CheckReturnValue
Ignored return value of method that is annotated with @CheckReturnValue

Severity
ERROR

Alternate names: ResultOfMethodCallIgnored, ReturnValueIgnored

The problem

The @CheckReturnValue annotation (available in JSR-3051 or in Error Prone) marks methods whose return values should be checked. This error is triggered when one of these methods is called but the result is not used.

@CheckReturnValue may be applied to a class or package 2 to indicate that all methods in that class or package must have their return values checked.

For convenience, we provide an annotation, @CanIgnoreReturnValue, to exempt specific methods or classes from this behavior. @CanIgnoreReturnValue is available from the Error Prone annotations package, com.google.errorprone.annotations.

If you really want to ignore the return value of a method annotated with @CheckReturnValue, a cleaner alternative to @SuppressWarnings is to assign the result to a variable that starts with unused:

public void setNameFormat(String nameFormat) {
  String unused = format(nameFormat, 0); // fail fast if the format is bad or null
  this.nameFormat = nameFormat;
}

Ignored contexts

@CheckReturnValue is ignored under the following conditions (which saves users from having to use either an unused variable or @SuppressWarnings):

  1. Calls from Mockito.verify() or Stubber.when(); e.g., Mockito.verify(t).foo() or doReturn(val).when(t).foo() (where foo() is annotated with @CheckReturnValue). Here, the method calls are just used to program the mock object, not to be consumed directly.

  2. Code that does exception testing with JUnit, where the intent is that the method call should throw an exception:

    • Uses of JUnit 4.13 or JUnit5’s assertThrows methods:

      assertThrows(IndexOutOfBoundsException.class, () -> list.get(-1));
      
    • The try/execute/fail/catch pattern

       try {
         list.get(-1);
         fail("Expected a IndexOutOfBoundsException to be thrown on a negative index");
       } catch (IndexOutOfBoundsException expected) {
       }
      
    • JUnit’s ExpectedException

      expectedException.expect(IndexOutOfBoundsException.class);
      list.get(-1); // If this throws IOOBE, the test passes.
      

Suppression

Suppress false positives by adding the suppression annotation @SuppressWarnings("CheckReturnValue") to the enclosing element.

  1. Of note, the JSR-305 project was never fully approved, so the JSR-305 version of the annotation is not actually official and causes issues with Java 9 and the Module System. Prefer to use the Error Prone version. 

  2. To annotate a package, create a package-info.java file in the package directory, add a package statement, and annotate the package statement.