Question

In my code,

for(City city : country.getCities()){
   // do some operations
}

Using country.getCities() is costly? Will JVM maintain the stacktrace for every call..?

List<City> cityList = country.getCities();
for(City city : cityList){
   // do some operations
}

What is the best way to use?

Answer 1

No, this loop:

for(City city : country.getCities())

will only call country.getCities() once, and then iterate over it. It doesn’t call it for each iteration of the loop. In your case it’s equivalent to:

for (Iterator<City> iterator = country.getCities().iterator();
     iterator.hasNext(); ) {
    City city = iterator.next();
    // do some operations
}

There’s no benefit in rewriting it as per your second snippet.

See section 14.14.2 of the JLS for more details.