About Question enthuware.ocajp.i.v7.2.949 :

[Danail]

Hello,

In expression (b2 != b1 = !b2) first is evaluated !b2, but in "Explanation" section you said that first "b2 != b1" is evaluated.

The whole order of evaluation is:
1. !b2 -> true
2. b2 != b1 -> false
3. false = true -> compile time error

Please, correct me if I am wrong.

[Danail]

I don't know why according to http://docs.oracle.com/javase/tutorial/ ... ators.html "Operator Precedence":
unary operator "!" has more priority than equality operator "!=" ?

Hello,

In expression (b2 != b1 = !b2) first is evaluated !b2, but in "Explanation" section you said that first "b2 != b1" is evaluated.

The whole order of evaluation is:
1. !b2 -> true
2. b2 != b1 -> false
3. false = true -> compile time error

Please, correct me if I am wrong.

[baptize]

! has higher precedence than !=

[openmic62]

When I took the test I answered "Compile time error", but got flagged for an incorrect answer. So, I copied the code from the test into a main method of a class I created.

Code: Select all

class T2Q04_QID0949{
	public static void main(String[] args){
		boolean b1 = false;
		boolean b2 = false;
		if (b2 != b1 = !b2){
			System.out.println("true");
		}else{
			System.out.println("false");
		}
	}
}

I did indeed get a compiler error.

Code: Select all

T:\src\main\java>javac -d ..\..\..\target\classes  T2Q04_QID0949.java
T2Q04_QID0949.java:10: error: unexpected type
                if (b2 != b1 = !b2){
                       ^
  required: variable
  found:    value
1 error

T:\src\main\java>javac -version
javac 1.7.0_07

I think Enthuware needs to fix this question, unless they can prove me wrong.
The correct answer is: Compile time error

[admin]

I see that "Compile time error" is indeed set as the correct answer.

[javanaut]

Yes, I wanted to see what the forum had to say about this question. The explanation does not mention anything about how the ! unary operator has more precedence that the equality != or the assignment = operator.

So it seems like:

• 1.

  Code: Select all

  !b2

  is evaluated and returns true
  2.

  Code: Select all

  b2 != b1

  is evaluated which evaluates to true != false which returns true
  
  3. Then the literal boolean value true is tried to be used like a variable container to take the value of the expression

  Code: Select all

  !b2

  but it cannot do this as a literal is not a variable so the code fails and results in a compile time error

What does the enthuware hive mind think?

Regards,

javanaut

[admin]

Yes, unary operator has more precedence. But even with that logic, the answer will not be any different.

(b2 != b1 = !b2) =>
(b2 != b1 = true) => (!= has more precendence than =)
(false = true) Which is illegal

Remember that there will actually be no execution because of compilation failure. We are just trying to hypothetically execute the expression and see whether it boils down to an illegal situation or not. Here, we see that it does resolve to an illegal expression and so it will not compile.

HTH,
Paul.

[javanaut]

Cool.

And yes, I guess the compiler goes through this logic in nanoseconds to see that it will not compile. Thank-you for the reply Paul.

Regards,

Javanaut

[agupta108]

Answer is correct, but explaination is a little incorrect in Enthuware:
"!b2 -> true" will be executed first,
and then "b2 != b1 -> false" will be executed,
resulting "false = true" in a Compile Time Error