Framework. Set of cooperating classes/interfaces. Example: Swing package is framework for problem domain of GUI programming

Transcription

1 Frameworks 1

2 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 2

3 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 3

4 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) 4

5 Inversion of Control Framework classes, not application-specific classes, control flow of execution Framework determines which methods to call and when 5

6 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 6

7 Applet characteristics No main() method Programmer overrides some or all of the following methods: init() start() stop() destroy() paint() 7

8 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 8

9 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() 9

10 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 10

11 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 11

12 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 12

13 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) 13

14 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() 14

15 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 15

16 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 16

17 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) 17

18 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 18

19 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 treess, respectively to store the set elements LinkedList and ArrayList: implementations of the List interface 19

20 Collections Framework 20

21 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 21

22 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 22

23 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 23

24 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? 24

25 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() 25

26 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 26

27 Optional operations API documentation tags some methods as optional operations Default implementations throw UnsupportedOperationException Collection methods add() and remove() are examples 27

28 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 28

29 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 29

30 Beyond the API: Greenfoot Greenfoot: a framework for teaching Java programming to beginners From the makers of BlueJ Problem domain is simple game programming 30

31 Greenfoot (user) interface 31

32 Behind Greenfoot is Java 32

33 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 33

34 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 34

35 Simple Graph Editor Classes & Interfaces 35

36 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 36

37 Division of responsibility Drawing shapes of nodes & edges: application Hit testing (element hit by mouse click): application Drawing toolbar: framework Mouse listening: framework 37

38 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 38

39 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 39

40 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. 40

41 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. 41

42 Prototype pattern 42

43 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 43

44 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 44

45 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 45

46 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() 46

47 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 47

48 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 48