Represents the executive of a discrete-event simulator. This class maintains a simulation clock, an event list, and starts executing the events in the appropriate order. Its methods permit one to start, stop, and (re)initialize the simulation, and read the simulation clock.
Usually, a simulation program uses a single simulation clock which is represented by an instance of this class. For more convenience and compatibility, this class therefore provides a mechanism to construct and return a default simulator which is used when an event is constructed without an explicit reference to a simulator, and when the simulator is accessed through the {@link Sim} class.
Note that this class is NOT thread-safe. Consequently, if a simulation program uses multiple threads, it should acquire a lock on a simulator (using a synchronized block) before accessing its state. Note however, that one can launch many simulations in parallel with as many threads, as long as each thread has its own Simulator.