Post on 16-Dec-2015
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IteratorIterator
Design PurposeDesign Purpose– Provide a way to access the Provide a way to access the
elements of an aggregate object elements of an aggregate object sequentially without exposing its sequentially without exposing its underlying representationunderlying representation
Design Pattern SummaryDesign Pattern Summary– Encapsulate the iteration in a Encapsulate the iteration in a
class pointing (in effect) to an class pointing (in effect) to an element of the aggregateelement of the aggregate
IteratorIterator
Interface Iterator {// move to first item:void first(); // if it is at last item:boolean isDone(); // move point to next item:void next(); // Return the current:Object currentItem();
}
Using IteratorUsing Iterator
/* To perform desiredOperation() on items in thecontainer according to the iteration (order) i: */Iterator i = IteratorObject; for(i.first(); !i.isDone(); i.next())
desiredOperation(i.currentItem());
Iterator Sample CodeIterator Sample Code
// Suppose that we have iterators for forward and// backward order: we can re-use print_employees()List employees = new List();ForwardListIterator fwd
= employees.createForwardIterator();ReverseListIterator bckwd
= employees.createBackwardIterator();
// print from front to back client.print_employees(fwd); // print from back to front client.print_employees(bckwd);
Using IteratorUsing Iterator
Class Client extends … {
print_employees(Iterator it) { for(it.first(); !it.isDone(); it.next()) {
print(i.currentItem()); } }
// other operations
}
Iterator Class ModelIterator Class ModelClient
Iteratorfirst()next()isDone()currentItem()
ContainercreateIterator()append()remove()
ConcreteIterator
return new ConcreteIterator(this)
ConcreteContainer
createIterator()
Iterator - QuestionIterator - Question
What is the difference between the two clientsWhat is the difference between the two clientsClass Client1 {Class Client1 {void operation( Container c) { Iterator it = c.createIterator(); for (it.first(); it.isDone(); it.next()) { Object item = it.currentItem(); // process item }}} } Class Client2 {Class Client2 {void operation( Iterator it) {for (it.first(); it.isDone(); it.next()) { Object item = it.currentItem(); // process item }}}}
Iterator - QuestionIterator - Question
Client1
Iterator Container
Client2
Client1 is dependent upon two interfaces: Iterator and Container
Client2 is dependent upon only one interface: Iterator
Iterator in Java - 1Iterator in Java - 1
Interface Iterator<E> {Interface Iterator<E> {
// check if there are more elements. // check if there are more elements.
boolean hasNext();
// Returns the next element in the iteration. // Returns the next element in the iteration.
E E next() () // Removes from the underlying collection // Removes from the underlying collection
// the last element returned by the iterator// the last element returned by the iterator
// (optional operation). // (optional operation).
void void remove();();
} }
Iterator in Java - 2Iterator in Java - 2
import java.util.Iterator;
public class ArrayListViaListWithIterator<E> { private Node head, tail; private int count; public ArrayListViaLinkedListWithInnerIterator() { head = null; tail = null; count = 0; } public Iterator<E> iterator() { return new ALIterator(); } public void add(int i, E e) { … } public E get(int i) { … } public boolean isEmpty() { … } public E remove(int i) { … } public E set(int i, E e) { … } public int size() { … }
Iterator in Java - 3Iterator in Java - 3
private class Node { private E item; private Node next;
Node(E i, Node n) { … } public E getItem() { … } public void setItem(E item) {{ … } public Node getNext() { … } public void setNext(Node next) { … } } // end of Node
Iterator in Java - 4Iterator in Java - 4 private class ALIterator implements Iterator<E> { private Node cursor; public AIIterator() { cursor = head; } public boolean hasNext() { return cursor != null; } public E next() { Node tmp = cursor; cursor = cursor.getNext(); return tmp.getItem(); } public void remove() { throw new RuntimeException("not supported"); } } // end of Iterator} // end of ArrayList
ObserverObserver
Design PurposeDesign Purpose– Arrange for a set of objects to Arrange for a set of objects to
be affected by a single object.be affected by a single object. Design Pattern SummaryDesign Pattern Summary
– The single object aggregates the The single object aggregates the set, calling a method with a set, calling a method with a fixed name on each member.fixed name on each member.
Observer - StructureObserver - Structure
1..n
for o: observers o.update(this);
observers
SubjectAttach(Observer)Detach(Observer)Notify()
ObserverUpdate(Subject)
ConcreteSubject
stategetState()setState()
ConcreteObserver
observerStateUpdate(Subject s)
// change stateNotify();
…s.getState();…
Observer: Sequence Observer: Sequence diagramdiagram
Client O1:Observer
:Subject O2: Observer
setState() notify()
getState()
getState()
update()
update()
Observer in Observer in JavaJavaObservablenotifyObservers()attach(Observer)detach(Observer)
Observerupdate(Observable, Object )
MyObservablesubjectState
MyConcreteObserverobserverStateupdate(…)
subject
ObserverObserver: Key Concept: Key Concept
-- to keep a set of objects up to date with the state of a designated object.
MediatorMediator
Design PurposeDesign Purpose– Avoid references between Avoid references between
dependent objects.dependent objects. Design Pattern SummaryDesign Pattern Summary
– Capture mutual behavior in a Capture mutual behavior in a separate class.separate class.
Mediator Sequence Mediator Sequence DiagramDiagram
Client A:Colleague_A
:Mediator
B: Colleague_B
request()
takeAction_1()
mediate()
:Mediator
C: Colleague_A
takeAction_2()
takeAction_3()
CommandCommand
Design PurposeDesign Purpose– Increase flexibility in calling for Increase flexibility in calling for
a service e.g., allow undo-able a service e.g., allow undo-able operations.operations.
Design Pattern SummaryDesign Pattern Summary– Capture operations as classes.Capture operations as classes.
Command - Command - StructureStructure
Client Invoker+request()
ConcreteCommand
execte()State
Commandexecute();
receiver.action()
receiverReceiveraction()
A B: A composed in B
Command - ExampleCommand - Example
Cut:ButtonClicked()
CopyCommand
execte()
Commandexecute();
doc.copy()
doc
Documentcopy()cut()
command
command.execute()
CutCommandexecte()
doc
doc.cut()
Copy:Button
Clicked()
command.execute()
command
Cut:MenuItem
Clicked()
command.execute()
Command – Sequence Command – Sequence DiagramDiagram
Cut:Button
CopyCommand
Command
DocumentCutCommand
Copy:Button
Cut:MenuItem
selected() execute()cut()
clicked()cut()
execute()
clicked()copy()
execute()
Key Concept - Key Concept - CommandCommand
-- to avoid calling a method directly (e.g., so as to record or intercept it).
IteratorIterator visits members of a collection visits members of a collection MediatorMediator captures behavior among captures behavior among
peer objectspeer objects ObserverObserver updates objects affected by updates objects affected by
a single objecta single object CommandCommand captures function flexibly captures function flexibly
(e.g. undo-able)(e.g. undo-able)
SummarySummary