What makes an object effectively immutable in Java?

Its externally observable state cannot change after construction. Use private fields, initialize them fully, avoid mutating methods, and prevent uncontrolled subclass behavior where necessary. A final reference only prevents reassignment; it does not make the referenced list or date immutable. Make defensive copies of mutable inputs and avoid exposing mutable internal references. Immutability simplifies sharing and reasoning, but static mutable state and unsafe publication can still create concurrency problems. Records provide concise data carriers, but their components can still reference mutable objects.