1. Week 6, Class 2: Observer Pattern
11/15/2018
Week 6, Class 2: Observer Pattern
THESE ARE CUT SLIDES THAT I MAY USE IN A FUTURE RENDITION OF THE CLASS
17q2 1-3,6-7,9-12
SE-2811
Slide design: Dr. Mark L. Hornick
Content: Dr. Hornick
Errors: Dr. Yoder
Dr. Yoder

2. SE-2811
11/15/2018
If you think the internet is not working in its current incarnation, you can’t change the system through think-pieces and F.C.C. regulations alone. You need new code [... you need cryptocurrency!].
Stephen Johnson, "Beyond the Bitcoin Bubble,” New York Times, Jan 15th 2018
[Interpretation added]
18q2-7-1
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

3. Java locks are re-entrant
public void example(Object lock, …) { synchronized (lock) { ... doSomething(lock, ...) } public void doSomething(Object lock, ...) { synchronized (lock) { // If I already have the lock, I can enter code ... // locked on the same object } // So I won’t accidentally lock waiting for myself! }
SE-2811
Dr. Mark L. Hornick

4. Wait and Notify… wait() --- stop here until notified
SE-2811
11/15/2018
Wait and Notify…
wait() --- stop here until notified
notify() --- notify a waiting thread that it can continue
notifyAll() – wakes up all waiting threads
Like most threading methods notify() does not guarantee that any given thread will start next.
Details:
These are all methods of the Object class!
(In contrast to join())
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

5. Multi-threading in Swing
SE-2811
11/15/2018
Multi-threading in Swing
The Single-Thread Rule
Once a Swing component has been realized, all code that might affect or depend on the state of that component should be executed in the event-dispatching thread.
-- Sun Tutorial “Threads in Swing,”
Way Back Machine
What events should be handled by the event dispatching thread?
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

6. Multi-threading in Swing
SE-2811
11/15/2018
Multi-threading in Swing
“A Swing component that's a top-level window is realized by having one of these methods invoked on it:
setVisible(true)
show() /** Deprecated */
pack()”
From the “Obsolete” Java Swing Tutorial
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

7. Lambda Expression Syntax
SE2811
11/15/2018
Lambda Expression Syntax
Exercise: What parts does a regular method have?
Lambda Expression: A variety of syntaxes are available:
<arguments> -> <method body>
(Scanner in) -> {int x = in.nextInt(); return x+1;}
(int x) -> x+1
() -> System.out.println(“hi”)
x -> x+1
SE-2811
Dr. Mark L. Hornick
Dr. Josiah Yoder

8. The synchronized statement
synchronized(variable) { variable.get(…) // only one of these } variable.set(…) // statements can be variable = new V() // executed at a time…
SE-2811
Dr. Mark L. Hornick

9. What is the difference between
SE-2811
11/15/2018
What is the difference between
public synchronized void myMethod() {
// … }
and
public void myMethod() {
synchronized (this) {
} ?
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

10. SE-2811
11/15/2018
The synchronized statement (§14.17) computes a reference to an object; it then attempts to perform a lock action on that object and does not proceed further until the lock action has successfully completed. ...
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

11. SE-2811
11/15/2018
A synchronized method (§ ) automatically performs a lock action when it is invoked… If the method is an instance method, it locks the lock associated with the instance for which it was invoked (… “this” …). If the method is static, it locks the lock associated with the Class object that represents the class in which the method is defined. ...
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

12. Conclusion: public synchronized void myMethod()
SE-2811
11/15/2018
Conclusion:
public synchronized void myMethod()
And
synchronized (this) { // … }
are the same
public static synchronized void myMethod()
synchronized (SomeClass.class) { // … }
Are the same
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

13. Is it safe to have multiple blocks access the same data?
SE-2811
11/15/2018
Is it safe to have multiple blocks access the same data?
See e.g.
“The synchronized statement (§14.17) computes a reference to an object; it then attempts to perform a lock action on that object”
Yes or No?
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

14. SE-2811
11/15/2018
I have been programming C# for 10 months now. I am now learning multi-threading and seems to be working nicely. I have multi-dimension array like
string[,] test = new string[5, 13];
I have the threads calling methods that end up saving their outputs to different coordinates inside the array above, without any one thread writing to the same location as another thread.
So thread1 might write to test[1,10] but no other thread will ever write to test[1,10]
My question is: I have read about using locks on objects like my array, do I have to worry at all about locks even though my threads might access to the test array at the same time but never write to the same coordinates(memory location)?
So far in my testing I have not had any issues, but if someone more seasoned than myself knows I could have issue then I'll look into use locks.
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

15. What would you answer? string[,] test = new string[5, 13];
SE-2811
11/15/2018
What would you answer?
string[,] test = new string[5, 13];
thread1 might write to test[1,10] but no other thread will ever write to test[1,10]
do I have to worry at all about locks?
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

16. Wait and Notify… wait() --- stop here until notified
SE-2811
11/15/2018
Wait and Notify…
wait() --- stop here until notified
notify() --- notify a waiting thread that it can continue
notifyAll() – wakes up all waiting threads
Like most threading methods notify() does not guarantee that any given thread will start next.
Details:
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

17. Multi-threading in Swing
SE-2811
11/15/2018
Multi-threading in Swing
The Single-Thread Rule
Once a Swing component has been realized, all code that might affect or depend on the state of that component should be executed in the event-dispatching thread.
-- Sun Tutorial “Threads in Swing,”
Way Back Machine
What events should be handled by the event dispatching thread?
SE-2811
Dr. Mark L. Hornick
Dr. Yoder

18. Multi-threading in Swing
SE-2811
11/15/2018
Multi-threading in Swing
“A Swing component that's a top-level window is realized by having one of these methods invoked on it:
setVisible(true)
show() /** Deprecated */
pack()”
From the “Obsolete” Java Swing Tutorial
SE-2811
Dr. Mark L. Hornick
Dr. Yoder