EXERCISES SOFTWARE DEVELOPMENT I. 09 Objects: Inheritance, Polymorphism and Dynamic Binding 2018W

Transcription

1 EXERCISES SOFTWARE DEVELOPMENT I 09 Objects: Inheritance, Polymorphism and Dynamic Binding 2018W

2 Inheritance

3 I INHERITANCE :: MOTIVATION Real world objects often exist in various, similar variants Attributes (size, weight, color) Behavior Etc. Objects are often grouped/arranged hierarchically Corporate structure (head department chief group leader employee) Means of transportation Transport Vehicle Train Boat Plane Truck Car InterCity Tram

4 I INHERITANCE :: MOTIVATION (2) Example Goals of inheritance public class Car { Date regdate; int color; float luggagespace; void drive () {... void opentrunk () {... Express commonalities Classification Specialization Re-use of code (this is only a positive side effect, not the main aim of inheritance!) public class Truck { Date regdate; int color; float loadingspace; void drive () {... void loading () {... Software Development I // 2018W // 4

5 I INHERITANCE :: MOTIVATION (3) Graphical view class Vehicle { Date regdate; int color; Superclass Parent class Base class Derived class Extended class Child class Subclass void drive () {... class Truck extends Vehicle { float loadingspace; Class Car extends Vehicle { float luggagespace; void loading () {... void opentrunk () {... Software Development I // 2018W // 5

6 INHERITANCE :: WHAT DOES IT STAND FOR? Inheritance is Declaration of new classes based on already existing classes New classes own ( inherit ) all attributes and functions/methods of the underlying base classes Source code of base class(es) can remain unchanged New/inherited class(es) can be extended with further attributes, additional methods Methods already defined in a base class can be changed in the inherited class Refinement = complement the functionality of the base class Replacement = replace/overwrite the original method/functionality Software Development I // 2018W // 6

7 INHERITANCE :: TERMS AND DEFINITIONS Every class in Java has one and only one direct superclass (concept of single inheritance) In the absence of any other explicit superclass (i.e., absence of keyword extends ), every class is implicitly a subclass of Object Object itself has no superclass Classes can be derived from classes that are derived from classes that are derived from classes, and so on, and ultimately derived from the topmost class (i.e., Object); such a class is said to be descended from all the classes in the inheritance chain Superclass // base class versus Subclass (derived class) Base class = existing class Subclass = derived class Inheritance hierarchy Direct superclass Indirect superclass Root (class) Software Development I // 2018W // 7

8 INHERITANCE :: TERMS AND DEFINITIONS Implicit inheritance Every class in Java inherits all characteristics of the class Object Class Object defines some methods that are useful for all types of classes, e.g., equals(), clone(), etc. Software Development I // 2018W // 8

9 INHERITANCE :: TERMS AND DEFINITIONS Implicit inheritance (2) As the Object class sits at the top of the class hierarchy tree, every class is a descendant, direct or indirect, of this class. Every class you use or write inherits the instance methods of Object You need not use any of these methods, but, if you choose to do so, you may need to override them with code that is specific to your class Useful methods inherited from Object are protected Object clone() throws CloneNotSupportedException Creates and returns a copy of this object public boolean equals(object obj)indicates whether some other object is "equal to" this one protected void finalize() throws Throwable Called by the garbage collector on an object when garbage collection determines that there are no more references to the object public final Class getclass() Returns the runtime class of an object (static vs. dynamic type see Polymorphism) public int hashcode()returns a hash code value for the object public String tostring()returns a string representation of the object notify, notifyall, and wait methods of Object all play a part in synchronizing the activities of independently running threads in a program (not discussed here) Software Development I // 2018W // 9

10 I INHERITANCE :: INHERITANCE RELATION The Is-A Relation An object of a subclass is also an object of the superclass ( Is-A relation ) Example: superclass Vehicle, subclass Truck: A truck is a vehicle An object of the subclass can be used anywhere where an object of the superclass can be used (=substitution); inaccessibility of attributes In addition to the characteristics of the superclass, the declaration of the subclass adds new characteristics (attributes, methods) A subclass is always a specialization of the superclass Layers of abstraction hierarchy of classes A Vehicle B Car C Cabrio Software Development I // 2018W // 10

11 I INHERITANCE :: INHERITANCE RELATION Example: Is-A Relation between 'Car' and 'Vehicle public static void main (String[] args) { Car c = new Car(); Vehicle v; v=c; v.luggagespace = 3; v.regdate = new Date(); v.setdate(new Date()); c=v; //ok; a car ('c') 'Is-A' vehicle ('v ) //luggagespace (class Car) not accessible!! //ok; regdate defined in Vehicle! // not possible; not every vehicle is a car // Error: "Type mismatch: cannot convert from Vehicle to Car Software Development I // 2018W // 11

12 I INHERITANCE :: TERMS AND DEFINITIONS Explicit inheritance Using the keyword extends, a Java class can be extended subclass extend superclass, for example class Cabrio extends Car { // Car extended from Vehicle int foldingtoptime;... Class TwoSeatCabrio extends Cabrio { boolean auxilaryrearseats;... // Object Transport Vehicle Car Cabrio TwoSeatCabrio Instantiation TwoSeatCabrio car1 = new TwoSeatCabrio(); car1.color = "red"; // Instance variable of "Vehicle car1.foldingtoptime = 20; // Instance var. of "Cabrio car1.auxilaryrearseats = false; // Instance var. of "TwoSeatCabrio Software Development I // 2018W // 12

13 INHERITANCE :: JAVA SPECIFICS Inheritance in Java compared to other programming languages Java only allows single inheritance, i.e., the keyword extends is always followed by a single class name (the superclass) other programming languages such as C++ allow multiple inheritance, but not without imposing some problems (e.g., "diamond problem") Java only allows to declare subtypes of classes (Car Vehicle), but not to confine for example data types as compared to e.g. the XML scheme (xs:integer xs:short) what about this statement? public class SmallInteger extends Integer { Error: "The type SmallInteger cannot subclass the final class Integer see the definition of Integer: public final class Integer extends Number 'final' classes cannot be subclassed (i.e., extended)! Software Development I // 2018W // 13

14 I INHERITANCE IN JAVA :: EXAMPLE COMICCHARACTER public class ComicCharacter { private String name; void print() { System.out.println(name); void dance() { System.out.println(name + " dances."); void sing() { System.out.println(name + " sings."); String getname() { return name; void setname(string name) { this.name = name; Software Development I // 2018W // 14

15 I INHERITANCE IN JAVA :: EXAMPLE COMICCHARACTER (2) public class SuperHero extends ComicCharacter { // inherits characteristics from ComicCharacter ('print', 'dance', 'sing'), // adds the functionality to 'fight' private String superpower; void fight() { System.out.println (getname()+" fights."); String getsuperpower() { return superpower; void setsuperpower(string superpower) { this.superpower = superpower; public class Wizard extends ComicCharacter { // ComicCharacter behavior extended with the possibility to 'fly' void fly() { System.out.println(getName()+" flies."); Software Development I // 2018W // 15

16 I INHERITANCE IN JAVA :: EXAMPLE COMICCHARACTER (3) public static void main(string[] args) { ComicCharacter cc = new ComicCharacter(); cc.setname("gonzo"); cc.dance(); SuperHero sh = new SuperHero(); sh.setname("arnie"); sh.dance(); sh.fight(); Wizard wd = new Wizard(); wd.setname("john"); wd.sing(); wd.fly(); What about this call is it allowed? cc = new SuperHero(); cc.dance(); Gonzo dances. Arnie dances. Arnie fights. John sings. John flies. null dances. Software Development I // 2018W // 16

17 Polymorphism and Dynamic Binding

18 I POLYMORPHISM :: OBJECTS WITH DIFFERENT FORMS Polymorphism refers (in general) to a principle in biology in which an organism or species can have many different forms or stages. This principle can also be applied to object-oriented programming (OOP) and the Java language. Subclasses of a class can define their own unique behaviors and yet share some of the same functionality of the parent class. Demonstration of polymorphism with ComicCharacter Object of a subclass (SuperHero, Wizard) is-a object of the superclass (ComicCharacter) Objects of subclasses can be used instead of objects of the base class (see Vehicle Car example above!) Object variables might be polymorph (having different types ) Static data type (type of declaration) Dynamic data type (corresponds to the actual class affiliation at run time) Object of a subclass can be assigned to a variable of (declared, static) type of the parent class (multilevel!) Software Development I // 2018W // 18

19 I POLYMORPHISM :: EXAMPLE COMICCHARACTER Dynamic (runtime) type: SuperHero Static (declaration) type: ComicCharacter public static void main(string[] args) { ComicCharacter cc; cc = new SuperHero(); // A SuperHero "is-a" ComicCharacter cc.dance(); // Each ComicCharacter is able to dance cc.fight(); // A ComicCharacter can't fight! Attention! Polymorphism applies only to one direction public static void main(string[] args) { SuperHero sh; ComicCharacter cc; cc = new ComicCharacter(); sh = cc; // Not every ComicCharacter "is-a" // SuperHero! sh = new SuperHero(); // Other way round: ok (see above) cc = sh; Due to polymorphism, any object is assignable to a variable of type java.lang.object (but not the other way round!) To check the dynamic data type of a variable the instanceof operator can be used Software Development I // 2018W // 19

20 POLYMORPHISM :: THE "INSTANCEOF -OPERATOR The instanceof -operator checks at runtime the is-a relation of an objects' type in relation to a class, e.g. public static void main(string[] args) { SuperHero sh = new SuperHero(); System.out.println(sh instanceof SuperHero); // true System.out.println(sh instanceof ComicCharacter); // true System.out.println(sh instanceof Wizard); // false System.out.println(sh instanceof Object); // true Software Development I // 2018W // 20

21 I POLYMORPHISM :: EXAMPLE COMICCHARACTER public static void main(string[] args) { ComicCharacter cc; cc = new SuperHero(); // A SuperHero "is-a" ComicCharacter System.out.println(cc instanceof ComicCharacter); // true System.out.println(cc instanceof SuperHero); // true System.out.println(cc instanceof Object); // true cc.dance(); // Each ComicCharacter is able to dance cc.fight(); // A ComicCharacter can't fight! Exception in thread "main" java.lang.error:? Unresolved compilation problem: The method fight() is undefined for the type ComicCharacter Only attributes and methods of the superclass (ComicCharacter) are universally accessible! Software Development I // 2018W // 21

22 POLYMORPHISM :: EXAMPLE ARRAY OF VEHICLES Until now, arrays were declared with a single, fixed data type (int[], char[], ) An array of vehicles could only store instances of class Vehicle Vehicle[] veh = new Vehicle[10]; With inheritance and polymorphism it is now possible to store different vehicle objects in that array (classes Motorbike, Car, Truck are extensions of class Vehicle): Vehicle[] veh = new Vehicle[10]; // heterogenous array veh[0] = new Vehicle(); // assignment as usual veh[1] = new Truck(); // assignment of a truck object veh[2] = new Car(); // assignment of a car object veh[3] = new Motorbike(); // assignment of a motorbike In each of the array elements are by default only attributes/methods of the base class (Vehicle) accessible! If, for example, class Motorbike defines a method performwheelie() (that is not defined in class Vehicle), then the following method call would be illegal! veh[3].performwheelie(); // not accessible ((Motorbike)veh[3]).performWheelie(); // ok with typecast ((Motorbike)veh[2]).performWheelie(); // runtime error! Software Development I // 2018W // 22

23 I INHERITANCE :: METHOD REPLACEMENT/ REFINEMENT Method replacement A method defined (and implemented) in the base class is fully replaced by another method in the subclass class Smurf extends ComicCharacter { private String color; void Smurf() { color = "Blue"; void sing() { System.out.println(color + " Smurfs don't sing!"); Method replacement can be prohibited (restricted) if the method in the parent class is defined as final (keyword before method name) Software Development I // 2018W // 23

24 I INHERITANCE :: METHOD REPLACEMENT/ REFINEMENT (2) Method refinement Complementing/extending the method body of a method already defined in the parent class In the subclass 1. Invoke the overridden method from the superclass through the use of the keyword "super" 2. Add additional statements Example: class Smurf extends ComicCharacter { super.dance() invokes the (overriden) dance() method from the superclass void dance() { ComicCharacter super.dance(); System.out.println("The other Smurfs applaud"); (Although discouraged, "super" can also be used to refer to a hidden field ) Software Development I // 2018W // 24

25 DYNAMIC BINDING :: INTRODUCTION + doit () Suppose you have 3 Java classes: A, B, and C where class B extends A and class C extends B (class inheritance hierarchy: A on top, then B in the middle, and finally C on the bottom); all 3 classes implement the instance method void doit(); public static void main(string[] args) { A x = new A(); x.doit(); // doit() from A A y = new B(); y.doit(); // doit() from??? What version of the y.doit() method is actually executed and why? C Given the class structure, the question is which version of the doit() method will be executed the one in class A, B, or C? + doit () Var. y is an object of type A, but it is instantiated as an object of class B The version of the doit() method that s executed by the object y is the one in class B because of what is known as dynamic binding in Java. Dynamic binding basically means that the method implementation that is actually called is determined at run time, and not at compile time. Dynamic binding (also known as late binding): the method that will be run is chosen at run time. A B + doit () Software Development I // 2018W // 25

26 I DYNAMIC BINDING :: EXAMPLE COMICCHARACTER Method that will be called is determined at run time (depending on dynamic type of the object) public static void main(string[] args) { ComicCharacter cc = new ComicCharacter(); cc.setname("papasmurf"); cc.sing(); sm = new Smurf(); sm.sing(); cc = sm; cc.sing(); // sing() method of class ComicCharacter // sing() method of class Smurf; different // behavior (method replaced in class Smurf) // cc is of (static) type ComicCharacter; // sing() method of class Smurf!! cc = new SuperHero(); cc.sing(); // sing() method of class SuperHero; the // method is not overridden in that class, // i.e. the sing method of the superclass // ComicCharacter is invoked Papasmurf sings. Blue Smurfs don't sing! null sings. Software Development I // 2018W // 26

27 I ABSTRACT CLASSES AND INTERFACES :: DECLARATION W/O IMPLEMENTATION An abstract method is a method declared without an implementation, i.e., with only a signature and no implementation body (no {-braces) - like this: public abstract void draw(int x, int y); An abstract class is a class that is declared abstract It may or may not include abstract methods Abstract classes cannot be instantiated, but they can be subclassed when an abstract class is subclassed, the subclass usually provides implementations for all of the abstract methods in its parent class. However, if it does not, the subclass must also be declared abstract (and implementation of the remaining methods must be provided in further subclasses) If a class includes at least one abstract method, the class itself must be declared abstract Abstract class interface! (see the following examples) unlike interfaces, abstract classes can contain fields that are not static and final, and they can contain implemented methods such abstract classes are similar to interfaces, except that they provide a partial implementation, leaving it to subclasses to complete the implementation if an abstract class contains only abstract method declarations, it should be declared as an interface instead Software Development I // 2018W // 27

28 ABSTRACT CLASSES :: EXAMPLE 'GEOMETRICFIGURES (1) Case 1: Abstract class/methods abstract class GeometricFigure { abstract public double area(); public void printarea() { System.out.print("Area: "+ this.area()); Abstract class may contain methods with implementation, but at least one abstract method... class Circle extends GeometricFigure { double radius; public Circle (double radius) { super(); this.radius = radius; public double area() { return Math.PI * radius * radius; A nonabstract class has to provide an implementation for all the abstract methods in the superclass. Concrete methods might be overridden or extended. public void printarea() { super.printarea(); System.out.println(" (Circle)"); Software Development I // 2018W // 28

29 ABSTRACT CLASSES :: EXAMPLE 'GEOMETRICFIGURES (2A) Case 1: Abstract class/methods class Square extends GeometricFigure { double len; public Square(double len) { this.len = len; public double area() { return len*len; If not overridden, method printarea() from the super class GeometricFigure will be used! Area: (Circle) Area: 25.0 // public void printarea() { // super.printarea(); // System.out.println(" (Square)"); // public static void main(string[] args) { GeometricFigure [] figures = new GeometricFigure[2]; figures[0] = new Circle (5); figures[1] = new Square (5); figures[0].printarea(); figures[1].printarea(); Software Development I // 2018W // 29 Basic instantiation): - Circle c = new Circle(5); - c.printarea(); - Square sq = new Square(5); - sq.printarea(); Dynamic binding: Method printarea() that will be called is determined at run time (depending on dynamic type)

30 ABSTRACT CLASSES :: EXAMPLE 'GEOMETRICFIGURES (2A) Case 1: Abstract class/methods class Square extends GeometricFigure { double len; public Square(double len) { this.len = len; Area: (Circle) Area: 25.0 public double area() { return len*len; // public void printarea() { // super.printarea(); // System.out.println(" (Square)"); // public static void main(string[] args) { GeometricFigure [] figures = new GeometricFigure[2]; figures[0] = new Circle (5); figures[1] = new Square (5); figures[0].printarea(); figures[1].printarea(); Software Development I // 2018W // 30 Dynamic binding: Method printarea() that will be called is determined at run time (depending on dynamic type)

31 INTERFACES :: EXAMPLE 'GEOMETRICFIGURES (1) Interface Definition and Implementation interface GeometricFigure { // constant declarations, if any public class Circle implements GeometricFigure { double radius; public double area(); public void printarea(); Method declarations: Method signatures have no {-braces and are terminated with a semicolon. A nonabstract class has to provide an implementation for all method signatures specified in the interface. public Circle (double radius) { super(); this.radius = radius; public double area() { return Math.PI * radius * radius; public void printarea() { System.out.println("Area: "+ this.area() + Software Development I // 2018W // 31

32 INTERFACES :: EXAMPLE 'GEOMETRICFIGURES (2) Provide an Implementation for Square and instantiate a Circle and a Square and run the printarea Method on both. interface GeometricFigure { public double area(); public void printarea(); public static void main(string[] args) { GeometricFigure [] figures = new GeometricFigure[ figures[0] = new Circle (5); figures[1] = new Square (5); class Circle implements GeometricFigure { double radius; public Circle (double radius) { super(); this.radius = radius; public double area() { return Math.PI * radius * radius; figures[0].printarea(); figures[1].printarea(); public void printarea() { System.out.println("Area: "+ this.area() + " (Circle)"); Software Development I // 2018W // 32

33 INTERFACES :: EXAMPLE 'GEOMETRICFIGURES (3) Provide an Implementation for Square and instantiate a Circle and a Square and run the printarea Method on both. class Square implements GeometricFigure { double len; public Square(double len) { this.len = len; public double area() { return len*len; public static void main(string[] args) { GeometricFigure [] figures = new GeometricFigure[ figures[0] = new Circle (5); figures[1] = new Square (5); figures[0].printarea(); figures[1].printarea(); public void printarea() { System.out.println("Area: "+ this.area() + " (Square)"); Software Development I // 2018W // 33

34 INTERFACES :: EXAMPLE 'GEOMETRICFIGURES (4) Provide an Implementation for Square and instantiate a Circle and a Square and run the printarea Method on both. interface GeometricFigure { public double area(); public void printarea(); public static void main(string[] args) { GeometricFigure [] figures = new GeometricFigure[ figures[0] = new Circle (5); figures[1] = new Square (5); class Square implements GeometricFigure { double len; public Square(double len) { this.len = len; public double area() { return len*len; figures[0].printarea(); figures[1].printarea(); public void printarea() { System.out.println("Area: "+ this.area() + " (Square)"); Software Development I // 2018W // 34

35 INTERFACES :: EXAMPLE 'GEOMETRICFIGURES Provide an Implementation for Square and instantiate a Circle and a Square and run the printarea Method on both. interface GeometricFigure { // constant declarations, if any class Square implements GeometricFigure { double len; public double area(); public void printarea(); public Square(double len) { this.len = len; class Circle implements GeometricFigure { double radius; public Circle (double radius) { super(); this.radius = radius; public double area() { return len*len; public void printarea() { System.out.println("Area: "+ this.area() + " (Square)"); public double area() { return Math.PI * radius * radius; public void printarea() { System.out.println("Area: "+ this.area() + " (Circle)"); public static void main(string[] args) { GeometricFigure [] figures = new GeometricFigure[2]; figures[0] = new Circle (5); figures[1] = new Square (5); figures[0].printarea(); figures[1].printarea(); Software Development I // 2018W // 35

36 INTERFACES: EXAMPLE 'GEOMETRICFIGURES (5) Summary (incl. dynamic binding) // TESTS (Abstract class): // 1. Not allowed (error: "cannot instantiate the type GeometricFigure") GeometricFigure gf = new GeometricFigure(); // 2. Allowed; Square/Circle "is-a" GeometricFigure Circle c = new Circle(5); GeometricFigure gf = new Square(5); // also allowed: new Circle(5); // 3. Calls the printarea()-method of Square-class c.printarea(); gf.printarea(); // printarea()-method selected with concept of dynamic binding! // TESTS (Interface): // 1. Not allowed (error: "cannot instantiate the type GeometricFigure") GeometricFigure gf = new GeometricFigure(); // 2. Allowed; Square/Circle "is-a" GeometricFigure Square sq = new Square(5); GeometricFigure gf = new Circle(5); // also allowed: new Square(5); // 3. Calls the printarea()-method of Circle-class sq.printarea(); gf.printarea(); // printarea()-method selected with concept of dynamic binding! Software Development I // 2018W // 36

37 EXERCISES SOFTWARE DEVELOPMENT I 09 Objects: Inheritance, Polymorphism and Dynamic Binding 2018W