Object-Oriented Programming in Java

Transcription

1 CSCI/CMPE 3326 Object-Oriented Programming in Java inheritance Dongchul Kim Department of Computer Science University of Texas Rio Grande Valley

2 Lab8 Please at the top of the program, as a comment include at a minimum your name, the date, and the lab number.

3 Lab8-1 Make a program to display the Person object s fields in a class (Lab8_1.java), which includes main method. Part 1 Define Person class in Person.java file. Define three member fields in Person class (with any variable types). Define a constructor with three argument to initialize three member fields In Person class, define three member methods that display each field. Define a constructor that has three arguments for the fields in Person class. Generate an instance of Person object in Lab8_1 class. Display the instance s fields using three methods of the Person class. Part 2 In Person class, define three member methods that allow you to update each member field. These member methods should have an argument to update but no return value. Note In Lab8_1 class, please update the instance s fields generated in part 1 using new three update methods. After updates, please display updated the member fields using the methods. Field and method name is up to you.

4 Lab8-1 Example of Person class and its methods public class Person { String name; // field int age; double height; Person( ) // constructor { name = ; age = ; height = ; void display_name() // method for display field. You need two more methods to display fields other than name { System.out.println(name); void update_name( ) // method for update field. You need two more methods to update fields other than name { name = ;

5 Lab8-2 Make a program to display the menu and sub-menu using while loop, conditional statement, and Menu class. The program has two files for Lab8_2.java and Menu.java Lab8_2 class includes main method. The Menu class has more than three methods to display a main menu, sub-menu, and message. These methods do not have any input argument and return value but only display the menu or message. In Lab8_2 class (Lab8_2.java), please implement a code to repeatedly display menu, sub-menu, and message with Person class (Person.java). Example of the program s input and output is as follows. (next slide)

6 Main Menu 1. Utility 2. Game 3. Multimedia 4. Exit Please input: 1 Utility 1. Calculator Note 4. Main menu Please input: 1 I am sorry. It s not ready yet. Utility 1. Calculator Note 4. Main menue Please input: 2 I am sorry. It s not ready yet. Utility 1. Calculator Note 4. Main menue Please input: 4 Main Menu 1. Utility 2. Game 3. Multimedia 4. Exit Please input: 2 Game 1. Poker 2. Blackjack 3. Main menu Please input: 1 I am sorry. It s not ready yet. Game 1. Poker 2. Blackjack 3. Main menu Please input: 3 Main Menu 1. Utility 2. Game 3. Multimedia 4. Exit Please input: 3 Multimedia 1. Music player 2. Camera 3. Download Youtube 4. Main menu Please input: 4 Main Menu 1. Utility 2. Game 3. Multimedia 4. Exit Please input: 7 I am sorry. Please input correclty. Main Menu 1. Utility 2. Game 3. Multimedia 4. Exit Please input: 4 Thank you! Bye~

7 Lab8-2 Example of the method to display menu in Menu class public void display_main_menu() { System.out.println( Main Menu ); System.out.println( 1. Utility ); System.out.println( 2. Game ); System.out.println( 3. Multimedia ); System.out.println( 4. Exit );

8 Lab8-3 Make a program to write text on a file and read it. You can implement all code in Lab8_3.java. This lab is to practice only file I/O. (If you want, you can define any class with separate java file.) Once the program starts, an user can input multiple IDs and Passwords iteratively. Each pair of ID and Password is stored in ID.txt and Password.txt sequentially. After finishing ID/PASS registration, the program reads two text files and displays all IDs and Passwords as follows.

9 Program Start ID:abcd Pass:3434 Successfully stored! Do you want to input more(y/n):y ID:ks3410 Pass:8321 Successfully stored! Do you want to input more(y/n):y ID:apple77 Pass:DG23.32d Successfully stored! Do you want to input more(y/n):y ID:Knight Pass:1234 Successfully stored! Do you want to input more(y/n):n Member ID and Password List ID:abcd Pass:3434 ID:ks3410 Pass:8321 ID:apple77 Pass:DG23.32d ID:Knight Pass:1234 Thank you!

10 Submission Please give a demo to TA

11 Inheritance

12 Inheritance Inheritance can be defined as the process where one class acquires the properties (methods and fields) of another. With the use of inheritance the information is made manageable in a hierarchical order. The class which inherits the properties of other is known as subclass (derived class, child class) and the class whose properties are inherited is known as superclass (base class, parent class).

13 Inheritance Real-life objects are typically specialized versions of other more general objects. Car Sport car SUV Shape Triangle Square Person Kid Adult Employee Presidnet Manager Inheritance is a code re-use issue. we can extend code that is already written in a manageable manner.

14 extends extends is the keyword used to inherit the properties of a class. Below given is the syntax of extends keyword. class Super{ class Sub extends Super{......

15 Inheritance Contains those attributes and methods that are shared by all Cars. Car Sports Car Mini Van Contains those attributes and methods that specific to a Sports Car. Contains those attributes and methods that are specific to a Mini Van.

16 Inheritance Contains those attributes and methods that are shared by all Doctors. Doctor Surgeon FamilyDoctor Contains those attributes and methods that specific to a Sprot Surgeon. Contains those attributes and methods that are specific to a Family Doctor.

17 Inheritance The subclass inherits fields and methods from the superclass without any of them being rewritten. New fields and methods may be added to the subclass. The Java keyword, extends, is used on the class header to define the subclass. public class Sports_car extends Car public class Surgeon extends Doctor

18 Example public class Doctor { boolean worksathospital; void treatpatient() { // perform a checkup public class Surgeon extends Doctor{ void treatpatient() { // perform surgery void makeincision() { // make incision (yikes!) public class FamilyDoctor extends Doctor { boolean makeshousecalls; void giveadvice() { // give homespun advice

19 Example public class Main{ public static void main(string[] args) { Surgeon s1 = new Surgeon(); s1.worksathospital=true; // it s a field of Doctor class s1.makeincision(); // method of Surgeon class

20 super It is used to differentiate the members of superclass from the members of subclass, if they have same names. It is used to invoke the superclass constructor from subclass.

21 class Super_class{ int num = 20; //display method of superclass public void display(){ System.out.println("This is the display method of superclass"); public class Sub_class extends Super_class { int num = 10; //display method of sub class public void display(){ System.out.println("This is the display method of subclass"); public void my_method(){ //Instantiating subclass Sub_class sub = new Sub_class(); //Invoking the display() method of sub class sub.display(); //Invoking the display() method of superclass super.display(); //printing the value of variable num of subclass System.out.println("value of the variable named num in sub class:"+ sub.num); //printing the value of variable num of superclass System.out.println("value of the variable named num in super class:"+ super.num); public static void main(string args[]){ Sub_class obj = new Sub_class(); obj.my_method();

22 this Within an instance method or a constructor, this is a reference to the current object the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this.

23 Example public class Person{ String name; int age; char gender; public Person(String name, int age, char gender){ this.name = name; this.age = age; this.gender = gender;