Question

Why is the following syntax correct:

x = y+++y;

Where it means y++ + y or y + ++y which means y * 2 + 1 (not sure about this, though: very ambiguous)

But this syntax is incorrect:

x = y+++++y;

Which should mean y++ + ++y, which means y * 2 + 2

Is there a reason for the incorrectness of this syntax? (Edit: thank you for explaining why it is invalid syntax, but that is not my intention with this question.)

(Edit: ofcourse I’m not using this in real code, purely in interest of parsers/lexers; but I wonder why the parser doesn’t like this; the last example even looks less ambiguous than the first one.)

(Edit:

    int i = 0;
    int j = (i = 3)+++i;

Is invalid too, though it seems very unambiguous to me, (i = 3) is a value, thus (value + value) and then the ++i value token.)

Answer 1

The parsing is greedy, that is , it looks for the longest matching token first. This simplify the implementations a lot (presumably). Also the Java language spec(3.2) says

Java always uses the longest possible
translation at each step, even if the
result does not ultimately make a
correct Java program, while another
lexical translation would

So, for y+++++y; the parser/tokenizer will break it down something like this:

• variable y
• operator ++ (as there is no +++ operator, ++ is the longest that match the syntax of java)
• operator ++ (as there is no +++ operator, ++ is the longest that match the syntax of java)
• operator + (This was the first thing that matches the syntax now)
• variable y

Effectively it is parsed as (y++) (++) (+y)
the ++ operator is defined for a variable, however the first expression (y++) returns a value. You can’t apply the next operator (++) to a value.

This means that x = y+++y; would be parsed as y++ + y, which is nothing wrong with.