Annotation Interface Nullable


An element annotated with Nullable claims null value is perfectly valid to return (for methods), pass to (parameters) or hold in (local variables and fields). Apart from documentation purposes this annotation is intended to be used by static analysis tools to validate against probable runtime errors or element contract violations.

By convention, this annotation applied only when the value should always be checked against null because the developer could do nothing to prevent null from happening. Otherwise, too eager Nullable usage could lead to too many false positives from static analysis tools.

For example, Map.get(Object key) should not be annotated Nullable because someone may have put not-null value in the map by this key and is expecting to find this value there ever since. It could be annotated as UnknownNullability or left unannotated.

On the other hand, the Reference.get() should be annotated Nullable because it returns null if object got collected which can happen at any time completely unexpectedly.

If a method overrides a superclass method, and the superclass method specifies the nullability on parameter or return type, then the subclass method should specify the same nullability, either directly or indirectly via NotNullByDefault. The only exception is the covariant return type nullability: if the superclass method is declared to return nullable value, then subclass may declare to return a not-null value.

The tools may issue a warning if the nullability for a subclass method contradicts from the specified nullability of a superclass method.

See Also:
  • Optional Element Summary

    Optional Elements
    Modifier and Type
    Optional Element
    Description
     
  • Element Details

    • value

      Returns:
      textual reason when the annotated value could be null, for documentation purposes.
      Default:
      ""