Valid annotation for parent is invalid for child

Question

Hello I have parent class with argument with nullable annotation.

class Parent {

  @Nullable
  String name;

  Parent(@Nullable Strign name) {
    this.name = name;
  }

  Driver createDriver() {
    return new CommonDriver(name);
  }
}

I have multiple children classes, for most of them, the "name" argument can be null, but there are some for which it cannot.

class ChildC extends Parent {

  ChildC(@NotNull String name){
    super(name);
  }

  @Override
  Driver createDriver() {
    return new ChildCDriver(name);
  }
}

and now I have issue (code inspection from intelliJ) inside ChildCDriver where name is @NotNull

Can this be solved somehow ?

Answer 1

This is reasonable code, but IntelliJ is not powerful enough to prove that the code is correct. You need to suppress the warning. Click on the line, press Alt+Enter, and find "Suppress" in that menu or a submenu.

The Nullness Checker can verify your code. The complete code appears below. The @FieldInvariant annotation expresses that the field has a more precise type in the subclass.

Without the @FieldInvariant annotation, the Nullness Checker issues this warning on line 27:

error: [argument.type.incompatible] incompatible types in argument.
    return new ChildCDriver(name);
                            ^
  found   : @Initialized @Nullable String
  required: @Initialized @NonNull String

With the @FieldInvariant annotation, the Nullness Checker proves the code is correct.

The below code example uses the Checker Framework's @NonNull and @Nullable annotations, but the Nullness Checker also supports @NotNull so you can continue using JetBrains annotations in your code.

import org.checkerframework.checker.nullness.qual.NonNull;
import org.checkerframework.checker.nullness.qual.Nullable;
import org.checkerframework.framework.qual.FieldInvariant;

class Parent {

  final @Nullable String name;

  Parent(@Nullable String name) {
    this.name = name;
  }

  Driver createDriver() {
    return new CommonDriver(name);
  }
}

@FieldInvariant(qualifier = NonNull.class, field = "name")
class ChildC extends Parent {

  ChildC(@NonNull String name) {
    super(name);
  }

  @Override
  Driver createDriver() {
    return new ChildCDriver(name);
  }
}

interface Driver {}

class CommonDriver implements Driver {
  CommonDriver(@Nullable String name) {}
}

class ChildCDriver implements Driver {
  ChildCDriver(@NonNull String name) {}
}