Why do Consumers accept lambdas with statement bodies but not expression bodies?

by

First, it’s worth looking at what a Consumer<String> actually is. From the documentation:

Represents an operation that accepts a single input argument and
returns no result. Unlike most other functional interfaces, Consumer
is expected to operate via side-effects.

So it’s a function that accepts a String and returns nothing.

Consumer<String> p = ""::equals;

Compiles successfully because equals can take a String (and, indeed, any Object). The result of equals is just ignored.*

p = s -> "".equals(s);

This is exactly the same, but with different syntax. The compiler knows not to add an implicit return because a Consumer should not return a value. It would add an implicit return if the lambda was a Function<String, Boolean> though.

p = s -> true;

This takes a String (s) but because true is an expression and not a statement, the result cannot be ignored in the same way. The compiler has to add an implicit return because an expression can’t exist on its own. Thus, this does have a return: a boolean. Therefore it’s not a Consumer.**

p = s -> ("".equals(s));

Again, this is an expression, not a statement. Ignoring lambdas for a moment, you will see the line System.out.println("Hello"); will similarly fail to compile if you wrap it in parentheses.

*From the spec:

If the body of a lambda is a statement expression (that is, an expression that would be allowed to stand alone as a statement), it is compatible with a void-producing function type; any result is simply discarded.

**From the spec (thanks, Eugene):

A lambda expression is congruent with a [void-producing] function type if …
the lambda body is either a statement expression
(ยง14.8)
or a void-compatible block.

More Related Contents:

  1. Cannot use Java 8 method with lambda arguments without specifying type arguments
  2. Difference between final and effectively final
  3. Java 8 Lambda function that throws exception?
  4. How can I throw CHECKED exceptions from inside Java 8 streams?
  5. Java 8 lambdas, Function.identity() or t->t
  6. Very confused by Java 8 Comparator type inference
  7. Precise definition of “functional interface” in Java 8
  8. Break or return from Java 8 stream forEach?
  9. Lambda Expression and generic defined only in method
  10. Reflection type inference on Java 8 Lambdas
  11. java.lang.NullPointerException is thrown using a method-reference but not a lambda expression
  12. Lambda expression to convert array/List of String to array/List of Integers
  13. Filter values only if not null using lambda in Java8
  14. Retrieving a List from a java.util.stream.Stream in Java 8
  15. Java “lambda expressions not supported at this language level”
  16. Java8: ambiguity with lambdas and overloaded methods
  17. Java 8, Streams to find the duplicate elements
  18. How do I use the new computeIfAbsent function?
  19. How to get a range of items from stream using Java 8 lambda?
  20. Java8 Lambdas and Exceptions
  21. Why can method reference use non-final variables?
  22. Possibility to explicit remove Serialization support for a lambda
  23. How to make a lambda expression define toString in Java 8?
  24. Java 8 grouping using custom collector?
  25. How can I throw CHECKED exceptions from inside Java 8 lambdas/streams?
  26. How to apply multiple predicates to a java.util.Stream?
  27. Parallel stream from a HashSet doesn’t run in parallel
  28. A peculiar feature of exception type inference in Java 8
  29. Non-interference exact meaning in Java 8 streams
  30. Creating String representation of lambda expression [duplicate]

Leave a Comment