Your source code can contain various annotations. For example, @Nullable and @NotNull annotations are used, when IntelliJ IDEA supposes that a certain element can become Null. The @NonNls annotation is used to ignore the hard-coded string literals.

To use annotations
  1. Make sure that annotations.jar that can be found in the lib folder under the IntelliJ IDEA installation, is added to the module dependencies.

    For @javax.annotation.ParametersAreNonnullByDefault annotations, javax should be added to the module libraries.

    You can also use annotations from JSR-305 and FindBugs. In this case, make sure they are added to the classpath.

    You can configure @Nullable, @NotNull and @Contract annotations in the Inspections page of the Preferences dialog (Constant conditions and exceptions - Configure annotations).

  2. Introduce the desired annotation before the package/class/field/variable/method/method parameter declaration.