Acknowledgements. Materials were borrowed from

Transcription

1 Composite Design Pattern The composite pattern allows clients to treat either individual objects or collections of objects in a uniform manner; this is a structural design pattern For example, an operating system treats a directory (the collection) or a file (the individual object) in the same manner This pattern results in a tree like structure where the internal nodes are the composites and the leaf nodes are leaves that cannot be expanded further.

2 Acknowledgements Materials were borrowed from Design Patterns in Java by Steven Metsker and William Wake (textbook for course) Head First Design Patterns by Elisabeth Freeman, Eric Freeman, Bert Bates, and Kathy Sierra

3 Another Example: AWT Widgets

4 MachineComponent at Oozinoz

5 Solution 5.2 This is the code associated with a leaf Notice the recursive call inside the while loop

6 Adding More Operations

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8 Solution 5.3

9 Tree Structure Here is one characterization of a tree structure There are other equally valid characterization of trees that you learned in data structures, give one of these alternative characterizations

10 A tree is an undirected simple graph G that satisfies any of the following equivalent conditions: G is connected and has no cycles. G has no cycles, and a simple cycle is formed if any edge is added to G. G is connected, and it is not connected anymore if any edge is removed from G. Any two vertices in G can be connected by a unique simple path. If G has finitely many vertices, say n of them, then the above statements are also equivalent to any of the following conditions: G is connected and has n 1 edges. G has no simple cycles and has n 1 edges

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12 Solution 5.4

13 Testing for Tree Structure Some composite structures may contains cycles; this makes algorithms more difficult to write For example, to traverse a composite structure with cycles, you must keep track if a node has already been visited or not; if it has been visited, the algorithm must back off and try another alternative Your task here is to write code that will check whether a given composite is a tree or if it contains cycles

14 Solution 5.5

15 Another Complete Example This example comes from the Head First Design Patterns book There are several restaurants in town each with their own menu

16 A Class Diagram with Methods

17 This abstract class provides default implementations; notice that no individual method is abstract.

18 The MenuItem is the leaf of our composite tree structure

19 We use an ArrayList for the component part of the composite structure The print method only names the menu and does not print the contents

20 Adding an Iterator to the print method The iterator associated with an ArrayList will help us print out all items in the list. Notice that if we call print on the top level component than the entire menu will be printed using a lot of separate iterators each associated with an individual array list.

21 The Waitress handles the Menus Since the waitress is trained to work in all three restaurants, she must be able to print all three menus.

22 Adding a few select items is shown We can even add a submenu to an existing menu

23 The output from the waitress

24 A First Look at a true Iterator Our print method was able to print all items in all menus, but it is not considered to be an iterator Why not? An iterator for a composite pattern shown return each item in the composite Even if we use the built in iterator for each array list, an iterator for the composite structure must be able to descend down into the structure and return from lower levels to continue the iteration at an upper level What data structure would you choose to remember all these postponed obligations?

25 We use a stack structure to remember postponed iterations at higher levels

26 Testing our Composite Iterator We want to visit all items in all menus and only print out the vegetarian items as a single list of items

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