Apex variable declaration and for loop performance

August 27, 2016

I found this a few times in a different projects and now I’d share this common mistake with you.

String someVariable = '';

// 50 lines of code between declaration and first use

for (Account acc: accounts) {
    someVariable = acc.Some_Field__c;
    // rest of the loop
}

Actually I really hate to find something like this in my codebase. What’s wrong with this code? Just one moment – the string variable which is on scope of for loop is declared outside of the for loop. That was an optimisation for some old compilers for C/C++ a many years ago. But modern compilers can optimise your code much better that you. So, just let them do that.

for (Account acc: accounts) {
    String someVariable = acc.Some_Field__c;
    // rest of the loop
}

This code will be much effective that the first one. I did a few tests on Apex.

String s1;
Datetime start = System.now();
for (Integer i = 0; i < 100000; i++) {
    s1 = String.valueOf(i);
}
System.debug(System.now().getTime() – start.getTime());

with results:

and run

Datetime start = System.now();
for (Integer i = 0; i &amp;lt; 100000; i++) {
    String s1 = String.valueOf(i);
}
System.debug(System.now().getTime() – start.getTime());

with the following results:

Runs through 300.000 iterations:

*Results was sorted for better readability

As you can see the code which use a declaration of variables inside the loop works faster or with the same time of execution. But such code is much more supportable and readable.