Animal{ public void eat(){ System.out.println("Eating!"); class Cat extends Animal{ public void meow (){ System.out.println("Meowing!"); public class TestInheritance{ public static void main(string args[]){ Cat cat = new Cat(); cat.eat(); cat.meow(); Eating! Meowing!

3 Task 1.2. Modify the above program as highlighted below, save, compile, and run it. class Animal{ public Animal(){ System.out.println("Animal Constructor called!"); public void eat(){ System.out.println("Eating!"); class Cat extends Animal{ public Cat(){ System.out.println("Cat Constructor called!"); public void meow (){ System.out.println("Meowing!"); public class TestInheritance{ public static void main(string args[]){ Cat cat = new Cat(); cat.eat(); cat.meow(); Animal Constructor called! Cat Constructor called! Eating! Meowing! Task 1.3: Can you explain what s happening here and why? (Just write as a comment in the code). (Note that, you have not created any object of the Animal class but its constructor has been called)

4 Task 1.4. (Use of super) Modify the above program as highlighted below, save, compile, and run it. class Animal{ int animalage; public Animal(){ System.out.println("Animal Constructor called!"); public Animal(int animalage){ this.animalage = animalage; System.out.println("Animal Constructor called using super!"); public int getanimalage() { return animalage; public void eat(){ System.out.println("Eating!"); class Cat extends Animal{ public Cat(int catage){ super(catage); public Cat(){ System.out.println("Cat Constructor called!"); public void meow (){ System.out.println("Meowing!"); public int getcatage() { return this.animalage; public class TestInheritance{ public static void main(string args[]){ Cat cat = new Cat(2); cat.eat(); cat.meow();

5 System.out.println("Cat's age: "+cat.getcatage()); Animal Constructor called using super! Eating! Meowing! Cat's age: 2 Task 1.5: Now, in your code change the access modifier of int animalage according to the following and observe the corresponding results (a) public int animalage; Save, compile, and run the modified program (b) protected int animalage; Save, compile, and run the modified program (c) private int animalage; Save, compile, and run the modified program Note that, a subclass does not inherit the private members of its base class. However, if the base class has public or protected methods for accessing its private fields, these can also be used by the subclass. Therefore, in this case you can use the following getcatage() implementation to make it work. public int getcatage() { return this.getanimalage();

6 Task 1.6: (Use of super continued) Modify the above program according to the following: In the Cat class: Declare a String data field named color private String color; Modify the Cat(int catage) constructor to Cat(int catage, String color){ //implement the code here, you must use super to call the Animal constructor Write a getcolor() method that return color In the main method, modify the code according to the following: Cat cat = new Cat(2, "Blue"); cat.eat(); cat.meow(); System.out.println("Cat's age: "+cat.getanimalage()); System.out.println("Cat's color: "+cat.getcolor()); Animal Constructor called using super! Eating! Meowing! Cat's age: 2 Cat's color: Blue

7 Task 1.7: (Use of overriding method) Modify the above program according to the following: In the Animal class, change the print statement in the eat() method according to the following: public void eat(){ System.out.println("Animal Eating!"); In the Cat class, implement the same eat() method according to the following: public void eat(){ System.out.println("Cat Eating!"); When you do this, Netbeans editor will suggest you to Annotation, you just accept the suggestion. Now save, compile, and run the modified program. Your main() method remains the same as below: public static void main(string args[]){ Cat cat = new Cat(2, "Blue"); cat.eat(); cat.meow(); System.out.println("Cat's age: "+cat.getanimalage()); System.out.println("Cat's color: "+cat.getcolor());

8 Animal Constructor called using super! Cat Eating! Meowing! Cat's age: 2 Cat's color: Blue Observation: Note that both versions of the eat() method are members of the Cat class. However, in the output you see Cat Eating! (and not the Animal Eating!). Can you explain, why? (Just write as a comment in the code).

9 Exercise 2: Design and Implementation You should work in small groups of 2/3 students Your starting point will be to consider a simple example based on people. Suppose you are developing a software system for UWE. They have already designed, written, coded and tested a class called Person. An object in this class holds personal information about somebody using his name and address. A Person also has methods that can construct a new person from given data, print the information in a standard format, set the name and address and so on. Now UWE also wants a Lecturer class, a Student class, and a GraduateStudent class that are all different kinds of People! A UML class diagram of the Person class and its corresponding Java code implementation are shown below. Note that all the member variable are private and methods are public. -firstname: String -lastname: String -address: String Person +Person() +Person(firstName: String, lastname: String, address: String) +setfirstname(firstname: String): void +getfirstname(): String +setlastname(lastname: String): void +getlastname(): String +setaddress(address: String): void +getaddress(): String +tostring(): String

10 public class Person { private String firstname; private String lastname; private String address; public Person() { public Person(String firstname, String lastname, String address) { this.firstname = firstname; this.lastname = lastname; this.address = address; public String getfirstname() { return firstname; public void setfirstname(string firstname) { this.firstname = firstname; public String getlastname() { return lastname; public void setlastname(string lastname) { this.lastname = lastname; public String getaddress() { return address; public void setaddress(string address) { this.address = public String tostring() { return "Person's name is " + getfirstname() +" "+ getlastname() +"\n Address is "+ getaddress(); public static void main(string args[]) {

11 Person p = new Person(); System.out.println(p); p.setfirstname("abdur"); p.setlastname("rakib"); p.setaddress("uwe Frenchay Campus"); System.out.println(p); Person p1 = new Person("Jun", "Hong", "UWE Frenchay Campus"); System.out.println(p1); Task 2.1: Study the Person class diagram and the corresponding Java code. Write, save, compile, and run the above code. /* file name should be Person.java in the basicoop2 package*/ Person's name is null null Address is null Person's name is Abdur Rakib Address is UWE Frenchay Campus Person's name is Jun Hong Address is UWE Frenchay Campus Observation: carefully look at the tostring() method to understand how it works (if necessary look at the lecture slides or ask your tutor).

12 Task 2.2. Design three classes Lecturer, Student, and GraduateStudent using pen and paper (or you can use MS Word/PowerPoint editor). The classes Lecturer and Student extend Person, and GraduateStudent extends Student. Draw the complete UML class diagram considering all the four classes. Your three newly designed classes contain the following: Lecturer class: One integer data field representing lecturer ID A no-argument constructor that creates a default lecturer A constructor that creates a lecturer with the specified name, address, and the lecturer ID (note that you need to access name and address from the base class) All the appropriate set and get methods A method named tostring() that returns a string description for the lecturer Student class: One integer data field representing student ID A no-argument constructor that creates a default student A constructor that creates a student with the specified name, address, and the student ID (note that you need to access name and address from the base class) All the appropriate set and get methods A method named tostring() that returns a string description for the student GraduateStudent class: One String data field representing supervisor s name (usually, a graduate student works under an assigned supervisor) A no-argument constructor that creates a default graduate student A constructor that creates a graduate student with the specified name, address, the student ID, and the supervisor s name (note that you need to access name, address, and student ID from the base class) All the appropriate set and get methods A method named tostring() that returns a string description for the graduate student

13 Task 2.3: Implement your classes Extend your program by implementing the three new classes. (a) Implement your Lecturer class first, make the class Lecturer extend the class Person by adding extends Person to the right of the class declaration. This is an inheritance relationship, so Lecturer now immediately inherits all the public members and methods of B. When implementing its constructor that takes THREE parameters, use two parameters to call the Person constructor by using the super keyword. Your constructor should look like this public Lecturer(String firstname, String lastname, String address, int lecturerid){ //add code In the //add code section, if you place any code before the super, you should note the resulting compiler error. The problem is that the super class constructor must be called before any other code. Your tutor may explain why this must be so. If you have done this mistake, then move the call to the super class constructor to the start of the method and the error will go away. Copy the remaining lecturerid value to the member variable of the same name, using the "this" keyword. Override the tostring() method to return all the three variables. When implementing this, you must use super to call base class get methods. For example, your code should look like public String tostring() { return "Lecturer's name is " + super.getfirstname() +" "+ super.getlastname() +"\n Address is "+ super.getaddress() +"\n Lecturer ID: " +getlecturerid(); Save, compile, and run your code by creating the following object and printing its information Lecturer lecturer = new Lecturer ("Abdur", "Rakib","UWE Frenchay Campus",1234); System.out.println(lecturer); Lecturer's name is Abdur Rakib Address is UWE Frenchay Campus

14 Lecturer ID: 1234 (b) Similarly, implement other two classes. Save, compile, and run your code by creating the following objects and printing their information Lecturer lecturer = new Lecturer ("Abdur", "Rakib","UWE Frenchay Campus",1234); System.out.println(lecturer); Student student = new Student ("Peter", "Miller","UWE Frenchay Campus",5678); System.out.println(student); GraduateStudent gradstudent = new GraduateStudent ("Dan", "Fielding","UWE Frenchay Campus",1357,"Andy King"); System.out.println(gradstudent); Lecturer's name is Abdur Rakib Address is UWE Frenchay Campus Lecturer ID: 1234 Students's name is Peter Miller Address is UWE Frenchay Campus Student ID: 5678 Students's name is Dan Fielding Address is UWE Frenchay Campus Student ID: 1357 Supervisor s name: Andy King

15 Congratulations! Today, you have learned an important feature of the object-oriented programming languages. Now please help your fellow classmates who have been struggling to complete these exercises OR if you like: (i) (ii) Change the access specifier of the member variables of the Person class and play with them. Extend your program by adding new classes. For example, UnderGradaute, PermanentLecturer, and PartTimeLecturer.