What is Immutable Object?

Immutable objects, once created, cannot have their fields changed. Any modification creates a new object instead. This eliminates a class of bugs related to shared mutable state in concurrent programs — no locks are needed because the data cannot change. In Python, tuples and frozensets are immutable. In Java, Strings are immutable. Functional programming languages (Haskell, Clojure) default to immutability. Benefits include simpler reasoning about code, safe sharing between threads, reliable hash keys, and easier undo/redo implementations. The trade-off is potential memory overhead from creating new objects for every change, though structural sharing can mitigate this.