How does the keyword synchronized interact with the monitor when used on a method?

In Java, the keyword synchronized, when applied to a method, means that when this method is called, the thread must acquire the monitor (lock) of the object to which the method belongs:

• For non-static methods, the monitor is the object itself (this).
• For static methods, the monitor is the class object (Class).

When a thread calls a synchronized method, it attempts to acquire the monitor. If the monitor is already held by another thread, the calling thread blocks until the monitor is released.

Thus, synchronized on a method provides mutual exclusion (mutex) for all calls to that method on a single object (or class for static methods), preventing multiple threads from executing the method's code simultaneously.

Example:

public synchronized void increment() {
    count++;
}

Here, increment can only be executed by one thread at a time on an object, as the thread must acquire the object's monitor before executing.