Why are additional parentheses used next to the synchronized keyword in code?

In Java, the keyword synchronized is used to lock an object to ensure thread-safe access to shared resources. The parentheses next to synchronized usually indicate the object on which synchronization occurs.

For example:

synchronized(this) {
    // critical section
}

Here, this is the object on which the monitor is taken. The parentheses are mandatory because synchronized requires an expression in parentheses that specifies the lock object.

If you write just synchronized without parentheses, it will be a syntax error. Therefore, the additional parentheses are not just syntax but an essential part of the construct, indicating on which object the synchronization takes place.