Operator precedence issue leads to "error: unexpected type"

Question

Since there have been so many operator precedence questions recently, I started playing with some code and came up with this:

int x = someNumber;
int y = --x++;

This gives:

Error: unexpected type

required: variable
found: value

I tried this because I was interested to learn how java would deal with the fact that postfix has a higher operator precedence than prefix. This seems like the above statement would lead to a contradiction, which I guess is handled by this error.

My question is two-fold:

1. Why this error? What does it mean, exactly?
2. Why does postfix have a higher precedence than prefix? I'm sure there's a good reason for it, but I haven't been able to come up with one. Perhaps it would fix this undefined behavior, but it would somehow give rise to more problems?

Answer 1

The cause of the error is that x++ produces a value, and you can't apply a decrement operator to a value, only to a variable. For example if x=41, x++ evaluates to 41, not to the variable x, and --(41) is meaningless.

As to why postfix has higher precedence than prefix, my guess is that it is to avoid ambiguity with other operators while parsing. For example, the compiler can report a syntax error for x--x instead of parsing it as x-(-x).

Answer 2

try

int y = 2++;

you will get the same error. Post/pre operator are applied on variable not on some number.Thats why you get error

 Error: unexpected type

Because it expects a variable not some number. Assume your number is 3

 int x = 3;
 int y = --x++;

int y become 2++ after applying -- operator on java (as java operator works 
from left   to  right)

I don't know what exactly you are trying to ask in second question. But take scenario

  int y = -x---x;

here also it will be operated from left to right which comes to

 (-x--)-(x) so answer will be -3 so dont get confused by postfix and prefix