An immutable object cannot be modified after it was created. Instead, a copy of the object is returned.

• Thread safety: they cannot change its state, so they cannot be corrupted by thread interference or observed in an inconsistent state.
• Atomicity of failure: if an immutable object throws an exception, it's never left in an undesirable or indeterminate state.
• Protection against null reference errors.

Immutable classes require a separate object for each distinct value. Creating these objects can be costly, especially if they are large.

• methods that alters the object state should return new objects
• make copies of caller provided data (except primitives or immutable objects)
• ensure the class cannot be overridden (make the class final, or use static factories and keep constructors private)