This type qualifier indicates how much of an object has been fully initialized. An object is fully initialized when each of its fields contains a value that satisfies the field's declaration.

An expression of type @UnknownInitialization(T.class) refers to an object that has all fields of T (and any super-classes) initialized. Just @UnknownInitialization is equivalent to @UnknownInitialization(Object.class).

A common use is

void myMethod(@UnknownInitialization(MyClass.class) MyClass this, ...) { ... }

which allows myMethod to be called from the MyClass constructor. See the manual for more examples of how to use the annotation (the link appears below).

Reading a field of an object of type @UnknownInitialization might yield a value that does not correspond to the declared type qualifier for that field. For instance, consider a non-null field:

@NonNull Object f;

In a partially-initialized object, field f might be null despite its @NonNull type annotation.

What type qualifiers on the field are considered depends on the checker; for instance, the org.checkerframework.checker.nullness.NullnessChecker considers NonNull. The initialization type system is not used on its own, but in conjunction with some other type-system that wants to ensure safe initialization.