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In a thread-safe class, all field accesses that can be caused by calls to public methods must be properly synchronized.
Protect the field access with a lock. Alternatively mark the field as volatile if the write operation is atomic. You can also choose to use a data type that guarantees atomic access. If the field is immutable, mark it as final.
Java Language Specification, chapter 17: Threads and Locks.
Java concurrency package: java.util.concurrent.