David Evans cs.virginia/~evans

1. Lecture 13: Concurring Concurrently David Evans http://www.cs.virginia.edu/~evans CS201j: Engineering Software University of Virginia Computer Science

2. Menu • Subtyping Rules Review • Overriding and Overloading • Concurrency CS 201J Fall 2002

3. Substitution Principle … (in client code) MysteryType1 mt1; MysteryType2 mt2; MysteryType3 mt3; … (anything could be here) mt1 = mt2.m (mt3); • If the Java compiler is happy with this code, which of these are guaranteed • to be true: • The apparent type of mt2 is MysteryType2 • At the last statement, the actual type of mt2 is MysteryType2 • MysteryType2 has a method named m • The MysteryType2.m method takes a parameter of type MysteryType3 • The MysteryType2.m method returns a subtype of MysteryType1 • After the last statement, the actual type of mt1 is MysteryType1 CS 201J Fall 2002

4. … (in client code) MysteryType1 mt1; MysteryType2 mt2; MysteryType3 mt3; … (anything could be here) mt1 = mt2.m (mt3); • If the Java compiler is happy with this code, which of these are guaranteed • to be true: • The apparent type of mt2 is MysteryType2 • At the last statement, the actual type of mt2 is MysteryType2 • MysteryType2 has a method named m • The MysteryType2.m method takes a parameter of type MysteryType3 • The MysteryType2.m method returns a subtype of MysteryType1 • After the last statement, the actual type of mt1 is MysteryType1 TRUE: the apparent type is obvious from the declaration. FALSE: we only know the actual type <= MysteryType2 TRUE FALSE: we only know it takes a parameter >= MysteryType3 TRUE: the assignment type checking depends on this FALSE: we only know that the actual type <= MysteryType1 CS 201J Fall 2002

5. Subtyping Rules … (in client code) MysteryType1 mt1; MysteryType2 mt2; MysteryType3 mt3; … mt1 = mt2.m (mt3); class A { public RA m (PA p) ; } If A is MysteryType2, what do we know about RA and PA? RA must be a subtype of MysteryType1: RA <= MysteryType1 MysteryType3 must be a subtype of PA: PA >= MysteryType3 CS 201J Fall 2002

6. Subtyping Rules … (in client code) MysteryType1 mt1; MysteryType2 mt2; MysteryType3 mt3; … mt1 = mt2.m (mt3); class A { public RA m (PA p) ; } class B extends A { public RB m (PB a); } If B <= A, what do we know about RB and PB? RB must be a subtype of RA: RB <= RA PA must be a subtype of PB: PB >= PA CS 201J Fall 2002

7. Substitution Principle … (in client code) MysteryType1 mt1; MysteryType2 mt2; MysteryType3 mt3; … mt1 = mt2.m (mt3); class A { public RA m (PA p) ; } class B extends A { public RB m (PB a); } Substitution Principle: Parameters PB >= PA Preconditions pre_A  pre_B Result RB <= RA Postconditions post_B  post_A CS 201J Fall 2002

8. Substitution Principle / Eiffel … (in client code) MysteryType1 mt1; MysteryType2 mt2; MysteryType3 mt3; … mt1 = mt2.m (mt3); class A { public RA m (PA p) ; } class B extends A { public RB m (PB a); } Substitution Principle Eiffel Parameters PB >= PA PB <= PA Preconditions pre_A  pre_B pre_B pre_A Result RB <= RA RB <= RA Postconditions post_B  post_A post_B  post_A CS 201J Fall 2002

9. Overloading and Overriding • Overriding: replacing a supertype’s method in a subtype • Dynamic dispatch finds method of actual type • Overloading: providing two methods with the same name but different parameter types • Statically select most specific matching method of apparent type CS 201J Fall 2002

10. Overloading Example public class Overloaded extends Object { public int tryMe (Object o) { return 17; } public int tryMe (String s) { return 23; } public boolean equals (String s) { return true; } } public boolean equals (Object) is inherited from Object CS 201J Fall 2002

11. public class Overloaded { public int tryMe (Object o) { return 17; } public int tryMe (String s) { return 23; } public boolean equals (String s) { return true; } } Overloading static public void main (String args[]) { Overloaded over = new Overloaded (); System.err.println (over.tryMe (over)); System.err.println (over.tryMe (new String ("test"))); Object obj = new String ("test"); System.err.println (over.tryMe (obj)); System.err.println (over.equals (new String ("test"))); System.err.println (over.equals (obj)); Object obj2 = over; System.err.println (obj2.equals (new String ("test"))); } 17 23 17 true false false CS 201J Fall 2002

12. Overkill • Overloading and overriding together can be overwhelming! • Avoid overloading whenever possible: names are cheap and plentiful • One place you can’t easily avoid it: constructors (they all have to have the same name) CS 201J Fall 2002

13. My Favorite C++ Program (On notes, for experts only) #include<stdio.h> class A { public: void other () { printf("is an empty func in A\n"); }; virtual void other (class A *a) { printf("In A\n"); } }; class B: public A { public: void other (class B *b) { printf("In B\n"); } }; class C: public A { public: void other (class C *c) { printf("In C\n"); } }; void main(void) { A a; B b; C c; A *aPtr = &a; B *bPtr = &b; C *cPtr = &c; aPtr = bPtr; aPtr->other(bPtr); bPtr->other(); } CS 201J Fall 2002

14. Concurrency CS 201J Fall 2002

15. Our computer can only do one instruction at a time, why would we want to program pretending it can do many things at once? CS 201J Fall 2002

16. Concurrent Programming • Some problems are clearer to program concurrently: • Modularity • Don’t have to explicitly interleave code for different abstractions (especially: user interfaces) • High-level interactions – synchronization, communication • Modeling • Closer map to real world problems: things in the real world aren’t sequential CS 201J Fall 2002

17. Concurrency in Java public class Thread implements Runnable { // OVERVIEW: A thread is a thread of execution in a program. // The Java Virtual Machine allows an application to have // multiple threads of execution running concurrently. public Thread (Runnable target) // Creates a new Thread object that will run the target. public void start () // Starts a new thread of execution. … many other methods } CS 201J Fall 2002

18. public class Thread implements Runnable { public Thread (Runnable target) public void start () … many other methods } Making a Thread // from PS5 Grid class: public void startObjects() // EFFECTS: Start all object threads. { Enumeration els = simobjects.elements (); while (els.hasMoreElements ()) { SimObject current = (SimObject) els.nextElement (); Thread simObjectThread = new Thread (current); simObjectThread.start (); } } What do you know about SimObject type? CS 201J Fall 2002

19. Runnable • public interface Runnable { • public void run() • When an object implementing interface Runnable is • used to create a thread, starting the thread causes the • object's run method to be called in that separately • executing thread. The general contract of the method run is that it may take any action • whatsoever. • } So, to be a subtype of Runnable, SimObject must have a method void run () with no preconditions and any postconditions it wants. CS 201J Fall 2002

20. Making a Runnable abstract public class SimObject implements Runnable { … public void run () // EFFECTS: Executes one turn by calling the // executeTurn method, and sleeps for a time // and repeats. { while (true) { executeTurn (); delay (TURN_DELAY + random.nextInt(TURN_RANDOM)); } } CS 201J Fall 2002

21. Actually… abstract public class SimObject implements Runnable { … public void run () // REQUIRES: this has been initialized //@also_requires isInitialized // EFFECTS: Executes one turn by calling the // executeTurn method, and sleeps for a time // and repeats. { … } We are violating the substitution principle! SimObject.run() has a stronger precondition than Runnable.run(). CS 201J Fall 2002

22. Concurrency • Making a concurrent Java program is easy: Make a subtype R of Runnable new Thread (new R ()).start () • Making a concurrent Java program that behaves correctly is really, really hard! CS 201J Fall 2002

23. Scheduling Meetings • Alice wants to schedule a meeting with Bob and Colleen “When can you meet Friday?” “When can you meet Friday?” Bob Alice Colleen “11am or 3pm” “9am or 11am” Picks meeting time Reserves 11am for meeting Reserves 11am for meeting “Let’s meet at 11am” “Let’s meet at 11am” CS 201J Fall 2002

24. Partial Ordering of Events • Sequential programs give use a total ordering of events: everything happens in a determined order • Concurrency gives us a partial ordering of events: we know some things happen before other things, but not total order • Alice asks to schedule meeting before Bob replies • Alice asks to schedule meeting before Colleen replies • Bob and Colleen both reply before Alice picks meeting time • Alice picks meeting time before Bob reserves time on calendar CS 201J Fall 2002

25. Race Condition “When can you meet Friday?” “When can you meet Friday?” Bob Alice Colleen Doug “When can you meet Friday?” “9, 11am or 3pm” “9am or 11am” “9, 11am or 3pm” Picks meeting time Reserves 11am for Doug “Let’s meet at 11am” “Let’s meet at 11am” “Let’s meet at 11am” “I’m busy then…” CS 201J Fall 2002

26. Preventing Race Conditions • Use locks to impose ordering constraints • After responding to Alice, Bob reserves all the times in his response until he hears back (and then frees the other times) CS 201J Fall 2002

27. Locking “When can you meet Friday?” “When can you meet Friday?” Bob Alice Colleen Doug “When can you meet Friday?” “9, 11am or 3pm” “9am or 11am” Picks meeting time Locks calendar “Let’s meet at 11am” “3pm” “Let’s meet at 11am” “Let’s meet at 3” CS 201J Fall 2002

28. Deadlocks “When can you meet Friday?” Doug Bob Alice Colleen Locks calendar for Doug, can’t respond to Alice “When can you meet Friday?” “When can you meet Friday?” “When can you meet Friday?” “9, 11am or 3pm” Can’t schedule meeting, no response from Bob Locks calendar for Alice, can’t respond to Doug Can’t schedule meeting, no response from Colleen CS 201J Fall 2002

29. Why are threads hard? • Too few ordering constraints: race conditions • Too many ordering constraints: deadlocks • Hard/impossible to reason modularly • If an object is accessible to multiple threads, need to think about what any of those threads could do at any time! • Testing is even more impossible than it is for sequential code • Even if you test all the inputs, don’t know it will work if threads run in different order CS 201J Fall 2002

30. Charge • Computers are single-threaded machines that provide their owner the illusion of multiple threads. • Brains are multi-threaded machines that provide their owner with the illusion of a single thread. • Practice with races/deadlocks on Tuesday, no class on Thursday • Return exams CS 201J Fall 2002