PA103 - Object-oriented Methods for Design of Information Systems
How to Think When Applying
Design Patterns
© Radek Ošlejšek
Fakulta informatiky MU
oslejsek@fi.muni.cz
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Initial model
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getLines() : Set<OrderLine>
Order
-product
-price
-numPieces
OrderLine
*
lines
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Goals
 Goals: extend the model so that:
 A) We are able to sort order lines by name, date, price,
and other criteria.s
 B) We have various types of orders, e.g. for retail
customers vs. bulk customers. They differ in stored data.
 C) both.
 Approach:
 Ad A) Employ behavioral design patterns because sorting
is behavior.
 Ad B) Employ structural design patterns because we have
to deal with spreading pieces of information across
classes.
 Ad C) Put them together.
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Goal A: Sorted lines
 Design decision: Should be the Order class
responsible for sorting lines or is it the responsibility of
that who wants it?
 Probably the second case is correct. But to demonstrate
the application of design pattern we give this responsibility
to Order class.
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Sorted lines: Naive solution
 Adding a new ordering requires to change the existing Order
class. Two naive solutions:
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getLines(sorting) : SortedSet<OrderLine>
Order
-product
-price
-numPieces
OrderLine
*
lines
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getLinesByProduct() : SortedSet<OrderLine>
+getLinesByPrice() : SortedSet<OrderLine>
Order
-product
-price
-numPieces
OrderLine
*
lines
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Sorted lines: Iterator
 Iterators providing lines in a specific sequence.
 But how to say what iterator is to be used?
 By parameter or providing more variants of the getIterator()
method?
 Similar problem to the previous naive solution.
Nevertheless, the code of the getIterator() is more simple
than in the naive approach
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getIterator() : Iterator
Order
-product
-price
-numPieces
OrderLine
*
+current() : OrderLine
+next() : boolean
Iterator
+current() : OrderLine
+next() : boolean
SortByProductIterator
+current() : OrderLine
+next() : boolean
SortByPriceIterator
<<instantiate>>
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Sorted lines: Strategy
 A client decides which strategy to use!
-product
-price
-numPieces
OrderLine
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getLines(SortingStrategy strategy) : SortedSet<OrderLi...
Order*
+sort(Set lines) : SortedSet<OrderLine>
SortingStrategy
1
+sort(Set lines) : SortedSet<OrderLin...
SortingByProduct
+sort(Set lines) : SortedSet<OrderLin...
SortingByPrice
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Sorted lines: Strategy in Java = Comparator
 … because java.util.Comparator is nothing else than „sorting
strategy“ for sorted Java collections.
-product
-price
-numPieces
OrderLine
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getLines(Comparator c) : SortedSet<OrderLin...
Order
java.util
*
+compare(o1, o2) : int
+equals(Object obj) : boolean
<<Interface>>
Comparator
+compare(OrderLine o1, Orderline o2) :...
ProductComparator
+compare(OrderLine o1, Orderline o2) :...
PriceComparator
1
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Goal B: Variable Orders
… just make sub-classes
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getLines() : Set<OrderLine>
Order
-product
-price
-numPieces
OrderLine
*
lines
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getLines() : Set<OrderLine>
Order
-product
-price
-numPieces
OrderLine
*
BulkOrder RetailOrder
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Goal C: Variable Orders With Sorting Strategies
… explosion of sub-classes => apply Bridge
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getLines() : SortedSet<OrderLine>
Order
BulkOrder RetailOrder
+getLines() : SortedSet<OrderLine>
BulkOrderSortedByProduct
+getLines() : SortedSet<OrderLine>
BulkOrderSortedByPrice
+getLines() : SortedSet<OrderLine>
RetailOrderSortedByProduct
+getLines() : SortedSet<OrderLine>
RetailOrderSortedByPrice
-product
-price
-numPieces
OrderLine
*
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Goal C: Bridge in Action
 OK, but Bridge, as a structural pattern, does not explain neither
who chooses the Items sub-class nor who sets the schoosen
sub-class to Order. These dynamic aspects are defined by
behavioral patterns.
 In this case, the Items class can be understood as a sorting
strategy.
-shippingAddress
-date
-items : Items
+addItem(product)
+removeItem(product)
+getLines() : SortedSet<OrderLine>
<<bridge-abstraction>>
Order
BulkOrder RetailOrder
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getLines() : SortedSet<OrderLine>
<<bridge-implementor>>
Items1 -product
-price
-numPieces
OrderLine
*
+getLines() : SortedSet<OrderLine>
ItemsSortedByProduct
+getLines() : SortedSet<OrderLine>
ItemsSortedByPrice
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Goal C: Strategy in Action (Again ;)
 Is it Bridge or Strategy? It depents on the point of view. Static
view => Bridge, dynamic view => Strategy.
 Only sorting by product or by price is allowed. But what if we
want to combine sorting methods?
 Define combined strategy or use Decorator
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getLines(SortingStrategy strategy) : SortedSet<OrderLine>
Order
+sort(Set lines) : SortedSet<OrderLine>
<<Interface>>
SortingStrategy
+sort(Set lines) : SortedSet<OrderLin...
SortingByProduct
+sort(Set lines) : SortedSet<OrderLin...
SortingByPrice
-product
-price
-numPieces
OrderLine *
1
BulkOrder RetailOrder
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MUSpring 2016
Goal C: Decorator in Action
 This is not the Bridge anymore.
 Drawback: SortedOrder has not the specific methods of
BulkOrder and RetailOrder. Therefore, clients have to distinguish
between them.
 Drawback: Implementation of OrderSortedByPrice taking into
account OrderSortedByProduct, for instance, would be much
complicated that introducing a new specialized sub-class.
-lines : Set<OrderLine>
+addItem(product)
+removeItem(product)
+getLines() : SortedSet<OrderLine>
Order
+specificBulkMethod()
BulkOrder
+specificRetailMethod()
RetailOrder
SortedOrder
1
OrderSortedByProduct OrderSortiedByPrice
-product
-price
-numPieces
OrderLine
*
PA103: Object-oriented Methods for Design of Information Systems IS © R. Ošlejšek, FI MU 14Spring 2016
Questions?