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Let s examine this program in detail The program creates three classes The first is SumArray It defines the method SumIt( ), which sums an integer array The second class is MyThread, which uses a static object called sa that is of type SumArray Thus, only one object of SumArray is shared by all objects of type MyThread This object is used to obtain the sum of an integer array Notice that SumArray stores the running total in a field called sum Thus, if two threads use SumIt( ) concurrently, both will be attempting to use sum to hold the running total Because this will cause errors, access to SumIt( ) must be synchronized Finally, the class Sync creates two threads and has them compute the sum of an integer array Inside SumIt( ), the lock statement prevents simultaneous use of the method by different threads Notice that lock uses lockOn as the object being synchronized This is a private object that is used solely for synchronization Sleep( ) is called to purposely allow a task-switch to occur, if one can but it can t in this case Because the code within SumIt( ) is locked, it can be used by only one thread at a time Thus, when the second child thread begins execution, it does not enter SumIt( ) until after the first child thread is done with it This ensures the correct result is produced To understand the effects of lock fully, try removing it from the body of SumIt( ) After doing this, SumIt( ) is no longer synchronized, and any number of threads can use it concurrently on the same object The problem with this is that the running total is stored in sum, which will be changed by each thread that calls SumIt( ) Thus, when two threads call SumIt( ) at the same time on the same object, incorrect results are produced because sum reflects the summation of both threads, mixed together For example, here is sample output from the program after lock has been removed from SumIt( ):.

Child #1 starting Running total for Child Child #2 starting Running total for Child Running total for Child Running total for Child Running total for Child Running total for Child Running total for Child Running total for Child Running total for Child Running total for Child Sum for Child #1 is 29 Child #1 terminating Sum for Child #2 is 29 Child #2 terminating #1 is 1 #2 #1 #2 #1 #2 #1 #2 #1 #2 is is is is is is is is is 1 3 5 8 11 15 19 24 29

The set k consists of all points with x-coordinate equal to 4 This is the set of all points that lie 4 units to the left of the y-axis We exhibit k in Figure 110 It is a vertical line

As the output shows, both child threads are using SumIt( ) at the same time on the same object, and the value of sum is corrupted The effects of lock are summarized here: For any given object, once a lock has been acquired, the object is locked and no other thread can acquire the lock Other threads trying to acquire the lock on the same object will enter a wait state until the code is unlocked When a thread leaves the locked block, the object is unlocked

Part II:

Although locking a method s code, as shown in the previous example, is an easy and effective means of achieving synchronization, it will not work in all cases For example, you might want to synchronize access to a method of a class you did not create, which is itself not synchronized This can occur if you want to use a class that was written by a third party and for which you do not have access to the source code Thus, it is not possible for you to add a lock statement to the appropriate method within the class How can access to an object of this class be synchronized Fortunately, the solution to this problem is simple: Lock access to the object from code outside the object by specifying the object in a lock statement For example, here is an alternative implementation of the preceding program Notice that the code within SumIt( ) is no longer locked and no longer declares the lockOn object Instead, calls to SumIt( ) are locked within MyThread