Vrftjkry

In Java (using Java 8 currently), I can write this and all will compile nice and well:

Supplier<Long> asd = () -> {

    throw new RuntimeException();

};

Yet, I cannot write this:

Supplier<Long> asd = () -> throw new RuntimeException(); // This won't compile :(

Does anyone know why Java's implementation does not allow such style (expression lambda) and only the statement/code-block style lambda?

I mean, since the lambda is only throwing the RuntimeException, why isn't the JDK able to infer the lambda expression as:

new Supplier<Long>() {

    @Override

    public Long get() {

        throw new RuntimeException();

    }

};

Is this documented somewhere in the specs/docs? Is this added only in JDK > 8?

From the language spec JLS 15.27.2:

A lambda body is either a single expression or a block (§14.2). 

throw is not a single expression (it's a statement); so you have to use it in a block.

What you wrote is an invalid lambda. There is a difference between the expression with the brackets {} and without. See the example. The following means that 1L is returned.

Supplier<Long> asd = () -> 1L;

which is equivalent to:

Supplier<Long> asd = () -> {

    return 1L;

}; 

However, when you write:

Supplier<Long> asd = () -> throw new RuntimeException();

It would be translated following which is an invalid lambda:

Supplier<Long> asd = () -> {

     return throw new RuntimeException();                    // invalid expression in Java

};

In a nutshell, you can understand () -> 1L as a shortcut for { return 1L; }.