class Person private String name private int age

class Person { private String name; private int age; private Person spouce; public Person(String name, int age){ this. name = name; this. age = age; } public Person get. Older. Person(Person other){ if (this. age > other. age){ return this; } return other; } public void marry(Person other){ this. spouce = other; other. spouce = this; } }

class Test { public static void main(String[] args){ Person alice = new Person("Alice", 30); Person bob = new Person("Bob", 35); Person older = bob. get. Older. Person(alice); bob. marry(alice); }

main alice 0 x 0100 bob 0 x 0200 name “Alice” age 30 spouce null name “Bob” age 35 spouce null

bob. marry(alice); marry other 0 x 0100 this 0 x 0200 main alice 0 x 0100 bob 0 x 0200 name “Alice” age 30 spouce null name “Bob” age 35 spouce null

public void marry(Person other){ this. spouce = other; other. spouce = this; } marry other 0 x 0100 this 0 x 0200 main alice 0 x 0100 bob 0 x 0200 name “Alice” age 30 spouce null name “Bob” age 35 spouce 0 x 0100

public void marry(Person other){ this. spouce = other; other. spouce = this; } marry other 0 x 0100 this 0 x 0200 main alice 0 x 0100 bob 0 x 0200 name “Alice” age 30 spouce 0 x 0200 name “Bob” age 35 spouce 0 x 0100

public class Professor { private String name; private int AFM; private Course lesson; public Professor(String name, int afm){ this. name = name; this. AFM = afm; } public void set. Lesson(Course c){ lesson = c; } public String to. String(){ return name + " " + AFM + " " + lesson; } }

import java. util. Array. List; import java. util. Scanner; public class Course { private String name; private int code; private int units; private Professor prof; private Array. List<Student. Record> student. List = new Array. List<Student. Record>(); public Course(String name, int code, int units){ this. name = name; this. code = code; this. units = units; } public void set. Prof(Professor p){ prof = p; p. set. Lesson(this); } }

public void set. Prof(Professor p){ prof = p; p. set. Lesson(this); } name “Prof. X” AFM 2012 lesson null set. Prof p 0 x 0010 this 0 x 0020 name “OOP” code 212 units 10 prof null

public void set. Prof(Professor p){ prof = p; p. set. Lesson(this); } name “Prof. X” AFM 2012 lesson null set. Prof p 0 x 0010 this 0 x 0020 name “OOP” code 212 units 10 prof 0 x 0010

public void set. Prof(Professor p){ prof = p; p. set. Lesson(this); } name “Prof. X” AFM 2012 lesson 0 x 0020 set. Prof p 0 x 0010 this 0 x 0020 name “OOP” code 212 units 10 prof 0 x 0010

© 2006 Pearson Addison-Wesley. All rights reserved A Class Hierarchy 7 -22

Η βασική κλάση public class Employee { private String name; private int AFM; public Employee( ) { … } public Employee(String the. Name, int the. AFM) { … } public Employee(Employee original. Object) { … } public String get. Name( ) { … } public void set. Name(String new. Name) { … } public int get. AFM( ) { … } public void set. AFM (int new. AFM) { … } public String to. String() { … } }

Η παράγωγη κλάση Hourly. Employee public class Hourly. Employee extends Employee { private double wage. Rate; private double hours; //for the month Νέα πεδία για την Hourly. Employee public Hourly. Employee( ) { … } public Hourly. Employee(String the. Name, int the. AFM, double the. Wage. Rate, double the. Hours) { … } public Hourly. Employee(Hourly. Employee original. Object) { … } public double get. Rate( ) { … } public void set. Rate(double new. Wage. Rate) { … } public double get. Hours( ) { … } public void set. Hours(double hours. Worked) { … } public double get. Pay( ) { return wage. Rate*hours; } public String to. String( ){ … } } Μέθοδος get. Pay υπολογίζει το μηνιαίο μισθό

Η παράγωγη κλάση Salaried. Employee public class Salaried. Employee extends Employee { private double salary; //annual Νέα πεδία για την Salaried. Employee public Salaried. Employee( ) { … } public Salaried. Employee(String the. Name, int the. AFM, double the. Salary) { … } public Salaried. Employee(Salaried. Employee original. Object ) { … } public double get. Salary( ) { … } public void set. Salary(double new. Salary) { … } public double get. Pay( ) { return salary/12; } public String to. String( ) { … } } Μέθοδος get. Pay υπολογίζει το μηνιαίο μισθό. Διαφορετική από την προηγούμενη

Constructor public class Employee { private String name; private int AFM; public Employee() { name = “no name”; AFM = 0; } public Employee(String the. Name, int the. AFM) { if (the. Name == null || the. AFM <= 0) { System. out. println("Fatal Error creating employee. "); System. exit(0); } name = the. Name; AFM = the. AFM; } }

public class Hourly. Employee extends Employee { private double wage. Rate; private double hours; //for the month public Hourly. Employee(String the. Name, int the. AFM, double the. Wage. Rate, double the. Hours) { super(the. Name, the. AFM); if ((the. Wage. Rate >= 0) && (the. Hours >= 0)) { wage. Rate = the. Wage. Rate; hours = the. Hours; } else { System. out. println( "Fatal Error: creating an illegal hourly employee. "); System. exit(0); } } } Με τη λέξη κλειδί super αναφερόμαστε στην βασική κλάση. Εδώ καλούμε τον constructor της Employee με ορίσματα το όνομα και το ΑΦΜ

public class Salaried. Employee extends Employee { private double salary; //annual public Salaried. Employee(String the. Name, int the. AFM, double the. Salary) { super(the. Name, the. AFM); if (the. Salary >= 0) salary = the. Salary; else { System. out. println( "Fatal Error: Negative salary. "); System. exit(0); } } }

public class Salaried. Employee extends Employee { private double salary; //annual public Salaried. Employee() { super(); salary = 0; } } Καλεί τον default constructor της Employee Η εντολή δεν είναι απαραίτητη σε αυτή την περίπτωση. Αν δεν έχουμε κάποια κλήση προς τον constructor της γονικής κλάσης, τότε καλείτε εξ ορισμού ο default constructor της Employee.

public class Salaried. Employee extends Employee { private double salary; //annual public Salaried. Employee(String the. Name, int the. AFM) { super(the. Name, the. AFM); salary = 0; } } Αν θέλουμε να αρχικοποιήσουμε το όνομα και το ΑΦΜ θα πρέπει να καλέσουμε τον αντίστοιχο constructor της γονικής κλάσης.

Constructor this • Όπως καλείται ο constructor super της γονικής κλάσης μπορούμε να καλέσουμε και τον constructor this της ίδιας κλάσης. public class Salaried. Employee extends Employee { private double salary; //annual public Salaried. Employee(String the. Name, int the. AFM, double the. Salary) { super(the. Name, the. AFM); if (the. Salary >= 0) salary = the. Salary; else{ System. out. println("Fatal Error: Negative salary. "); System. exit(0); } } Καλεί ένα άλλο constructor public Salaried. Employee(){ this(“no name”, 0, 0); } } της ίδιας κλάσης

Γιατί να μην κάνουμε κάτι πιο απλό? Κατευθείαν ανάθεση των πεδίων public class Salaried. Employee extends Employee { private double salary; //annual public Salaried. Employee(String the. Name, int the. AFM, double the. Salary) { name = the. Name; AFM = the. AFM; salary = the. Salary; } ΛΑΘΟΣ! } Οι παραγόμενες κλάσεις δεν έχουν πρόσβαση στα private πεδία και τις private μεθόδους της βασικής κλάσεις.

public class Employee { private void do. Something(){ System. out. println(“do. Something”); } } public class Salaried. Employee extends Employee { public void do. Something. More(){ do. Something(); System. out. println(“and more”); } } ΛΑΘΟΣ!

public class Employee { private String name; private int AFM; public Employee( ) { … } public Employee(String the. Name, int the. AFM) { … } public Employee(Employee original. Object) { … } public String get. Name( ) { … } public void set. Name(String new. Name) { … } public Date get. Hire. Date( ) { … } public void set. Hire. Date(Date new. Date) { … } public String to. String() { return (name + " " + AFM); } }

public class Hourly. Employee extends Employee { private double wage. Rate; private double hours; //for the month public Hourly. Employee( ) { … } public Hourly. Employee(String the. Name, int the. AFM, double the. Wage. Rate, double the. Hours) { … } public Hourly. Employee(Hourly. Employee original. Object) { … } public double get. Rate( ) { … } public void set. Rate(double new. Wage. Rate) { … } public double get. Hours( ) { … } public void set. Hours(double hours. Worked) { … } public double get. Pay( ) { return wage. Rate*hours; } public String to. String( ){ return (get. Name( ) + " " + get. AFM( ) + "n$" + wage. Rate + " per hour for " + hours + " hours"); } }

public class Salaried. Employee extends Employee { private double salary; //annual public Salaried. Employee( ) { … } public Salaried. Employee(String the. Name, int the. AFM, double the. Salary) { … } public Salaried. Employee(Salaried. Employee original. Object ) { … } public double get. Salary( ) { … } public void set. Salary(double new. Salary) { … } public double get. Pay( ) { return salary/12; } public String to. String( ) { … } { return (get. Name( ) + " " + get. AFM( ) + "n$" + salary + " per year"); } }

public class Salaried. Employee extends Employee { private double salary; //annual public Salaried. Employee( ) { … } public Salaried. Employee(String the. Name, int the. AFM, double the. Salary) { … } public Salaried. Employee(Salaried. Employee original. Object ) { … } public double get. Salary( ) { … } public void set. Salary(double new. Salary) { … } public double get. Pay( ) { return salary/12; } Έτσι καλούμε την to. String της βασικής κλάσης Πιο καλή υλοποίηση, μπορεί να έχει φωλιασμένες κλήσεις από προγονικές κλάσεις public String to. String( ) { … } { return (super. to. String( ) + "n$" + salary + " per year"); } }

Παράδειγμα χρήσης public class Inheritance. Demo { public static void main(String[] args) { Hourly. Employee joe = new Hourly. Employee("Joe Worker", 100, 50. 50, 160); System. out. println("joe's longer name is " + joe. get. Name( )); System. out. println("Changing joe's name to Josephine. "); joe. set. Name("Josephine"); Καλεί τις μεθόδους της Employee System. out. println("joe's record is as follows: "); System. out. println(joe); } } Καλεί την μέθοδο to. Strong της Hourly. Employee

public class Is. ADemo { public static void main(String[] args) { Salaried. Employee joe = new Salaried. Employee("Josephine", 100000); Hourly. Employee sam = new Hourly. Employee("Sam", 200, 50. 50, 40); System. out. println("joe's longer name is " + joe. get. Name( )); System. out. println("show. Employee(joe): "); show. Employee(joe); System. out. println("show. Employee(sam): "); show. Employee(sam); } Μπορούμε να καλέσουμε τη μέθοδο και με Hourly. Employee και με Salaried. Employee public static void show. Employee(Employee employee. Object) { System. out. println(employee. Object. get. Name( )); System. out. println(employee. Object. get. AFM( )); } }

public class Employee { private String name; private int AFM; public Employee(Employee other){ this. name = other. name; this. AFM = other. AFM; } } public class Salaried. Employee extends Employee { public Salaried. Employee(Salaried. Employee other){ super(other); this. salay = other. salary; } } Η κλήση του copy constructor της Employee (μέσω της super(other)) γίνεται με ένα αντικείμενο τύπου Salaried. Employee. Αυτό γίνεται γιατί Salaried. Employee is a Employee και το αντικείμενο other έχει και τους δύο τύπους.

public class Is. ADemo { public static void main(String[] args) { Salaried. Employee joe = new Salaried. Employee("Josephine", 100000); Hourly. Employee sam = new Hourly. Employee("Sam", 200, 50. 50, 40); System. out. println("joe's longer name is " + joe. get. Name( )); System. out. println("show. Employee(joe) invoked: "); show. Employee(joe); System. out. println("show. Employee(sam) invoked: "); show. Employee(sam); } public static void show. Employee(Employee employee. Object) { System. out. println(employee. Object); } } Θα καλέσει την to. String που αντιστοιχεί στο αντικείμενο που περάσαμε ως παράμετρο και όχι την to. String της Employee.