Odd example to use. It's always annoying to re-type the same thing over and over. But I recently went through a mountain of my Java code and replaced:

for (Object obj : list)

with

int size=list.size(); for (int index=0;index<size;index++)

I'm definitely very long suffering! It took a few hours but what you get with abstractions is a loss of control over implementation. When you use the tighter loop, the compiler turns everything (roughly) into this:

Iterator it=list.iterator(); //Which calls new ListItr(); for (it.hasNext()) { Object next=it.next();

The abstraction has to do that because it doesn't know whether the list (really, the java.util.Collection) is a LinkedList or an ArrayList or a Set. If your loop itself is in a tight loop, it isn't hard to call it millions and millions of times during program execution--which makes millions and millions of unnecessary allocations and method calls. There is a loss of performance which, at the end of the day, is more important than how much you enjoy your job.

Of course, other languages would allow you to specify an implementation for a given abstraction.