The Thread class provides the start() method, which starts a new thread (see Chapter 2). As we mentioned, a thread should be started only once. However, if a program calls the start() method of an already running thread, the IllegalThreadStateException is thrown.

The Thread class provides the setDaemon() method, which specifies whether the thread is a daemon thread (see Chapter 6). As we mentioned, the daemon status of a thread must be set before the thread is started. If the setDaemon() method is called when the thread is already running, the IllegalThreadStateException is thrown.

The Thread class provides the countStackFrames() method, which determines how deep in the call stack the thread is currently executing (see Appendix A). A thread must be suspended in order for this count to take place. If the thread is not suspended when this method is called, the Illegal-ThreadStateException is thrown.

The ThreadGroup class provides the destroy() method to allow the thread group to be destroyed (see Chapter 10). A ThreadGroup instance can only be destroyed when the group does not contain any threads and does not contain any groups that contain threads. If the destroy() method is called on a group that contains threads or is already destroyed, the IllegalThreadStateException is thrown.

The Thread class contains certain constructors that allow the thread to be placed into a specific thread group (see Chapter 10). The thread group that is passed to these constructors must not have been destroyed; if the constructor is passed a thread group that has been destroyed, the IllegalThreadStateException is thrown.

The Thread class provides the setPriority() method, which controls the priority assigned to the thread (see Chapter 6). The priority that is assigned must fall between the system minimum and maximum priorities. If the priority requested is not within this range, an IllegalArgumentException is thrown. (The setPriority() method may also throw a security exception; see the section Section 2.4.5,” later in this appendix.)

The Object class provides the wait() method, which allows a thread to wait for a notification condition (see Chapter 4). The wait() method must be called while the synchronization lock for the object is held. The wait() method is also overloaded with two other method signatures that allow the program to specify a timeout. If any of these methods is called without owning the synchronization lock, the IllegalMonitorStateException is thrown.

The Object class provides the notify() method, which allows a thread to send a notification signal to any threads waiting (see Chapter 4). The notify() method must be called while the synchronization lock for the object is held. The Object class also provides the notifyAll() method, which wakes up all the waiting threads. If either of these methods is called without owning the synchronization lock, the IllegalMonitorStateException is thrown.

The Thread class provides a version of the stop() method that allows the user to specify the object used to stop the thread (see Appendix A). Normally, programs do not use this method; however, if a program does use this method and passes a null object to stop a thread, the NullPointerException is thrown.

The ThreadGroup class provides a version of its constructor that allows the application to specify the parent group (see Chapter 10). If null is specified for the parent group, the NullPointerException is thrown.

The Thread class provides the checkAccess() method, which simply calls the security manager to determine if the thread can be accessed by the current thread group (see Chapter 10). A SecurityException is thrown if access is not permitted. For a complete list of methods that call the checkAccess() method, see Figure 10.1.

The ThreadGroup class provides the checkAccess() method, which simply calls the security manager to determine if the thread group can be accessed by the current thread group (see Chapter 10). A SecurityException is thrown if access is not permitted. For a complete list of methods that call the checkAccess() method, see Figure 10.1.

The Thread class provides the setPriority() method, which sets the scheduling priority of the thread. The priority requested must be less than the maximum priority of the thread group to which the thread belongs. If the priority is greater than this maximum priority, a SecurityException may be thrown (see Chapter 10).

The stop() method of the Thread class may throw a security exception under Java 2 and later releases if the stopThread permission has not been granted to the code that is calling the stop() method (see Chapter 10).

The run() method of the Thread class executes user-specific code and, hence, can throw any runtime exception the user code does not catch. Exceptions that the run() method throws are caught in the manner we describe in Appendix A.