CheckReturnValue
The result of this call must be used

Alternate names: ResultOfMethodCallIgnored, ReturnValueIgnored

The problem

When code calls a non-void method, it should usually use the value that the method returns.

Consider the following code, which ignores the return value of concat:

string.concat("\n");

That code is a no-op because concat doesn’t modify string; it returns a new string for the caller to use, as in:

string = string.concat("\n");

To avoid this bug, Error Prone requires callers to use the return value of concat and some other well-known methods.

Additionally, Error Prone can be configured to require callers to use the return value of any methods that you choose.

How to tell Error Prone which methods to check

Most methods are like concat: Calls to those methods must use their return values.

However, there are exceptions. For example, set.add(element) returns a boolean: The return value is false if element was already contained in set. Typically, callers don’t need to know this, so they don’t need to use the return value.

For Error Prone’s CheckReturnValue check to be useful, it needs to know which methods are like concat and which are like add.

@CheckReturnValue and @CanIgnoreReturnValue

The @CheckReturnValue annotation (available in JSR-305¹ or in Error Prone) marks methods whose return values must be used. 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 used.

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.