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Frameworks

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Framework

• Set of cooperating classes/interfaces – Structure essential mechanisms of a problem

domain – Programmer can extend framework classes,

creating new functionality • Example: Swing package is framework for

problem domain of GUI programming

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Frameworks Vs. Patterns

• Pattern: general design rule for solving a problem

• Framework: set of classes providing functionality in a particular domain; often uses multiple patterns

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Application Frameworks

• Implement services common to any type of application

• Supply sets of classes for programmer to augment to build applications (often by subclassing framework classes)

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Inversion of Control

• Framework classes, not application-specific classes, control flow of execution

• Framework determines which methods to call and when

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Framework example: Applets

• Applet: Java program originally intended to run within a web page to add interactions and dynamism

• Java source referenced by an HTML document • Most browsers no longer support applets • Package java.applet is a simple application

framework – consists of superclasses to make applets – programmer adds classes, overrides methods to create

actual applets

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Applet characteristics

• No main() method • Programmer overrides some or all of the

following methods: – init() – start() – stop() – destroy() – paint()

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How applets work

• Since an applet has no main() method, initialization of object takes place in method init(), which takes the place of both main() and the constructor

• Method init() is typically used to perform initializations of short duration; longer running initializations, e.g. loading a file across a network, should be done in separate threads

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Methods of Applet class

• Method init() is empty, and must be overridden by the child class

• Method start() is called to set up the applet to run – may create several threads – called when applet is loaded & initialized the

first time, resized, or when user returns to page from elsewhere

– paint() automatically called after start()

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Methods of Applet class

• Method stop() is called to stop the applet - usually means halting threads started by start() when, for example, the user leaves the page

• Method destroy() performs final cleanup before the applet is unloaded

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Methods of Applet class

• Control sequence is: – init() – start() – paint() – then stop() or destroy(), as appropriate

• Applet can transition many times between start() and stop() states

• May define all, some, or none of these methods when extending an Applet; should only define those necessary for specific applet

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More fun facts about applets

• Applet class is actually subclass of Panel - so inherits graphical component attributes

• Events, in particular, are handled same way as other graphical components, and window repainting same as application

• Default layout for applet is flow layout, not border layout

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Applets vs. Applications

• An applet has no main method, as it is structured to run inside another application

• The Java Virtual Machine runs applications, but not applets

• Can use appletviewer program to test and debug applet code

• Most applets are event-driven (but don’t have to be)

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Creating Applet/Application Hybrids

• Applet class pays no attention to static methods contained in class definition

• Key idea is that Applet is a Panel – can nest inner class within Applet to create Frame necessary for

application – only component created for this Frame will be Panel constructed

by Applet – Method main(), which is ignored when executing as applet, creates

instance of applet when run as an application – Applet then creates instance of Frame, placing itself in center – Constructor for Frame executes requisite init() and start()

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Applets as Framework

• Programmer for individual applet uses inheritance to extend the applet framework class to specific program

• Applet class deals with behavior common to all applets; programmer only fills in custom behavior for specific application

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Applets as Framework

• Inversion of control means applet programmer not concerned with overall control flow - fills in handlers for initialization, starting, stopping and painting, but has no control over when these methods are called

Frameworks in the Java API • As we have just seen, the Applet framework

is one example from the Java API; the entire set of swing classes is another (but much larger, won’t fit on one screen)

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Java Collections Framework

• Collection: an object that contains a group of objects

• Java API contains library of useful data structures to describe various Collection types

• Collections library also serves as framework for adding new collection classes that can interact with existing classes

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Java Collections Framework

• Collections framework specifies four interfaces: – Collection: general collection (bag) – Set: unordered collection of unique elements – SortedSet: ordered collection of unique elements – List: ordered collection of elements; can contain duplicates

• Several concrete classes as well - some examples: – HashSet and TreeSet: set implementations that use hashing and

balanced binary trees, respectively to store the set elements – LinkedList and ArrayList: implementations of the List interface

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Collections Framework

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Collection and Iterator

• Collection and Iterator are the fundamental interfaces of the collections framework

• A collection is any class that can store multiple elements; individual collection classes enforce different rules for how data are to be stored and located

• An iterator is a mechanism for visiting all elements in a collection

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Methods specified by Collection interface

boolean add(Object obj) boolean addAll(Collection c) void clear() boolean contains(Object obj) boolean containsAll(Collection c) boolean equals(Object obj) int hashCode()

boolean isEmpty() Iterator iterator() boolean remove(Object obj) boolean removeAll(Collection c) boolean retainAll(Collection c) int size() Object[] toArray() Object[] toArray(Object[] a

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AbstractCollection class

• Collection is a hefty interface: client programmer must implement 15 methods

• AbstractCollection class relieves client programmer of this burden; provides reasonable default implementations for almost all of these methods

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Example method from AbstractCollection

public Object[] toArray() { Object[] result = new Object[size()]; Iterator e = iterator(); for (int i=0; e.hasNext(); i++) result[i] = e.next(); return result; }

Note use of size() and iterator(); these are the two methods left undefined by the abstract class. Which pattern is in play here?

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Notes on AbstractCollection

• Methods like toArray() implement the template method pattern; since they are implementations, not just specifications, they must appear in a class

• Methods size() and iterator() are left undefined by AbstractCollection; most concrete collections derived from this class also override add() and remove()

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Adding new class to Collections framework

• Text uses example of Queue class; originally developed in chapter 3 without connection to framework

• Why use framework? – Framework provides a number of methods that work on

all Collections; addAll(), for example, does bulk addition of all elements from one collection to another

– If Queue is part of framework, such methods are automatically applicable to the new class

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Optional operations

• API documentation tags some methods as optional operations

• Default implementations throw UnsupportedOperationException

• Collection methods add() and remove() are examples

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Views

• View: object that implements an interface of collections framework, but permits only restricted access to data structure

• Optional operations exist to support views • Built-in Java type array has no methods; can apply

view to enhance array functionality

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Views

• Example: asList method – asList turns an array into a view object: collection that

implements List interface, so List methods can be applied to array elements

– does not copy array elements; get and set methods of view object access original array

– Can’t apply add() or remove() methods to view, because size of underlying array can’t be changed: this is why these methods are “optional”

Beyond the API: Greenfoot

• Greenfoot: a framework for teaching Java programming to beginners – From the makers of BlueJ – Problem domain is simple game programming

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Greenfoot (user) interface

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Behind Greenfoot is Java …

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Graph editor framework

• Problem domain: interactive editing of graphs

• Graph: collection of nodes and edges arranged in certain shape

• Examples of graphs: – class relationship diagrams – electronic circuit diagrams – flow charts

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Graph editor framework

• Encapsulates aspects common to all graph editing applications – user interface – event handling

• Application programmer extends graph editor framework, defines specific behavior for nodes and edges

Simple Graph Editor Classes & Interfaces

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User interface Toolbar includes: • grabber tool for selecting elements • buttons for each node/edge type • menus:

• loading/saving diagram • deleting selected elements

Drawing area: • Mouse used for drawing; behavior depends on current tool:

• if node, clicking empty space inserts new node • if grabber, clicking on element selects it; dragging operation moves selected node & connected edges • if edge, drag from one node to another inserts edge

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Division of responsibility

• Drawing shapes of nodes & edges: application

• Hit testing (element hit by mouse click): application

• Drawing toolbar: framework • Mouse listening: framework

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Division of responsibility

• Application programmer must inform framework about node & edge types of particular graph type – concrete graph gives framework prototype objects – toolbar queries graph for prototypes, adds buttons for

each one – nodes & edges draw themselves in paintIcon method of

button icon object – when user inserts new node or edge, object

corresponding to selected tool is cloned & added to graph

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Prototype pattern

• Teaches how a system can instantiate classes that are not known when system is built

• Nature of node & edge types unknown when framework code is designed - use prototype pattern to solve this problem

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Prototype pattern

• Context: – A system instantiates objects of classes that are

not known when the system is built. – You do not want to require a separate class for

each kind of object. – You want to avoid a separate hierarchy of

classes whose responsibility it is to create the objects.

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Prototype pattern

• Solution: – Define a prototype interface type that is

common to all created objects. – Supply a prototype object for each kind of

object that the system creates. – Clone the prototype object whenever a new

object of the given kind is required.

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Prototype pattern

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Prototype pattern applied to graph editor

• Name in pattern: – Prototype – ConcretePrototype1 – Creator

• Name in graph editor: – Node – CircleNode – Instance of GraphPanel

that handles mouse operation for adding new nodes

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Framework classes

• Node and Edge interfaces include methods: – draw(): used in painting graph – contains(): tests whether mouse click falls

within range of an element – getBounds(): returns rectangle enclosing

element – clone(): clones prototypes when inserting new

elements

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Framework classes

• Edge interface includes methods getStart() and getEnd() which return nodes connected by edge

• Node interface method getConnectionPoint() returns optimal attachment point for edge

• Edge method getConnectionPoints() yields 2 end points of edge; used by grabbers to mark currently selected edge

• Framework also supplies AbstractEdge class for programmer convenience

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Framework classes

• Graph class: – collects nodes and edges – has methods for adding, removing, finding and

drawing nodes and edges – is abstract; subclasses must override methods

that populate the toolbar: public abstract Node[] getNodePrototypes() public abstract Edge[] getEdgePrototypes()

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Framework classes

• GraphFrame class manages the toolbar, menu bar, and graph panel

• Toolbar class holds toggle buttons for node and edge icons

• GraphPanel class displays graph, handles mouse events for editing commands

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Building Graph Editor Application

• For each node and edge type, define class that implements Node or Edge

• Define subclass of Graph with methods getNodePrototypes and getEdgePrototypes defined to supply prototype objects for graph elements

• Supply a main() method