Will the value 2000 be returned if two parallel threads simultaneously increment a volatile variable initially set to zero?

No, the value 2000 is not guaranteed when two threads simultaneously increase a volatile variable. The volatile keyword ensures visibility of changes to the variable between threads, but not atomicity of operations. Increment (++) is a compound operation (read, increase, write) that is not atomic. To correctly increment in a multithreaded environment, atomic classes such as AtomicInteger should be used:

import java.util.concurrent.atomic.AtomicInteger;

AtomicInteger counter = new AtomicInteger(0);

// In threads
counter.incrementAndGet();

Thus, without atomic operations, the result may be less than expected.