What causes a race condition, and how do you make Java code thread-safe?

Defines races through timing-dependent shared-state operations and evaluates confinement, immutability, synchronization, and atomic tools.

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A race condition occurs when correctness depends on the timing or interleaving of concurrent operations. counter++ is a read, calculation, and write, so two threads can lose an update even if individual integer reads and writes are atomic. Thread-safe code preserves its invariants under concurrent access. Common strategies are thread confinement, immutable state, synchronization around a compound action, atomic variables, or concurrent collections.

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Thread-safety reasoning across compound operations, invariants, and ownership.

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Thread safety is a property of the complete state transition and access protocol, not of one field keyword.
A race condition occurs when correctness depends on the timing or interleaving of concurrent operations. counter++ is a read, calculation, and write, so two threads can lose an update even if individual integer reads and writes are atomic. Thread-safe code preserves its invariants under concurrent access. Common strategies are thread confinement, immutable state, synchronization around a compound action, atomic variables, or concurrent collections.
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The lock or atomic operation must cover the whole invariant. Protecting balance but not the related ledger update can still expose inconsistent state. A thread-safe collection makes its own operations safe, but remains a compound race; use or according to their contract. Reducing shared mutable state is often simpler than adding more locks.

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Calling counter++ atomic.
Locking only one field of a multi-field invariant.
Assuming a concurrent collection makes compound sequences atomic.

What causes a race condition, and how do you make Java code thread-safe?

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