In Java, a ThreadGroup is a way to group multiple threads together. A ThreadGroup provides a convenient way to manage a collection of threads as a single unit. Here's an overview of how ThreadGroups work in Java.

Creating a ThreadGroup

You can create a ThreadGroup by instantiating the ThreadGroup class. You can specify a name for the ThreadGroup when you create it. If you don't specify a name, the ThreadGroup will be given a default name.

ThreadGroup group = new ThreadGroup("myGroup");

You can also specify a parent ThreadGroup when you create a new ThreadGroup. If you don't specify a parent ThreadGroup, the new ThreadGroup will be assigned the same parent as the Thread that creates it.

ThreadGroup parent = new ThreadGroup("parentGroup");
ThreadGroup child = new ThreadGroup(parent, "childGroup");

Adding Threads to a ThreadGroup

You can add a Thread to a ThreadGroup by specifying the ThreadGroup as the first argument to the Thread constructor.

ThreadGroup group = new ThreadGroup("myGroup");
Thread thread = new Thread(group, myRunnable);

You can also add a Thread to a ThreadGroup after it has been created using the ThreadGroup.add() method.

ThreadGroup group = new ThreadGroup("myGroup");
Thread thread = new Thread(myRunnable);
group.add(thread);

You can get a list of the threads in a ThreadGroup by calling the ThreadGroup.enumerate() method. The method returns an array of Threads that are in the ThreadGroup.

ThreadGroup group = new ThreadGroup("myGroup");
Thread thread1 = new Thread(group, myRunnable);
Thread thread2 = new Thread(group, myRunnable);
Thread[] threads = new Thread[group.activeCount()];
group.enumerate(threads);

Handling Uncaught Exceptions

A ThreadGroup can also handle uncaught exceptions that occur in its threads. You can set an uncaught exception handler for a ThreadGroup by calling the ThreadGroup.setUncaughtExceptionHandler() method.

ThreadGroup group = new ThreadGroup("myGroup");
group.setUncaughtExceptionHandler(myExceptionHandler);

If a Thread in the ThreadGroup throws an uncaught exception, the ThreadGroup's uncaught exception handler will be called.

Managing the Threads in a ThreadGroup

You can suspend, resume, or stop all of the threads in a ThreadGroup by calling the ThreadGroup.suspend(), ThreadGroup.resume(), or ThreadGroup.stop() methods.

ThreadGroup group = new ThreadGroup("myGroup");
group.suspend();
// All threads in the ThreadGroup are now suspended
group.resume();
// All threads in the ThreadGroup are now resumed
group.stop();
// All threads in the ThreadGroup are now stopped

You can also set the maximum priority for the threads in a ThreadGroup using the ThreadGroup.setMaxPriority() method.

ThreadGroup group = new ThreadGroup("myGroup");
group.setMaxPriority(Thread.MAX_PRIORITY);