When are braces optional in Java 8 lambda syntax?

Question

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?

Answer 1

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 returned, then all possible code paths must either return a value or throw a Throwable.

Answer 2

This just in: the EG has (mostly) made a decision on syntax.

After considering a number of alternatives, we decided to essentially adopt the C# syntax. We may still deliberate further on the fine points (e.g., thin arrow vs fat arrow, special nilary form, etc), and have not yet come to a decision on method reference syntax.

The C# syntax is:

lambda = ArgList Arrow Body
ArgList = Identifier
           | "(" Identifier [ "," Identifier ]* ")"
           | "(" Type Identifier [ "," Type Identifier ]* ")"
Body = Expression
           | "{" [ Statement ";" ]+ "}"

An expression evaluates to something, you can't have void expressions in Java. It is a statement, thus you need {} around it.

http://mail.openjdk.java.net/pipermail/lambda-dev/2011-September/003936.html