checkstyle – UnusedLambdaParameterShouldBeUnnamed

UnusedLambdaParameterShouldBeUnnamed

Since Checkstyle 10.18.0

Description

Ensures that lambda parameters that are not used are declared as an unnamed variable.

Rationale:

Improves code readability by clearly indicating which parameters are unused.
Follows Java conventions for denoting unused parameters with an underscore (_).

See the Java Language Specification for more information about unnamed variables.

Attention: Unnamed variables are available as a preview feature in Java 21, and became an official part of the language in Java 22. This check should be activated only on source code which meets those requirements.

Examples

To configure the check:

<module name="Checker">
  <module name="TreeWalker">
    <module name="UnusedLambdaParameterShouldBeUnnamed"/>
  </module>
</module>

Example:

class Example1 {

  int x;

  void test() {
    // violation below, 'Unused lambda parameter 'x' should be unnamed'
    Function<Integer, Integer> f1 = (x) -> this.x + 1;
    // ok below, parameter is unnamed
    Function<Integer, Integer> f2 = (_) -> this.x + 1;
    // ok below, parameter is used
    Function<Integer, Integer> f3 = (x) -> x + 1;
    // violation below, 'Unused lambda parameter 'y' should be unnamed'
    BiFunction<Integer, Integer, Integer> bf1 = (x, y) -> x + this.x;
  }
}