What does it mean for a class to be thread-safe, and what strategies can you use to achieve it?

A class is thread-safe if it behaves correctly when it is accessed concurrently by multiple threads, without requiring the calling code to add any extra synchronization of its own. There are several common strategies for achieving this. Making a class stateless -- with no instance variables, so every method call operates only on local variables -- automatically makes it thread-safe. Immutability, achieved through final fields, no setter methods, and defensive copies, also removes the possibility of unsafe concurrent mutation. Thread confinement restricts a piece of state so that only one thread ever touches it, for example using ThreadLocal. Explicit synchronization, using synchronized blocks, ReentrantLock, or atomic variables, protects mutable shared state directly. Finally, you can simply reuse existing thread-safe classes such as ConcurrentHashMap, BlockingQueue implementations, or AtomicInteger instead of writing your own synchronization logic.