What does the -> <- operator do?

Question

I recently came upon the following code:

IntPredicate neg = x -> x <- x;

What is this, some sort of reverse double lambda?

Answer 1

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));   // false
System.out.println(neg.test(0));   // false
System.out.println(neg.test(-4));  // true

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Just 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);.