Declare the class as `final`
Make all fields `private` and `final`
Initialize all fields through a constructor
Do not provide setter methods
If a field is mutable, create defensive copies in the constructor
If a getter returns a mutable object, return a defensive copy
Use immutable types for fields whenever possible
Do not allow subclassing that can alter behavior
Ensure the class state is fully initialized before use
Avoid exposing internal mutable references
Keep all methods side-effect free