prefix and postfix operators java

Question

I was trying unary postfix and prefix operators in java

Here's the code

int a=10;

This line of code does not give compile time error

System.out.println(a+++ a +++a);   

But this line does

System.out.println(a++ +++a);

whereas this line even doesn't

System.out.println(a+++ ++a);

I can't understand the pattern of how compiler interprets these queries.

Answer 1

In the case of

System.out.println(a++ +++a);

The compiler appears to be interpreting this as

System.out.println((a++)++ +a);

This doesn't work because the result of a pre/post increment/decrement expression is a value, not a variable. (It might also be seeing it as a+ ++(++a) but the outcome is the same).

Indeed, if you compile this with the Oracle compiler from command line, you get the following error:

UnaryOperatorTests.java:10: error: unexpected type
        System.out.println(a++ +++a);
                        ^
  required: variable
  found:    value
1 error

Which is much more indicative of what's going on when compared to the Eclipse compiler's message:

Invalid argument to operation ++/--

That said, you can get that same error from Eclipse by trying to do:

System.out.println(1++);

Adding a space thusly:

System.out.println(a++ + ++a);

seems to remove the ambiguity that confuses the compiler, and compiles as you might expect.