Shared coding conventions allow teams to collaborate effectively. While types for lambda arguments are optional, specifying them anyway makes the code clearer and easier to read.
Arrays.sort(rosterAsArray, (a, b) -> { // Noncompliant return a.getBirthday().compareTo(b.getBirthday()); } );
Arrays.sort(rosterAsArray, (Person a, Person b) -> { return a.getBirthday().compareTo(b.getBirthday()); } );
When the lambda has one or two parameters and does not have a block this rule will not fire up an issue as things are considered more readable in those cases.
stream.map((a, b) -> a.length); // compliant