Why must throw statements be enclosed with a full code block in a lambda body? [duplicate]

Question

If there is a single statement in a lambda function, we can omit defining the full code block for it:

new Thread(() -> System.out.println()); 

Why is that not the case for statements that throw exceptions? This yields a compilation error stating '{' expected:

new Thread(() -> throw new RuntimeException()); 

Of course, enclosing the lambda body in a code block works:

new Thread(() -> {     throw new RuntimeException(); }); 

Answer 1

AFAIK The jls says that the lambda body has to be:

expression or a block. Having it like this:

new Thread(() -> throw new RuntimeException()); 

is neither and the compiler somehow informs you about that.

Declaring it like this:

 new Thread(() -> {      throw new RuntimeException();  }); 

makes it a block. Here is the relevant part:

A block is a sequence of statements, local class declarations, and local variable declaration statements within braces.

Answer 2

In Java8, the grammar of a lambda body only accepts an expression or a block. Whereas throwing an exception is a statement, not an expression.

throwStatement: 'throw' expression ';' ;  lambdaBody: expression | block;  expression: lambdaExpression | assignmentExpression;  block : '{' blockStatements? '}' ; 

Maybe it can be enhanced by including throwStatement into lambdaBody in the next jdk version if it is needed. Indeed, we need that as you mentioned above. for example:

lambdaBody: expression | block | throwStatement;