In Java, inter-thread communication is the process of coordinating the activities of multiple threads that need to work together to accomplish a task. Inter-thread communication in Java is typically achieved using the wait(), notify(), and notifyAll() methods of the Object class.

1. wait(): The wait() method causes the current thread to wait until another thread calls the notify() or notifyAll() method on the same object. The wait() method is typically used to suspend a thread until a certain condition is met. Here's an example:

synchronized(sharedObject) {
    while(!condition) {
        sharedObject.wait();
    }
    // Thread resumes execution
}

In this example, the thread acquires the lock on the sharedObject monitor and enters a loop, waiting for the condition to be true. If the condition is not true, the thread calls the wait() method on the sharedObject monitor, which causes the thread to wait until another thread calls the notify() or notifyAll() method on the same object. Once the condition is true, the thread resumes execution.

2. notify(): The notify() method wakes up one of the threads that are waiting on the same object. If multiple threads are waiting, the thread that is awakened is arbitrary. Here's an example:

synchronized(sharedObject) {
    // Change the condition
    condition = true;
    sharedObject.notify();
}

In this example, the thread acquires the lock on the sharedObject monitor and changes the condition to true. The thread then calls the notify() method on the sharedObject monitor, which wakes up one of the threads that are waiting on the same object.

3. notifyAll(): The notifyAll() method wakes up all the threads that are waiting on the same object. Here's an example:

synchronized(sharedObject) {
    // Change the condition
    condition = true;
    sharedObject.notifyAll();
}

In this example, the thread acquires the lock on the sharedObject monitor and changes the condition to true. The thread then calls the notifyAll() method on the sharedObject monitor, which wakes up all the threads that are waiting on the same object.

In addition to the wait(), notify(), and notifyAll() methods, Java also provides other inter-thread communication mechanisms such as BlockingQueue, CountDownLatch, and CyclicBarrier, which can be used to coordinate the activities of multiple threads in more complex scenarios.