Sequential (default) - The semantics of the class are guaranteed only in the presence of a single thread of control. Only one thread of control can be executing in the method at any one time.

Guarded - The semantics of the class are guaranteed in the presence of multiple threads of control. A guarded class requires collaboration among client threads to achieve mutual exclusion.

Active - The class has its own thread of control.

Synchronous - The semantics of the class are guaranteed in the presence of multiple threads of control mutual exclusion is supplied by the class.

-Sequential (default)- The semantics of the class are guaranteed only in the presence of a single thread of control. Only one thread of control can be executing in the method at any one time. Guarded- The semantics of the class are guaranteed in the presence of multiple threads of control. A guarded class requires collaboration among client threads to achieve mutual exclusion. Active- The class has its own thread of control. Synchronous- The semantics of the class are guaranteed in the presence of multiple threads of control　mutual exclusion is supplied by the class.