Concepts

e2immu concepts

The best place to start reading about the immutability concepts is:

• The Road to Immutability →, PDF

There’s also a slide deck covering the basic concepts.

TL;DR#

• modification: changes to the object graph of an object’s fields.

• container: a type which does not make modifications to the arguments of its constructors and non-private methods. You can use it as safe storage for your objects.

• final fields: all fields are effectively final (either explicitly, or only modified during the construction phase). Java record types are a nice example.

• independence: a modification to one object has no effect on the other object. Fields of implicitly immutable type cannot be dependent, because the type has no means of modifying them.

• a type is immutable when the following four criteria are met:

  1. the type is final fields;
  2. its fields are not modified;
  3. its fields are either private, or of immutable type;
  4. constructor parameters are independent of the fields, as are the return values and parameters of non-private methods

• eventually immutable: immutability is achieved after a field changes from a before state into an after or final state. This blocks a number of modifying methods, which makes the type effectively immutable.

Primitives, Object, and String are immutable - once we ignore the synchronization methods of Object. Unmodifiable variants of collection classes, such as the result of Set.of() and List.copyOf(), are immutable as well.

The project proposes simple support classes such as SetOnce and EventuallyFinal that help propagate eventual immutability throughout your program.

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