Which one is better in calling a function: two times or storing the result in a variable?

Question

I have got this doubt many times, but didn't figure it out the correct soltion. This time I want clear it off. I have situation like

1. 
String sNumber="ksadfl.jksadlf";
if(sNumber.lastIndexOf('.')>0)
   //do something
...
...
if(sNumber.lastIndexOf('.')>1)
 //do something
...

2.
int index = sNumber.lastIndexOf('.');
if(index>0)
//do something
...
...
if(index>1)
//do something
...

what are the trade offs between first way and second way? which one is better in storing the result in a variable or calling the function two times?

Answer 1

In this example, the 2nd form is better (in most plausible situations¹) from a performance perspective, and (IMO) more readable.

In general, there are a couple of trade-offs to consider:

• Is readability more important than efficiency?
• How much of a "performance hit" is it to call the method twice versus once?

Also, you also need to consider the cases where the method may have side-effects, and whether it may give a different answer on two successive calls. In those cases, calling the method twice is semantically different to calling it once and storing the result in a temporary variable.

---

^{1 - The index variable makes the stack frame 1 word larger. Normally this doesn't matter, but if the code is in a recursive method that gets called in a deeply recursive fashion, that 1 extra word multiplied by a number of nested calls could result in a StackOverflowError.}

Answer 2

Since the question only talks about 2 lean lastIndexOf calls, there's actually little difference between the two. If it's a remote call via a proxy or something that does a database hit, then storing the method result into a variable would see you some improvement.

The good thing about calling the method again is that you'll always get the latest value of the index, in case it was manipulated somewhere in the earlier blocks. On most cases, #2 will be the right choice, but for your exact example above, it's #1.