When are braces optional in Java 8 lambda syntax?
java, java-8, lambda
Solution
You may omit the braces when the lambda body is a single expression or a void method invocation. Every expression evaluates to a value, and thus cannot be void.
If the body of the lambda is a block of statements (e.g. a series of calculations followed by a `return` statement), or the lambda has no value (i.e. has a `void` return type) and is not a single void method invocation, you must use the block form, which requires brackets.
In a block-style lambda, if a value is `return`ed, then all possible code paths must either `return` a value or `throw` a `Throwable`.
Problem
I realise that the Java 8 lambda implementation is subject to change, but in lambda build b39, I've found that braces can only be omitted when the lambda expression returns a non-void type. For example, this compiles: ``` public class Collections8 { public static void main(String[] args) { Iterable<String> names = Arrays.asList("Alice", "Bob", "Charlie"); names.filter(e -> e.length() > 4).forEach(e -> { System.out.println(e); }); } } ``` But removing the braces like this: ``` names.filter(e -> e.length() > 4).forEach(e -> System.out.println(e)); ``` gives the error ``` Collections8.java:6: error: method forEach in interface Iterable<T> cannot be applied to given types; names.filter(e -> e.length() > 4).forEach(e -> System.out.println(e)); ^ required: Block<? super String> found: lambda reason: incompatible return type void in lambda expression where T is a type-variable: T extends Object declared in interface Iterable ``` Can anyone explain what's going on here?