FieldInvariant (checker-framework 3.39.0-eisop1 API)

```
@Documented
@Retention(RUNTIME)
@Target(TYPE)
@Inherited
public @interface FieldInvariant
```
Specifies that a field's type, in the class on which this annotation is written, is a subtype of its declared type. The field must be declared in a superclass and must be final.
The @FieldInvariant annotation does not currently accommodate type qualifiers with attributes, such as @MinLen(1). In this case, the type system should implement its own field invariant annotation and override AnnotatedTypeFactory.getFieldInvariantDeclarationAnnotations() and AnnotatedTypeFactory.getFieldInvariants(). See MinLenFieldInvariant for an example.

See the Checker Framework Manual:
Field invariants

Required Element Summary

Required Elements
Modifier and Type	Required Element	Description
`java.lang.String[]`	`field`	The field that has a more precise type, in the class on which the `FieldInvariant` annotation is written.
`java.lang.Class<? extends java.lang.annotation.Annotation>[]`	`qualifier`	The qualifier on the field.

- Element Detail
  - qualifier
```
java.lang.Class<? extends java.lang.annotation.Annotation>[] qualifier
```
    The qualifier on the field. Must be a subtype of the qualifier on the declaration of the field.
- - field
```
java.lang.String[] field
```
    The field that has a more precise type, in the class on which the FieldInvariant annotation is written. The field must be declared in a superclass and must be final.