Java Inheritance. Classes implement the concept of ADT:

Transcription

1 Java Inheritance Rui Moreira Class ADT (Abstract Data Type) Classes implement the concept of ADT: n Provide a coherent representation for the declaration of structured data types and also the code for manipulating those data types, i.e., associate in the same concept (class) the member variables (attributes) and the member functions (methods) n Data and code are no longer separated concepts, i.e., we may think on data objects with implicit behaviour and that may change themselves we no longer have the separation between data structures and the functions for manipulating those structures (as with C or Pascal) Rui Moreira 2 1

2 Interface Interfaces define interaction contracts: n n n n Interfaces are mechanisms/devices that objects may use to interact (e.g., we interact with a TV through its public interface controls) An interface is a collection of methods which define the public behaviour exposed by several object (e.g., get, set, move, area, etc.) Interfaces expose classes/objects behaviour without revealing its internal implementation/details - encapsulation A class that implements an interface agrees to implement all the behaviour defined by the interface methods - contract Rui Moreira 3 Interface Definition Interface q defines the signature of a set of public methods q method defintion has no body/implementation /** AccountMoneyI: defines methods for manipulating the Account money */ public interface AccountMoneyI { public void whidraw(long amount); public void deposit(long amount); public long balance(long amount); /** AccountRelationI: defines methods for getting objects related with Account */ public interface AccountRelationI { public Client getclient(); public BankMananger getbankmananger(); AccountMoneyI (from bank) whidraw( ) deposit( ) balance( ) AccountRelationI getclient( ) getbankmanager( ) Rui Moreira 4 2

3 Multiple Interface Inheritance // A class can implement several interfaces public class AccountImpl implements AccountMoneyI, AccountRelationI { // Some attributes here... // Then all methods must be implemented public void whidraw(long amount){ public void deposit(long amount){ public long balance(long amount) ){ public Client getclient() ){ public BankMananger getbankmananger() ){ AccountRelationI getclient( ) getbankmanager( ) AccountImpl (from bank) balance : long accountnumber : String AccountImpl( ) AccountMoneyI (from bank) whidraw( ) deposit( ) getbalance( ) Rui Moreira 5 Class Inheritance n A Java class can inherit (extend) another class: /** Super-class Employee: represents a generic employee */ public class Employee { public String name = ; private Date birth = null; private float salary; /** Constructor */ public Employee(String n, Date b, float s) { /*... */ /** Sub-class Manager: Manager( ) represents a specific employee */ getsubordinates( ) public class Manager extends Employee { private Employee subordinates[]; /** Constructor */ public Manager(String n, Date b, float s, Employee[] sub) { /*... */ Manager subordinates[ ] : Employee Employee name : String = "" birth : Date = null salary : float Employee( ) getname( ) getbirth( ) setbirth( ) getsalary( ) setsalary( ) Salesman zone : String = null Salesman( ) getzone( ) Rui Moreira 6 3

4 Class Inheritance n Java allows only single class inheritance, i.e., extend only 1 class at a time (because of the diamond problem) n The subclass inherits only public attributes and public methods (i.e., private attributes and methods of super-class are not accessible to subclass) n Constructors are not inherited, therefore the subclass must implement its own constructors and explicitly call super-class constructors Rui Moreira 7 Operators: this and super n Java provides 2 operators: q q this: for referencing the class/object itself super: for referencing the super-class/object /** Sub-class Employee: represents a generic employee */ public class Employee { /* Atributes */ public String name = ; private Date birth = null; private float salary; /** Constructor */ public Employee(String n, Date b, float s) { this.name = n; // SET OBJECT ATRIBUTE name to n this.birth = b; // SET OBJECT ATRIBUTE birth to b this.salary = s; // SET OBJECT ATRIBUTE salary to s Rui Moreira 8 4

5 Operators: this and super n Java provides 2 operators: q this: for referencing the class/object itself q super: for referencing the super-class/object public class Manager { private Employee subordinates[]; /** Constructor */ public Manager(String n, Date b, float s, Employee[] sub) { super(n, b, s); // EXPLICITLY CALL SUPER-CLASS CONSTRUCTOR this.subordinates = sub; /** Another constructor */ public Manager(String n, Date b, float s, Employee[] sub) { super.name = n; // WE MAY SET PUBLIC ATRIBUTES //super.birth = b; NOT ALLOWED TO SET PRIVATE ATTRIBUTES!!!! super.setbirth(b); // ALTERNATIVELLY USE PUBLIC METHODS super.setsalary(s);// SET PRIVATE ATTRIBUTE VIA PUBLIC METHOD this.subordinates = sub; Rui Moreira 9 Object Polymorphism n An object can take several shapes // Variable emp is of type Employee Employee emp = null; // Set reference to a new Manager emp = new Manager( John, new Date(2, 08, 1980), 1000, arrayemp); // We CAN ACCESS all attributes/methods of Employee s = emp.getsalary(); // THIS IS ALLOWED!!!! // But CANNOT ACCESS attributes/methods of Manager // because emp is of type Employee arraysubord = emp.getsubordinates(); // THIS IS NOT ALLOWED!!!! // Same reference variable can point to a different object emp = new Salesman( Mary, new Date(2, 08, 1980), 1000, Porto ); Rui Moreira 10 5

6 Cast n Explicit cast: force an object to be of a given type // We CAN ACCESS attributes/methods of Manager // by casting/masking emp to Manager subordinates = ((Manager)emp).getSubordinates(); // ALLOWED!!!! // We CAN ALSO ACCESS attributes/methods of Salesman // by casting/masking the object (emp) to Salesman zone = ((Salesman)emp).getZone(); // ALSO ALLOWED!!!! NB: n we CAN ONLY CAST objects that have/inherit the same base-type (i.e., same super-class). n we CANNOT CAST between different types of objects (i.e., which do not share same super-class)!! Rui Moreira 11 Cast n Explicit cast: restore the funcionality of a generic object // Generic object can point to any kind of object Object obj = new Rectangulo(2, 2, 4, 1); if (obj instanceof Rectangulo){ // CAST - Force rec to point to an object Rectangulo Rectangulo rec = (Rectangulo)obj; // Now we can access the attributes/methods of Rectangulo inside = rec.isinside(1, 1); Rui Moreira 12 6

7 Object Polymorphism n instanceof - check instance type Employee name : String = "" birth : Date = null salary : float // printinfo(): prints information about a given Employee // instance which depends on its type public void printinfo(employee e){ if (e instanceof Employee) { e.description(); // Prints employee info else if (e instanceof Manager) { e.description(); // Prints manager info // Obtain manager subordinates sub = ((Manager)e).getSubordinates(); else if (e instanceof Salesman) { e.description(); // Prints salesman info // Obtain Salesman zone zone = ((Salesman)e).getZone(); Manager subordinates[ ] : Employee Manager( ) getsubordinates( ) Employee( ) getname( ) getbirth( ) setbirth( ) getsalary( ) setsalary( ) Salesman zone : String = null Salesman( ) getzone( ) Rui Moreira 13 Method Polymorphism - Overload n Overload methods: q the same method name can be used to identify/implement different methods q each of the methods must have different parameters q each method can have the same or different return values public class PrintClass { // Same method name with different parameters/arguments public void print(int i){ System.out.print( i = +i); public void print(float f){ System.out.print( f = +f); public void print(string s){ System.out.print( s = +s); Rui Moreira 14 7

8 Method Polymorphism Override n Override Methods (redefine behaviour of super-class methods): q the sub-class may add new methods to the super-class but can also redefine/override methods of the super-class (change the behaviour of existing methods) // Method description() is provided by Employee // and overridden by Manager and Salesman to add specific info public class Manager { public String description(){ // Add additional Manager info return super.description()+ : +this.subordinates.length; public class Salesman { public String description(){ // Add additional Salesman info return super.description()+ : +this.zone; Rui Moreira 15 Exercises Implement the following class diagram (NB: you may change the classes already defined in previous examples/slides, e.g., Rectangulo, Triangulo, Circulo) Ponto x : int = 0 y : int = 0 Ponto( ) getx( ) gety( ) setx( ) sety( ) drawponto( ) move( ) extends 1..* Rectangulo upleftponto : Ponto = null downrightponto : Ponto = null Rectangulo( ) getupleftponto( ) getdownrightponto( ) setupleftponto( ) setdownrightponto( ) move( ) isinside( ) area( ) draw( ) tostring( ) GeoFigureI getcolor( ) changecolor( ) draw( ) tostring( ) implements GeoFigure color : int extends Circulo centerponto : Ponto = null diameter : int = 0 getcenterponto( ) getdiameter( ) setcenterponto( ) setdiameter( ) move( ) isinside( ) area( ) draw( ) tostring( ) extends Triangulo upponto : Ponto = null downponto : Ponto = null middleponto : Ponto = null getupponto( ) getdownponto( ) getmiddleponto( ) setupponto( ) setdownponto( ) setmiddleponto( ) move( ) isinside( ) area( ) draw( ) tostring( ) Rui Moreira 16 8