Interface Patterns

Adapter

Provides the interface a client expects, using the services of a class with a different interface

Note

Avoid using object adapter if possible because the adaptee could change and make private things the adapter uses. Errors like this are less likely to be picked up at compile time.

Example

JTable – You adapt an array of your custom objects to provide the TableModel interface that JTable needs

Facade

Provides a simplified interface making a subsystem easy to use. It lies somewhere between a toolkit and a complete application.

Example

JOptionPane provides a simplified way to input choices based on JPanel and JButtons

Composite

Lets clients treat individual objects and compositions of objects uniformly.

Note composites may include more than one leaf class or composites with varied implementations of the operation method. However all must implement the component interface.

Example

Tree and directory structures.

Bridge

Decouples an abstraction from its implementations so that they can vary independently.

Example

Database drivers / Printer drivers

Responsibility Patterns

Singleton

Ensures a class has only a single instance… Resists the distribution of responsibility.

Note

Use a private constructor and create a method that calls it only once. Ensure the method is declared final.

Example

J2ME runtime class.

Drivers that are responsible for controlling hardware.

Observer

Example

JSlider.addChangeListener()

Event passing in the model - view – controller (MVC) pattern

Mediator

Takes responsibility off several objects. Defines an object that encapsulates how a set of objects interacts.

Note

The attached objects being mediated are usually of different classes and often send events to the mediator. Defining the interfaces the pattern uses (lower diagram) seems wise.

Example

Accessing a database to reflect real world changes.

Creating a GUI with lots of inter-related GUI components

Proxy

Provides a surrogate or placeholder for another object.

Examples

Image proxies – for while the true image is being downloaded

Network resource proxys

RMI / CORBA – for remote method invocation

Dynamic proxies using JAVA reflection

Defines a one to many dependency so when an object changes all its dependents are notified and update themselves automatically.

Note

In some cases a simplified subject allows only a single observer to be attached. Attempting to attach another will overwrite the first one or cause an exception to be thrown.

Chain of responsibility

Avoids coupling the sender of a request to receiver. The request is passed along a chain until it reaches an object able to handle it.

Example

Parser chains to decode an input that could be in several formats.

Customer support telephone networks

Traversing a composite (such as up a scene graph to find your position)

Event handling in windows Visual C++

Flyweight

Example

String (in JAVA)

Font information in a book

Uses sharing to support large numbers of fine grained objects efficiently. Multiple clients share the immutable part of an object.

Construction Patterns

Builder

Separates construction of a complex object from its representation, so the same process can create different representations.

Example

JAVA StringBuilder class

Parsers that take an input stream then use the information to build an object or representation.

Factory

Example

Object.toString() in the JAVA Object interface

The Iterator() method in the JAVA collection interface

Threadpools

Abstract factory

The intent of abstract factory or kit is to allow the creation of families of related or dependent objects, without specifying their concrete classes.

Example

Creating user interface components with different look & feels by using UIManager and UI delegates.

Prototype

Provides objects by copying an example

Example

copy constructors

Objects supporting the clone interface.

Lets a class developer define an interface for creating an object while retaining control over which class to instantiate. Also lets a class defer initialisation to subclasses.

Note

A factory is also frequently used to isolate a client from having to manage the reuse of objects.

Note

JAVA Objects default clone() method produces a shallow copy not a deep copy. You may need to override this method to ensure the objects state information is replicated correctly.

Memento

Without violating encapsulation, captures and externalizes an objects internal state so it can be restored to this state later.

Example

REDO and UNDO in applications.

Saving and restoring a position in a game.

Operation Patterns

Template

Defines an algorithm in an operation, deferring the definition of some steps so the subclass or other classes can supply them.

Example

Hooks

Sorting collections (you supply the comparator method)

State

Example

Sequencing complex GUIs (multi-input & multi-state)

State-machines

Strategy

Encapsulates different approaches in separate classes that each implement a common operation

Example

Various sorting algorithms that use different sorting techniques but all implement a method called sort() that achieve the same result.

Command

Encapsulates a request as an object, allowing a user or service to determine when to evoke it, add parameters to a request, queue or log requests and support undoable operations.

Example

Menu items that allow an ActionListener to be attached, which implements ActionPerformed() if the menu item is triggered.

Note

To avoid unnecessary state creation, context usually holds all the state objects, and a context reference is passed to the state object which uses it to obtain the state objects and methods for updating the present state of the context.

Note

The above UML diagram shows a subclass with an overridden sub-method. Alternatively the sub-method could be passed in using the command design pattern

Distributes state specific logic across classes that represent an objects state (encapsulating state logic into separate classes). The object appears to change its class whenever its state changes.

Note

In some cases a reference to the target is passed in as a parameter when the command is constructed, and is held within the command. The target often has an interface defined on which the command acts to achieve its functionality.

Interpreter

Lets you compose executable objects according to a set of composition rules. Defines the representation of a language. Defers the creation of executable objects until runtime.

Example

BASIC, JAVA, CNC machine control programs

Note

Expressions can be run in a particular context (such as producing results in a particular window)

Extension Patterns

Decorator

Attaches additional responsibilities, fields and method variations to an object at runtime. It is an alternative to sub classing for extending functionality.

Example

Buffered file writer

Filters for streams

iterator

Example

Collection iterators

for (class element: collection) loops

Composite iterators

Visitor

Lets you define a new method or methods without changing the classes on which they operate.

Example

Taxis – you call them, they visit you, then you use their public automobile and your cash to go out to a bar and have some fun.

An iterator is an object that sequentially accesses the elements of another aggregated object without exposing its underlying representation.

Note

In the above UML concrete visitor calls accept on a targetObject. The targetObject then calls the appropriate visit method of the visitor passing a reference to itself. The visitor can then use public fields and methods of the targetObject or its own fields and methods to achieve what is desired.