What does the -> <- operator do?
There is no -> <- operator. That first -> is just lambda syntax, as introduced in Java 8, and that second <- is a misleading concatenation of 'smaller than' < and 'unary minus' -.
You can read it as IntPredicate neg = (x) -> (x < (-x));, i.e. it tests whether x is smaller than -x, which is the case for all (well, most) negative numbers, hence the name neg.
IntPredicate neg = x -> x <- x;System.out.println(neg.test(4)); // falseSystem.out.println(neg.test(0)); // falseSystem.out.println(neg.test(-4)); // trueJust for completeness: This test is not only (intentionally?) hard to understand, but -- as pointed out in the comments -- it also fails for Integer.MIN_VALUE (which is ==-Integer.MIN_VALUE). Instead, you should probably just use the much simpler IntPredicate neg = x -> (x < 0);.