What Do Two Question Marks in a Row in a Ternary Clause Mean?

Question

I recently saw this ternary operation statement in a piece of Java code:

int getVal(Integer number, boolean required) {
    Integer val = number == null ? required ? 1 : 2 : 3;
    return val;
}

I've never seen a ternary statement with two question marks in a row like that (without any parentheses). If I play with the input values, I can get 1 to return if number == null and 3 to return otherwise, but it doesn't seem to matter what required is, 2 never gets returned.

What does this statement mean (i.e. how should I read it as a word statement of true/false conditions) and what would the inputs need to be for 2 to be returned?

Answer 1

An expression of the form a ? b ? c : d : e uses the conditional operator twice. The only ambiguity is in which order.

It can not mean (a ? b) ? c : d : e because that would be a syntax error.

Therefore, it must mean a ? (b ? c : d) : e.

That is, it is equivalent to:

if (a) {
  if (b) {
    return c;
  } else {
    return d;
  }
} else {
  return e;
}

A more interesting case is

a ? b : c ? d : e

which could be read as

(a ? b : c) ? d : e 

or

a ? b : (c ? d : e)

To resolve this ambiguity, the Java language specification writes:

The conditional operator is syntactically right-associative (it groups right-to-left)