Why is void return type compatible with other datatypes in lambdas?

[Ryan Gayle]

Hello,
A new member here.
I have a query, that i was wondering if someone could help out with.

1)Here, the return type of single abstract method (SAM) of functional interface A is void.
But, when I use A to point to a lambda that returns a String, and doesnt return void, why does it work?

Here, the lambda calls toUpperCase(), which returns String.
As lambda is providing body for the SAM of A, why does this work?

interface A { public void abstractMethod(); }

class Demo
{
	static String s="hi"; 
	 
	public static void main(String[] args) {
		
		A ref = () -> s.toUpperCase();
		ref.abstractMethod();
		
	}
}

thanks.

[Paul Clapham]

You're always allowed to write code which ignores the value returned by a method. For example you could write

"hi".toUpperCase();

as a statement in Java. Looks like the same applies when a lambda calls a method and ignores its return value too.

[Paul Anilprem]

You might also find a similar query that was answered here helpful.

[Ryan Gayle]

Thanks guys. I understood the significance of 'statement expressions' and void-compatible lambdas.
So, if abstractmMethod() of A is void, the lambda's body is free to return a type String, if the statement is an 'expression statement'.

Thank you for your helpful replies guys.