Chair of Software Engineering. Languages in Depth Series: Java Programming. Prof. Dr. Bertrand Meyer. Exercise Session 9.

Transcription

1 Chair of Software Engineering Languages in Depth Series: Java Programming Prof. Dr. Bertrand Meyer Exercise Session 9 Andrei Vancea

2 Today s Exercise Session Pattern of the Day Chain of Responsibility Quizzes 2

3 Chain Of Responsibility Behavioral pattern Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request. Chain the receiving objects and pass the request along the chain until an object handles it. 3

4 Chain Of Responsibility - Structure Handler Defines an interface for handling requests. (optional) Implements the successor link. 4

5 Chain Of Responsibility - Structure ConcreteHandler Client Handles requests it is responsible for. Can access its successor. If the ConcreteHandler can handle the request, it does so; otherwise it forwards the request to its successor. Initiates the request to a ConcreteHandler object on the chain. 5

6 Chain Of Responsibility - Example *from 6

7 Chain Of Responsibility - Example 7

8 Chain Of Responsibility - Example 8

9 Chain Of Responsibility - Example 9

10 Chain Of Responsibility - Examplei 10

11 Chain Of Responsibility Consequences Reduce coupling The pattern frees the receiving object of knowing who should handle the request. Objects in the chain don t need to know the structure of chain. Added flexibility. You can add or change responsibilities for handling a request by changing the chain at run time. Reception is not guaranteed Nothing in the pattern guarantees that the request will be handled. A request can fall of the chain if the chain is not properly configured. 11

12 Lazy Initialization What does the program print? the program prints nothing; it just hangs. 12

13 Lazy Initialization When a thread is about to access a member of a class, it checks if the class has been initialized. There are three possible cases 1. The class is not yet initialized. 2. The class is being initialized by some thread other than the current thread. 3. The class is already initialized. What happens? (1) The initialization is started (2) The current thread waits for the initialization to complete (3) Nothing 13

14 Lazy Initialization t waits for the main thread to finish the init. main thread waits t to complete Deadlock! 14

15 Ping/Pong What does it print? Is it guaranteed to print the same thing every time you run it? It always print PongPing 15

16 Ping/Pong Before a synchronized static method executes, it obtains the monitor lock associated with its Class object The main thread acquires the lock on PingPong.class before creating the second thread As long as the main thread holds on to this lock, the second thread can't execute a synchronized static method Thus, the application should print PingPong The problem : This is NOT a multithreaded program The t.run method does not start the thread; it just executes the run method in the current thread Solution : change t.run to t.start 16

17 Dog vs Cats What does the program print? 17

18 Dog vs Cats 5 woofs and 5 meows 18

19 Dog vs Cats The problem is that Dog and Cat inherit the count field from a common superclass. A single copy of each static field is shared among its declaring class and all subclasses. Dog and Cat use the same count field. Solution : 19

20 Questions? 20

21 Exercise Session 10 Pattern of the Day Proxy Quizzes 21