This will require extra methods for traversal and object retrieval. These methods are put into an abstract Iterator interface.
Each element then implements a derivation of the Iterator , depending on how that element keeps its list ArrayIterator , LinkListIterator , etc. Functions for traversal and retrieval are put into the abstract Iterator interface. Future Iterators can be derived based on the type of list they will be iterating through, such as Arrays or Linked Lists. Thus, no matter what type of indexing method any implementation of the element uses, it will have the appropriate Iterator.
This is an implementation of the Iterator pattern. It allows the client to traverse through any object collection, without needing to access the contents of the collection directly, or be concerned about the type of list the collection's structure uses.
Now that traversal has been handled, it is possible to analyze the elements of a structure. It is not feasible to build each type of analysis into the element structure themselves; every element would need to be coded, and much of the code would be the same for similar elements. Instead, a generic CheckMe method is built into the element's abstract class. Each Iterator is given a reference to a specific algorithm such as spell check, grammar check, etc.
When that Iterator iterates through its collection, it calls each element's CheckMe , passing the specified algorithm. CheckMe then passes a reference to its element back to said algorithm for analysis.
Thus, to perform a spell check, a front-to-end iterator would be given a reference to a SpellCheck object. The iterator would then access each element, executing its CheckMe method with the SpellCheck parameter.
Each CheckMe would then call the SpellCheck , passing a reference to the appropriate element. In this manner, any algorithm can be used with any traversal method, without hard-code coupling one with the other. For example, Find can be used as 'find next' or 'find previous', depending on if a 'forward' iterator was used, or a 'backwards' iterator. In addition, the algorithms themselves can be responsible for dealing with different elements. For example, a SpellCheck algorithm would ignore a Graphic element, rather than having to program every Graphic -derived element to not send themselves to a SpellCheck.
Creational patterns are ones that create objects, rather than having to instantiate objects directly. This gives the program more flexibility in deciding which objects need to be created for a given case. These concern class and object composition. They use inheritance to compose interfaces and define ways to compose objects to obtain new functionality.
Most of these design patterns are specifically concerned with communication between objects. Criticism has been directed at the concept of software design patterns generally, and at Design Patterns specifically. When I see patterns in my programs, I consider it a sign of trouble. The shape of a program should reflect only the problem it needs to solve.
Any other regularity in the code is a sign, to me at least, that I'm using abstractions that aren't powerful enough-- often that I'm generating by hand the expansions of some macro that I need to write.
In an interview with InformIT in , Erich Gamma stated that the book authors had a discussion in on how they would have refactored the book and concluded that they would have recategorized some patterns, added a few additional ones and removed one of them Singleton altogether. Introduction, Chapter 1 [ edit ] Chapter 1 is a discussion of object-oriented design techniques, based on the authors' experience, which they believe would lead to good object-oriented software design, including: 'Program to an 'interface', not an ' implementation'.
Design Patterns Elements Of Reusable The authors refer to inheritance as white-box reuse , withwhite-box referring to visibility, because the internals of parent classes are often visible to subclasses. The danger is stated as follows: 'Because inheritance exposes a subclass to details of its parent's implementation, it's often said that 'inheritance breaks encapsulation'. Gang of Four They warn that the implementation of a subclass can become so bound up with the implementation of its parent class that any change in the parent's implementation will force the subclass to change.
The authors admit that delegation and parameterization are very powerful but add a warning: 'Dynamic, highly parameterized software is harder to understand and build than more static software. The seven problems including their constraints and their solutions including the pattern s referenced , are as follows: Document Structure [ edit ] The document is 'an arrangement of basic graphical elements' such as characters, lines, other shapes, etc.
Problems and Constraints Text and graphics should be treated the same way that is, graphics aren't a derived instance of text, nor vice versa The implementation should treat complex and simple structures the same way. It should not have to know the difference between the two. Specific derivatives of abstract elements should have specialized analytical elements.
Solution and Pattern A recursive composition is a hierarchical structure of elements, that builds 'increasingly complex elements out of simpler ones' pp. Formatting [ edit ] Formatting differs from structure. Problems and Constraints Balance between formatting quality, speed and storage space Keep formatting independent uncoupled from the document structure.
Solution and Pattern A Compositor class will encapsulate the algorithm used to format a composition. Embellishing the User Interface [ edit ] The ability to change the graphical interface that the user uses to interact with the document. Problems and Constraints Demarcate a page of text with a border around the editing area Scroll bars that let the user view different parts of the page User interface objects should not know about the embellishments Avoid an 'explosion of classes' that would be caused by subclassing for 'every possible combination of embellishments' and elements p.
Problems and Constraints The editor must implement standards of multiple platforms so that it is portable Easily adapt to new and emergent standards Allow for run-time changing of look-and-feel i.
Have a set of concrete subclasses for each abstract subclass that can have a different look-and-feel standard. ScrollBar having MotifScrollBar and PresentationScrollBar for Motif and Presentation look-and-feels Solution and Pattern Since object creation of different concrete objects cannot be done at runtime, the object creation process must be abstracted. Supporting Multiple Window Systems [ edit ] Just as look-and-feel is different across platforms, so is the method of handling windows.
Problems and Constraints The document editor must run on many of the 'important and largely incompatible window systems' that exist p. Due to differing standards, there will not be a common abstract class for each type of widget. Do not create a new, nonstandard windowing system Solution and Pattern It is possible to develop 'our own abstract and concrete product classes', because 'all window systems do generally the same thing' p.
User Operations [ edit ] All actions the user can take with the document, ranging from entering text, changing formatting, quitting, saving, etc. Problems and Constraints Operations must be accessed through different inputs, such as a menu option and a keyboard shortcut for the same command Each option has an interface, which should be modifiable Operations are implemented in several different classes In order to avoid coupling, there must not be a lot of dependencies between implementation and user interface classes.
Menus should be treated like hierarchical composite structures. Hence, a menu is a menu item that contains menu items which may contain other menu items, etc. Solution and Pattern Each menu item, rather than being instantiated with a list of parameters, is instead done with a Command object. Spell Check and Hyphenation [ edit ] This is the document editor's ability to textually analyze the contents of a document.
Problems and Constraints Allow for multiple ways to check spelling and identify places for hyphenation Allow for expansion for future analysis e. Solution and Pattern Removing the integer-based index from the basic element allows for a different iteration interface to be implemented. Patterns by Type [ edit ] Creational [ edit ] Creational patterns are ones that create objects, rather than having to instantiate objects directly. Abstract factory groups object factories that have a common theme.
Builder constructs complex objects by separating construction and representation. Factory method creates objects without specifying the exact class to create. Prototype creates objects by cloning an existing object. Singleton restricts object creation for a class to only one instance. May 18, at am. Krishna says:. May 25, at am.
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Si presenta in primo luogo la classe RandomWalker che rappresenta il viaggiatore. You are now stopped at km This place is 14 kms far from your last stop. Do you like this place? Starting trip This place is 33 kms far from your last stop. This place is 11 kms far from your last stop. This place is 91 kms far from your last stop. Observer Observer GoF pag. Esempio Ad un oggetto Subject vengono comunicati diversi numeri. Questo oggetto decide in modo casuale di cambiare il suo stato interno, memorizzando il numero ad esso proposto.
Applicazione del pattern Schema del modello Observer java. Observable java. Observer, e la classe java. Ha questo costruttore: - Observable Costruisce un Observable senza Observers registrati. Ogni istanza di questo ConcreteSubject riceve semplicemente un numero tramite il metodo receiveValue, e con una scelta a caso decide di copiare o meno questo valore nella propria variabile di stato value.
Observer; import java. Watcher : I see that the Subject holds now a 1. Main : Do you like the number 2? Subject : I have a number 1 now, and I not interested in the number 2.
Main : Do you like the number 3? Watcher : I see that the Subject holds now a 3. Main : Do you like the number 4? Watcher : I see that the Subject holds now a 4. Main : Do you like the number 5? Watcher : I see that the Subject holds now a 5.
Main : Do you like the number 6? Watcher : I see that the Subject holds now a 6. Main : Do you like the number 7?
Watcher : I see that the Subject holds now a 7. Main : Do you like the number 8? Watcher : I see that the Subject holds now a 8. Main : Do you like the number 9? Subject : I have a number 8 now, and I not interested in the number 9.
Main : Do you like the number 10? Subject : I have a number 8 now, and I not interested in the number 10 The Subject has changed 7 times the internal value. The Psychologist opinion is: The Subject likes little numbers. Altre idee interessanti riguardanti questo pattern sono presenti negli articoli di Lopez [11] e Bishop [2].
State State GoF pag. Descrizione Consente ad un oggetto modificare il suo comportamento quando il suo stato interno cambia. Si vuole definire un meccanismo efficiente per gestire i diversi comportamenti che devono avere le operazioni di un oggetto, secondo gli stati in cui si trovi.
Ogni classe ConcreteState rappresenta un singolo stato possibile del Context e implementa una interfaccia comune State contenente le operazioni che il Context delega allo stato.
Il suo costruttore richiama il costruttore della superclasse per la gestione del riferimento al rispettivo oggetto Clock. Al momento della creazione, ogni clock viene settato alle ore e con nello stato di visualizzazione normale.
Preme il tasto MODE : attiva lo stato di modifica delle ore. Preme il tasto MODE: attiva lo stato di modifica dei minuti 5. Preme il tasto MODE: ritorna allo stato di visualizzazione normale. Vale dire, una particolare operazione eseguita in uno stato, crea un nuovo stato e lo assegna come stato corrente.
Strategy Strategy GoF pag. Questo permette modificare gli algoritmi in modo indipendente dai clienti che fanno uso di essi. Tra i metodi di questa classe si ha definito uno che esegue la propria stampa. Questo metodo potrebbe stampare il vettore nel seguente modo chiamato, ad es. Questa interfaccia deve fornire un accesso efficiente ai dati del Context, richiesti da ogni ConcreteStrategy, e viceversa.
Il Context utilizza questa interfaccia per invocare gli algoritmi definiti in ogni ConcreteStrategy. Strategy Descrizione del codice Si implementa la classe MyArray Context che mantiene al suo interno un array di numeri, gestiti tramite i metodi setValue e getValue. Template Method Template method GoF pag.
Il Template Method consente ridefinire certi passi di un algoritmo senza cambiare la struttura di esso. Oltre a questo, il sistema gestisce delle quote che limitano il lotto da richiedere per volta. Implementazione Si presenta per primo la classe astratta ItemStock, che fornisce la logica generale del metodo remove. Lot Size: 5 I will try to take 6 units: Items cannot be removed. VRI1 Golden stone 20 Max. Lot Size: 5 I will try to take 3 units: Items removed.
VRI1 Golden stone 17 Max. GEN1 Common stone. On Hand: 0 In Transit: 10 Backorders: 3 Visitor Visitor GoF pag. Descrizione Rappresenta una operazione da essere eseguita in una collezione di elementi di una struttura.
Per esempio la collezione potrebbe essere un Vector che ha dentro di se oggetti String, Integer, Double o altri Vector. Applicazione del Pattern Schema del modello VisuitorExample java. La firma di ogni operazione identifica la classe che spedisce la richiesta di visita al ConcreteVisitor, e in questo modo il visitor determina la concreta classe da visitare. Finalmente il ConcreteVisitor accede agli elementi direttamente tramite la sua interfaccia.
Questo metodo soltanto riceve un riferimento ad un Visitor, e chiama la sua operazione di visita inviando se stesso come riferimento. Il problema di questo approccio riguarda il fatto che gli Element non potranno ereditare da nessuna altra classe. Tutti possono avere lo steso nome, dato che il metodo particolare da applicare viene identificato discriminato dal tipo di parametro specificato esempio, visit VisitableString s o visit VisitableFloat f.
Iterator; import java. Method; import java. Visitor System. Su di questo oggetto viene invocato il metodo getMethod che restituisce un oggetto Method, il quale fa riferimento a un metodo appartenente alla classe ReflectiveWatcherVisitor. The trick to "Iterator Observers": Factor out common code and make your Iterators observable.
Reflect on the Visitor design pattern. Implement visitors in Java, using reflection. Java Design Patterns. A tutorial. Upper Saddle River: Addison-Wesley When is a Singleton not a Singleton?
Addison-Wesley, Java 2 Tecniche Avanzate. McGraw-Hill, Composite pattern. Learn how to implement the Command pattern in Java. The singleton rule.
The Java tutorial: a practical guide for programmers. Implementing the Singleton Pattern in Java. Related Papers. Java - mattone dopo mattone. By Andrea Turel Caccese. By Ale Sa. Manuale pratico di Java. By Isromi Janwar.
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