Understanding Java Volatile Happens‑Before Rules for Write and Read Operations

The question asks whether a code segment where one thread executes a write method and another thread executes a read method can have the write (D) observed, and why.

The answer is yes: the Java volatile keyword provides a happens‑before visibility guarantee. Specifically, a write to a volatile variable happens‑before any later read of that same volatile variable.

Additional rules are explained:

For volatile variables, a write to a volatile variable happens‑before any subsequent read of that same volatile variable.

A write to a volatile variable also happens‑before any ordinary variable reads or writes that occur after that volatile write, and ordinary reads/writes cannot be reordered to occur after the volatile write.

Conversely, a read of a volatile variable happens‑after any ordinary variable reads or writes that occurred before that volatile read.

The happens‑before relation is transitive: if A happens‑before B and B happens‑before C, then A happens‑before C.

Applying these rules to the code, the sequence B → C → D establishes a happens‑before chain, so the read at D can see the latest value written at A.