short / long circuit operators functionally different?!

[Tilo Hemp]

Hi all!

Up to this morning, I thought that the logical operators || and | (or && and &, respectively) were functionally the same, as long as they do not change the state of any variables.

But then I came to page 321 of the excellent K&B book, which inspired me to write the code:

public class Ch5 {
 
public static void main(String[] args) {
 
                // long version
if(false & false | true)
System.out.println("true");
else
System.out.println("false");
 
                // short version
if(false && false | true)
System.out.println("true");
else
System.out.println("false");
}
 
}    

which prints

true
false

Pleas explain!

I have to apologize if this has been asked before, I tried the search function but there were too many / too few answers (depending on for what I searched).

Regards

[Barry Gaunt]

The & and | operators have higher precedence than && and ||.
If you bracket your expressions according to precedence you will see why you get the results you do.

[Craig Tyler]

This is as much of a precedence issue as it is the difference between short and "long" circuit operators. The short-circuit operators have a lower precedence, so the two expressions end up like so:

if((false & false) | true)
...
if(false && (false | true))

In the first expression, as the & operator is of higher precedence than |, the two falses get grouped together, but the entire expression results in a true value since "false | true" is true.

The second expression gives higher precedence to the | operator, so "false | true" get grouped together. Here, however, the short-circuit operator stops at the first false since it wouldn't matter what the second grouping would result in.

Hope this helps.

[Tilo Hemp]

ah, I see... so it is a precedence issue. Will such things be in the Java 5 exam? Because in the K&B book, it says the operator precedence is not on the exam except for +- vs. */ (as far as I understood - so I didn't have a deeper look onto it). And now there comes along this example based on operator precedence... does that mean its important after all?

Anyway, I suppose it would not be much effort learning it by heart...
[ February 22, 2006: Message edited by: Tilo Hemp ]

[Edisandro Bessa]

Hi Tilo,

Just in addition to Barry's post, the precedence for logical operator is :

^ 1st --> & 2nd --> | 3rd --> && 4th --> || 5th.

In the past, I had the same doubt and asked about it.

If you want to take a brief look at thread I started in the past here is the link:

http://www.coderanch.com/t/252813/java-programmer-SCJP/certification/Logical-Operators-Precedence

[Tilo Hemp]

Hi,

thanks a lot everybody! Although I am still not sure how much I will need it, it's good to have this information!

Greetings
Tilo
[ February 22, 2006: Message edited by: Tilo Hemp ]

[levani dvalishvili]

in practice I find short sircuit operations very convinient, when you have to do null checking, something like this

if(obj!=null && obj.someMethod()!=null)

or

 if(stringObject==null || stringObject.equals(""))

if you do these with & or with | operators and the obj is null code will throw nullponterexception and will breake right away.
[ February 22, 2006: Message edited by: levani dvalishvili ]