Encapsula)on, cont d. Polymorphism, Inheritance part 1. COMP 401, Spring 2015 Lecture 7 1/29/2015

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1 Encapsula)on, cont d. Polymorphism, Inheritance part 1 COMP 401, Spring 2015 Lecture 7 1/29/2015

2 Encapsula)on In Prac)ce Part 2: Separate Exposed Behavior Define an interface for all exposed behavior In Java, an interface is like a contract. Indicates that a certain set of public methods are available. One or more classes can indicate that they implement the interface. Name of interface can be used as a type name Just like class names are used as type names. Value of an interface type variable can be set to any object that is an instance of a class that implements the interface. Mark constructors as public

3 Interfaces in Java Like classes, should go in their own.java file Should have same name as file Only one public interface per file. Body of interface is a just list of method signatures. Implemen)ng classes MUST declare these methods as public Form: public interface InterfaceName {! type method1(parameters);! type method2(parameters);! // etc! }! Classes specify which interfaces they implement with implements modifier as in: public class ClassName implements InterfaceA, InferfaceB {!

4 Interface Naming Conven)ons Interface name must be different from class names that implement the interface. Conven)on A Start all interface names with I for interface. For example: ITriangle, IPoint Class names can be anything that is not in this form. Conven)on B Use generic abstrac)on name for interface. Make class names descrip)ve of implementa)on If no natural way to do this, simply append Impl to generic abstrac)on name to differen)ate. Personally, I generally go with conven)on B.

5 lec6. v6 Separates Point into an interface and an implemen)ng class. No)ce that distanceto() and equals() are part of behavior I want the abstrac)on to expose. Must be marked public No)ce that main method uses variables with type Point (the interface name), but that the actual object is created as an instance of a specific class that implements the interface Point. No)ce that Triangle only interacts with the methods specified in the Point interface.

6 Advantage of Encapsula)on Can provide different implementa)ons of the same behavior lec6.v7 Create a new implementa)on of Point based on polar coordinates.

7 Exposed vs Internal Behavior Exposed behavior should be reflected in the interface(s) that a class implements Recall that any method declared in an interface must be defined by an implemen)ng class as a public method. Internal behavior should be hidden Use private modifier on these methods to ensure that access only occurs within the class

8 lec6.v8 Con)nued applica)on of encapsula)on principle to Triangle by defining Triangle as an interface rewri)ng what used to be the class Triangle as the class PointTriangle that implements the interface hiding internal behaviors as private methods

9 Summing Up A Java file defines one public class or public interface. To support encapsula)on: External behavior of an abstrac)on defined as an interface. JavaBeans geders and seders for direct or derived proper)es. Other methods that are part of the abstrac)on. A class provides the implementa)on of one or more interfaces. All fields within a class are marked as private. Public constructor Methods that implement any interface(s) must be public. Internal methods marked as private.

10 Do you always need an interface? Best prac)ce is to separate an abstrac)on into an interface and a class that implements it. Allows you to have mul)ple classes that implement the interface in different ways. For simple classes where you know there will only be one implementa)on, you can get away without defining the interface separately. Should s)ll: mark fields as private, constructor as public, make a dis)nc)on between public methods for external behavior and private methods for internal behavior

11 Polymorphism Poly = many, morph = forms General principle of providing access to an abstrac)on or method in many forms Idea is that different forms fit different contexts Goal of the underlying func)onality is the same. In OO programming, principle is evident in a number of different places. Constructor overloading Method overloading

12 Constructors What happens when you don t define a constructor. Default constructor with no arguments. lec7.ex1 Creates new object with all fields set to default value Numeric fields set to 0 Boolean fields set to false String, Array, and any other sort of reference value field set to null.

13 An aside on puing more than one class in a file. A Java file can only have one public class. A class marked public can be imported into other code. Must match file name One or more non- public classes in a Java file can also be defined Only available to code in the same package. Does not have to match file name Oken used to define classes that are related to the public class in that file.

14 Constructor Overloading Can define mul)ple versions of the constructor. Dis)nguished from each other by type and number of parameters Must be some difference otherwise the compiler won t be able to tell them apart. When you use the constructor, the right one will be chosen based on the parameters provided. Note that once you define at least one constructor, the implicit, default no- argument constructor is not automa)cally available. lec7.ex2

15 Constructor Chaining Common padern is to chain one constructor off of another. To do this, the first line of code in the constructor must be the this keyword used as if a func)on with parameters. The matching constructor is called first and allowed to execute. Then any subsequent code is executed. Can chain mul)ple constructors one on to another lec7.ex3

16 Method Overloading Regular methods can also be overloaded Same method name defined more than once. Return type may or may not be the same. But usually is. Method type must be the same. Instance method or sta)c class method Same access modifier Parameter list must somehow be different Again, this is how the compiler knows which one is meant. Either different in number or type (or both) One version can call another No restric)ons on when No special syntax lec7.ex4, lec7.ex5

17 Why Overload? Provides access to constructor / method in a more context specific way. Limita)ons of overloading Does not handle the case when you have two different situa)ons that aren t dis)nguished by the number or type of parameters being passed.

18 Recap of Interfaces A contract for behavior. Defined by a set of method signatures. Acts as a data type. No implementa)on. Specific classes implement the interface

19 Song and Video as Media public interface Media { int getlengthinseconds(); double getlengthinminutes(); int getra)ng(); void setra)ng(int new_ra)ng); String getname(); } public class Song implements Media {... We expect Song and Video to have methods matching those specified in Media. public class Video implements Media {...

20 Is- A and cas)ng A class that implements an interface creates an is- a rela)onship between class data type and the interface data type. A implements B => A is a B Song is a Media Video is a Media Cas)ng allowed across an is- a rela)onship. Song s = new Song(); Media m; m = (Media) s; Video v = new Video(); Media m; m = (Media) v; Video v = new Video(); Song s; s = (Song) v; 20

21 Inheritance What is inheritance in real life? Characteris)cs / resources that you receive from your parents Get these automa)cally. Part of who you are. Similar idea in object- oriented programming. In Java, concept of inheritance applied to both interfaces and classes. Both signaled by the keyword extends Similar in concept, but details are dis)nctly different. Class inheritance more complex.

22 Extending Interfaces Adds methods to the contract. Original: parent interface, super interface New: subinterface, child interface, extended interface Created by using the extends keyword. public interface CompressedMedia extends Media { int getcompressedsize(); int getuncompressedsize(); Media uncompress(); }

23 Extension Creates Hierarchy Is- A rela)onship is transi)ve up the hierarchy. Methods for both must be provided. Media extends Compressed Media public class Song implements CompressedMedia {... Song s = new Song(); CompressedMedia cm = (CompressedMedia) s; Media m = (Media) s; OK because s is a Compressed Media OK because s is a Media by virtue of extension. Song s2 = (Song) m; Cas=ng from interface back to specific object type is allowed, but at run=me, if the object s type does not actually match, a run=me excep=on will be thrown.

24 Extension vs. Mul)ple Interfaces Interface extension appropriate when addi)onal methods make no sense without methods of the parent interface. Alterna)vely, can use mul)ple interfaces together as facets of an object.

25 Extension vs. Mul)ple Interfaces public interface Compressed { int getcompressedsize(); int getuncompressedsize(); Media uncompress(); } Instead of extending Media, Compressed is a separate interface and Song implements both. public class Song implements Compressed, Media {... public interface Media { int getlengthinseconds(); double getlengthinminutes(); int getra)ng(); void setra)ng(int new_ra)ng); String getname(); } Song s = new Song(); Media m = (Media) s; Song is a Media AND Song is a Compressed. Compressed c = (Compressed) s;

26 Subinterface Mul)ple Inheritance Mul)ple inheritance for interfaces is allowed. A subinterface can extend more than one exis)ng interface. In this case, just a union of all methods declared in all of the parent interfaces. lec7.ex6 Plus, of course, any addi)onal ones added by the subinterface.

27 Mo)va)ng Enumera)ons Oken need to model part of an object as one value from a set of finite choices Examples: Suite of a playing card Day of week Direc)ons of a compass One approach is to use named constants lec7.ex7 Drawbacks of this approach No type safety No value safety

28 Simple Java Enumera)ons General syntax: access_type enum EnumName {symbol, symbol,...};! Example: public enum Genre {POP, RAP, JAZZ, INDIE, CLASSICAL} Enumera)on name acts as the data type for the enumerated values. Enumerated values available as EnumName.symbol as in: Genre.POP Outside of the class Fully qualified name required as in: Song.Genre.POP Symbol names don t have to all caps, but that is tradi)onal lec7.ex8

29 Enumera)ons in Interfaces Enumera)ons can be defined within an interface. Useful when enumera)on is related to the interface as an abstrac)on and will be needed within any specific implementa)on.

30 Not so simple enumera)ons Java enumera)ons are actually much more powerful than this. Check out this tutorial for more: hdp://javarevisited.blogspot.com/2011/08/ enum- in- java- example- tutorial.html Also posted on course resources page in Piazza.

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