hertzsprung hertzsprung - 7 months ago 38
Java Question

When are braces optional in Java 8 lambda syntax?

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?

jpm jpm
Answer

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.