In Java multithreading, what is the purpose of the wait(), notify(), and notifyAll() methods on an object's monitor?

Introduction / Context:
In Java concurrency, it is common to have threads that must wait for certain conditions to become true, such as data becoming available in a queue or a task finishing. While synchronization controls mutual exclusion, it does not by itself provide a way for threads to sleep until a condition changes. The wait, notify, and notifyAll methods, defined in java.lang.Object, support classic wait notify coordination between threads. Interviewers often ask about these methods to see if you understand low level thread communication.

Given Data / Assumptions:

• Threads may share objects that act as monitors or locks.
• A thread may need to pause execution until another thread performs an action on shared data.
• Busy waiting loops that repeatedly check a condition waste CPU time.
• Java provides methods for threads to wait and be signaled efficiently using object monitors.

Concept / Approach:
The wait method causes the current thread to release the monitor lock for an object and enter the wait set associated with that object, suspending execution until another thread calls notify or notifyAll on the same object. The notify method wakes up one arbitrary thread from the wait set, while notifyAll wakes up all waiting threads. These methods must be called from within synchronized blocks or methods that hold the same object's monitor. Together, they provide a basic mechanism for implementing producer consumer queues, resource pools, and other coordination patterns by combining intrinsic locks with condition waiting and signaling.

Step-by-Step Solution:
Step 1: A thread enters a synchronized block or method that locks on a shared monitor object. Step 2: The thread checks a shared condition, such as whether a queue is empty or full. Step 3: If the condition indicates that it must wait, the thread calls wait on the monitor object, which releases the lock and suspends the thread in the wait set. Step 4: Another thread that modifies the shared state later enters a synchronized block on the same object, changes the condition, and calls notify or notifyAll. Step 5: Notified threads are moved from the wait set to a blocked state, compete again for the monitor lock, and when they reacquire it, they recheck the condition and continue execution.

Verification / Alternative check:
A simple producer consumer example proves the value of these methods. The producer thread adds items to a shared buffer and calls notifyAll after inserting data. The consumer thread waits using wait while the buffer is empty. When notifyAll is called, the consumer wakes, reacquires the lock, and sees that the buffer is no longer empty. Without wait and notify, the consumer would need to spin in a loop checking the buffer repeatedly, which would waste CPU and complicate code.

Why Other Options Are Wrong:
Option B is incorrect because file and socket closing are handled by APIs such as close methods and try with resources, not by wait and notify methods. Option C is wrong because these methods do not terminate the JVM; they only affect the scheduling of threads waiting on a particular monitor. Option D is clearly incorrect since formatting or printing strings to the console is unrelated to wait notify semantics; those concerns fall under I O and logging libraries.

Common Pitfalls:
Common pitfalls include calling wait or notify outside of a synchronized block on the same object, which throws an IllegalMonitorStateException. Another mistake is using notify when multiple different conditions are guarded by the same lock, which can lead to missed signals or threads waking unnecessarily; notifyAll can be safer but may reduce performance. Modern Java often uses higher level concurrency utilities such as BlockingQueue or Condition objects from java.util.concurrent to simplify these patterns, but understanding wait, notify, and notifyAll remains important for reading legacy code.

Final Answer:
The wait, notify, and notifyAll methods allow threads to coordinate by waiting on an object's monitor until a condition changes and being signaled by other threads that call notify or notifyAll on the same object.