Mutable-immutable class pattern on Java depending on package

Question

Is there any pattern in java that makes a class mutable inside a package and immutable outside of it?

Answer 1

Simply refrain from using any access modifier on all the fields and on those methods that modify the fields, e.g. String secret; or void setSecret(String secret) { ... }. A field/method that does not have an access modifier is package-private, which means that it can only be seen by code that resides in the same package.

However, as @JoachimSauer and @Darkman point out, such a class is not in fact immutable. Immutable means that an instance will never change (no matter by whom) after it has been constructed. The class will simply be encapsulated: only a limited part of your codebase (the package where the class is defined) may modify instances of the class.