Interfaces Advanced Programming Techniques

Transcription

1 Interfaces Advanced Programming Techniques Prof. Dr. Bernhard Humm FB Informatik, Hochschule Darmstadt 1

2 Agenda 1. Principles 2. Interface as a concept 3. Interfaces in Java 4. Advanced interface concepts 5. Literature 6. Quick Check 2

3 Orthogonality Dimension 2 Orthogonality: A change in one dimension has no offect on the other dimension. Dimension 1 Counter example: Helicopter flight control 3

4 The core of all design principles: Separation of Concerns / Orthogonality Dimension 2: GUI technology Orthogonality: Change of GUI technology (e.g., from Java servlet to HTML5/JavaScript) has no effect on DBMS technology chosen. Dimension 1: DBMS technology Separation of concerns: Separate a software solution into distinct sections, such that each section addresses a separate concern. 4

5 Information hiding / encapsulation David L. Parnas Information Hiding / Encapsulation: Hide implementation details of a class or a component from clients using it. Clients may access the class / component via its public interface only. 5

6 Discussion: Information hiding Advantages: - Encapsulated classes are easier to use (reduced complexity) - Implementation details may be modified without effects on other classes Disadvantages: - Access potentially slightly slower - Implementation details cannot be exploited by clients Strictly separte two roles / views: client view (service consumer) and implementation view (service provider) Client (service consumer) Interface Implementation (service provider) 6

7 Agenda 1. Principles 2. Interface as a concept 3. Interfaces in Java 4. Advanced interface concepts 5. Literature 6. Quick Check 7

8 Programming to interfaces Program to an interface, not an implementation Programming to interfaces is more than using the Java keyword interface It is a concept that can be used in all programming languages 8

9 Interfaces Interface: - Specifies the client view - Is simple - Is general: may be used in different contexts Implementation: - Implements the functionality specified by the interface - Is complex - Is specific cd Collections «interface» List + add() : void + get() : void + contains() : void + remove() : void «realize» ArrayList «realize» LinkedList 9

10 Analogy: class versus human being Human being Every human being has many different properties. Only some of them are of importance, depending on the context: - Traffic: driver s license - Hospital: health insurance - Work: CV, job profile, salary - Home: all sorts of other properties Class A class, e.g., Customer, may have many differnt propoerties. Only some of them are of importance, depending on the context: Customer management: business logic and properties SortedSet: Comparable Client / Server communication: Serializable A class may implement various interfaces, for various contexts 10

11 Schnittstellen or, put differently ;-) Patient Cat Pet class Cat implements Patient, Pet { } Roger King: My cat is object-oriented 11

12 Definition Interface and Operation An Interface is specified by: - (1) A unique name - (2) A set of operations - (3) An interface protocol, specifying the order of operation invocations or restrictions thereof An Operation is specified by: - (1) Signature (syntax): Name of the operation, parameters and return values, their data types, exceptions - (2) Semantics: Behaviour of the operation - (3) Non-functional properties: e.g., performance, availability, price, etc. 12

13 Interface specification Interface Name Operationen Signatur Semantik Interface name public interface OrderManager { Return type Operation name Input parameter and type Order placeorder (Customer c, Article a) throws NotAvailableException; } Pre-conditions, post-conditions, etc. (text) Exception Nicht-funktionale Eigenschaften Protokoll Text Text, sequence diagrams, etc. 13

14 Preconditions and postconditions Preconditions - Must be satisfied before operation invocation, e.g., restrictions on parameter values - Client s responsibility! - If not satisfied service provider may throw exception - Example web shop: article must be available before being ordered Postconditions - Must be satisfied after operation invocation, e.g., restrictions on result - Service provider s responsibility! - Client can assume correctness (may throw exception if not satisfied) - Example web shop: Invoice is generated after order 14

15 Agenda 1. Principles 2. Interface as a concept 3. Interfaces in Java 4. Advanced interface concepts 5. Literature 6. Quick Check 15

16 Java interfaces: Declaration Interfaces may contain: - Instance methods - Constants - Class methods (keyword static; since Java 8) - Default methods (feature for upward compatibility; since Java 8) No constructors No implementation (except default methods) Example: interface Customer { public String getname(); public void setname(string name); } 16

17 Java interfaces: Implementation by a class A class that implements an interface must implement all instance methods A class may implement multiple interfaces An interface may be implemented by multiple classes Example: class CustomerImpl implements Customer { public CustomerImpl(String name) {...} public String getname() {...} public void setname(string name) {...} } 17

18 Java interfaces: Usage Use of interface nearly everywhere in client code Use of class only in one position: constructor invocation Customer mycustomer = new CustomerImpl("Willy");... Map <String, Customer> mycustomers; mycustomers = new HashMap <String, Customer>(); mycustomers.put("1234", mycustomer);... class SomeOtherClass { public boolean processcustomer(customer cust) {... }; } 18

19 Programming to interfaces, not to classes Configuration: Specification of concrete class Client Interface <<uses>> <<implements>> Implementation 19

20 Interfaces: Roles and views import import Client (service consumption) // Declaration, configuration I i = new C(); // Usage i.m(); Class is visible in configuration only Interface public interface I { public void m (); } Implementation (service provision) public class C implements I { public void m() { } } 20

21 Collection and Map interfaces and classes in Java <<interface>> Collection <<interface>> Map <<interface>> List <<interface>> Set <<interface>> SortedMap HashMap TreeMap HashTable ArrayList Vector <<interface>> SortedSet LinkedList Stack TreeSet 21

22 *ables Adjustable Appendable Callable Cloneable Comparable Destroyable Externalizable Flushable Formatable Identifyable Iterable Joinable Manageable Printable Readable Referenceable Refreshable Runnable Scrollable Serializable Streamable Throwable Timeable Transferable Writeable 22

23 The principle of minimal assumption The principle of minimal assumption Program to an interface, not an implementation e.g., List instead of ArrayList, Map instead of HashMap. Use the most general interface which provides the operations required, e.g., Collection instead of List, Iterator instead of ListIterator 23

24 Discussion: When to provide a Java interface? The more Java interfaces the better the code quality? Should every Java class have an associated Java interface? If not: when does it make sense to provide a Java interface? Rule of thumb Provide a Java interface if different implementations are intended 24

25 The KISS principle Keep It Short and Simple! Discussion: give examples of KISS 25

26 Agenda 1. Principles 2. Interface as a concept 3. Interfaces in Java 4. Advanced interface concepts 5. Literature 6. Quick Check 26

27 Convention for this course: UML socket symbol used for required interfaces only Who defines the interface? Source: Siedersleben: Moderne Softwarearchitektur Chapter 3 Interface provided Client I Service provider e.g., log4j Logger e.g., log4j SimpleLogger Standard Interface e.g., Serializable Client I Service provider Client Call-back interface, e.g., Observer Interface required I Service provider e.g., SAP Adapter Interface required Interface provided Client I Adapter I Service provider 27

28 Dependency inversion: Observer pattern addobserver, deleteobserver, <<uses>> Observable Client (Observer) <<implements>> Observer Service provider (Observable) Update (call-back) Dependency inversion principle The service provider (Observable) exports the provided as well as the required interface. Unidirectional dependency (Observer Observable) despite of bidirectional method invocations 28

29 Agenda 1. Principles 2. Interface as a concept 3. Interfaces in Java 4. Advanced interface concepts 5. Literature 6. Quick Check 29

30 Literature Johannes Siedersleben (Hrsg.): Quasar: Die sd&m Standard-Architektur, Teil 1 da.de/fileadmin/personal/b.humm/publikationen/siedersleben_- _Quasar_1 sd_m_brosch_re_.pdf Abschnitt Schnittstellen 30

31 Agenda 1. Principles 2. Interface as a concept 3. Interfaces in Java 4. Advanced interface concepts 5. Literature 6. Quick Check 31

32 Quick Check Interfaces 1. What does orthogonality / separation of concerns mean? 2. What is the principle of information hiding / encapsulation? What are advantages and disadvantages? 3. What is the different between the client view (service consumer) and the implementation view (service provider)? 4. What does it mean to program to an interface, not to an implementation? 5. Define interface and operation 6. How to specify an interface? 7. What are preconditions and postconditions? 8. How do declare, implement, and use interfaces in Java? 32

33 Quick Check Interfaces (cont d) 9. What is the principle of minimal assumption? 10. Distinguish different kinds of interface usages depending on who is defining the interfaces 11. What is the difference between a provided interface and a required interface? 12. Explain the dependency inversion principle 13. How to decouple components via required interfaces and adapters? 33