Editing Monitor (synchronization) (section)

==Mutual exclusion==

While a thread is executing a method of a thread-safe object, it is said to ''occupy'' the object, by holding its [[lock (computer science)|mutex (lock)]].  Thread-safe objects are implemented to enforce that ''at each point in time, at most one thread may occupy the object''.  The lock, which is initially unlocked, is locked at the start of each public method, and is unlocked at each return from each public method.

Upon calling one of the methods, a thread must wait until no other thread is executing any of the thread-safe object's methods before starting execution of its method.  Note that without this mutual exclusion, two threads could cause money to be lost or gained for no reason. For example, two threads withdrawing 1000 from the account could both return true, while causing the balance to drop by only 1000, as follows: first, both threads fetch the current balance, find it greater than 1000, and subtract 1000 from it; then, both threads store the balance and return.