More efficient way to use "Expression" in enhanced "for" cycles

Question

Among the following forms, which the most efficient?

...
//assumindo que o método pegaListaPessoas() pesquisa na base de dados todos as pessoas
for (Pessoa pAux : pegaListaPessoas())
{
    //executo alguma coisa
}

or:

...
//assumindo que o método pegaListaPessoas() pesquisa na base de dados todos as pessoas
List<Pessoa> pessoas = pegaListaPessoas();
for (Pessoa pAux : pessoas)
{
    //executo alguma coisa
}

Answer 1

Always the best way for you to figure this out is to measure. What may seem one thing in theory can be shown another in a specific real case. In fact this can be considered premature optimization.

If you ask random people on the internet who don’t know an implementation, the exact conditions of your application and the performance requirements, don’t expect something very precise.

I cannot answer with absolute certainty because I do not know the implementation of pegaListaPessoas(), I don’t know the implementation of what it returns until because I don’t even know what it returns. But it could kick with good chance of hitting that makes no difference.

What does this method return? A Iterable that scans a cached list of database content? And the implementation of this interface is a List? Or is it a proper implementation where the next() will consult the database? If there is a performance problem, it may be in a different place than imagined.

Roughly the implementations must be compiled to the equivalent of

for (Iterator<Pessoa> i = pegaListaPessoas().iterator(); i.hasNext(); i.next()) {

and

List<Pessoa> pessoas = pegaListaPessoas();
for (Iterator<Pessoa> i = pessoas.iterator(); i.hasNext(); i.next()) {

In the first case, he calls the pegaListaPessoas() the first time, gets a iterator of the list created by the method, all this before the first ; and with each step it will catch the next item of iteration with the next() (after the second ;) and check if there are any other items with the hasNext(). Only the last two are executed at each step. Initialization is performed only once.

The only difference in the second case is that the list is generated before starting the loop.

In fact if you visualize it as a while becomes more obvious:

Iterator<Pessoa> i = pegaListaPessoas().iterator();
while (i.hasNext()){
     Pessoa p = i.next();
}

and

List<Pessoa> pessoas = pegaListaPessoas();
Iterator<Pessoa> i = pessoas.iterator();
while(i.hasNext()){
     Pessoa p = i.next();
}

I put in the Github for future reference.

You understand that the secret is iterator? And as far as I know one loop for in can only run on top of collections implementing the interface Iterable (perhaps with the exception of a array, String and other types that the language is aware of and use the index directly).