java - Sonar : Replace this lambda with a method reference

Question

This code sample

Collection<Number> values = transform(
        getValuatedObjects(),
        input -> getValueProvider().apply(input).getValue());

violates the sonarqube rule:

Replace lambdas with method references when possible

is it a sonar bug ? or can i really use a method reference ?

Answer 1

You can’t replace the lambda input -> getValueProvider().apply(input).getValue() with a method reference without changing the semantics.

A method reference replace a single method invocation, so it can’t simply replace a lambda expression consisting of more than one method invocation.

A lambda expression of the form input -> getValueProvider().apply(input) could be replaced by getValueProvider()::apply if, and only if, the evaluation time of getValueProvider() does not matter as in the lambda form the method is invoked on each lambda body evaluation while for the method reference it is invoked only once and the result captured.

This is similar to the difference between x -> System.out.println(x) and System.out::println where reading the contents of the field System.out happens at different times but usually it doesn’t matter. But you should be aware of the difference.

In your example, a third method getValue() is invoked. The only way to express that with method references needs a functional interface like Function which has methods like andThen and/or compose. However, the way Java?8 works, that would require casting the first method reference to the target interface to invoke the combining method which would be by no way easier to read that the lambda expression you have now: ((Function<X,Y>)getValueProvider()::apply).andThen(Y::getValue) where Y is the type, apply(input) returns.

Note that the rule says “Replace lambdas with method references when possible” which gives you room to say, “well, here it is impossible”, however, I’m not sure how much you can call it a “rule” then…