Logical Tests in Java

[Spab23]

Can anyone here confirm that Java doesn't necessarily perform each conditional in an "if" statement, if the first few parts of it will determine its outcome?

For example, if I have three conditionals ANDed together and the first one is false, then are the other two tested?

The reason I ask is I have a multiple OR conditional and one of the conditions might result in a NullPointerException if it is executed. I do have a test for nulls earlier in the expression.

My assumption is that:

if (action == null || (action.indexOf("string") > -1))

...will never give a NullPointerException, but

if ((action.indexOf("string") > -1) || action == null)

...will give a NullPointerException if "action" is null.

Am I right in assuming this?

Thanks for your help!

 

[jstreich]

In theory yes... Inless action.indexOf() is overridden and uses another pointer that happens to be null.

short and sweet:

if (A && B) // if A is true check B
            //    if B is true do this stuff
            //    else do that stuff
            // if A is false, do that stuff (without checking B)
{
  //this stuff
}
else
{
  //that stuff
}

[plug=shameless]

http://game-master.us/phpx-3.4.0/

[/plug]

 

[jstreich]

In theory yes... Inless action.indexOf() is overridden and uses another pointer that happens to be null.

short and sweet:

if (A && B) // if A is true check B
            //    if B is true do this stuff
            //    else do that stuff
            // if A is false, do that stuff (without checking B)
{
  //this stuff
}
else
{
  //that stuff
}

With OR:

if (A && B) // if A is true do this stuff (without checking B)
            // if A is false, check B
            //     if B is true do this stuff
            //     else do that stuff
{
  //this stuff
}
else
{
  //that stuff
}

[plug=shameless]

http://game-master.us/phpx-3.4.0/

[/plug]

 

[sedj]

Spab23, yes your assumption is correct - I believe its called "short circuit conditional statements" or something like that.

jstreich,

I think you need to be careful what types you perform bitwise operations on

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[Diancecht]

Another way to call it it's a lazy strategy, i've been working with that for a while.

The way to use the eager evaluation in the same case could be the use of bit to bit & operation.

Cheers,

Dian

 

[Spab23]

Thanks for all of your confirmations!

 

[jstreich]

jstreich,

I think you need to be careful what types you perform bitwise operations on

That isn't bitwise, it's logical... but the second example should have been || not &&.

[plug=shameless]

http://game-master.us/phpx-3.4.0/

[/plug]

 

[sedj]

jstreich,

Yes you're quite right, my bad

Here's a link for anyone else getting their bitwise operators confused with their conditionals :

http://java.sun.com/docs/books/tutorial/java/nutsandbolts/bitwise.html

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http://www.primrose.org.uk