java编译器可以优化循环以尽早返回吗？

Question

I'm working with an external library that decided to handle collections on its own. Not working with it or updating is outside my control. To work with elements of this third party "collection" it only returns iterators.

A question came up during a code review about having multiple returns in the code to gain performance. We all agree (within the team) the code is more readable with a single return, but some are worried about optimizations.

I'm aware premature optimization is bad. That is a topic for another day.

I believe the JIT compiler can handle this and skip the unneeded iterations, but could not find any info to back this up. Is JIT capable of such a thing?

A code sample of the issue at hand:

public void boolean contains(MyThings things, String valueToFind) {
    Iterator<Thing> thingIterator = things.iterator();
    boolean valueFound = false;
    while(thingIterator.hasNext()) {
        Thing thing = thingIterator.next();
        if (valueToFind.equals(thing.getValue())) {
            valueFound = true;
        }
    }
    return valueFound;
}

VS

public void boolean contains(MyThings things, String valueToFind) {
    Iterator<Thing> thingIterator = things.iterator();
    while(thingIterator.hasNext()) {
        Thing thing = thingIterator.next();
        if (valueToFind.equals(thing.getValue())) {
            return true;
        }
    }
    return false;
}

Answer 1

We all agree the code is more readable with a single return.

Not really. This is just old school structured programming when functions were typically not kept small and the paradigms of keeping values immutable weren't popular yet.

Although subject to debate, there is nothing wrong with having very small methods (a handful of lines of code), which return at different points. For example, in recursive methods, you typically have at least one base case which returns immediately, and another one which returns the value returned by the recursive call.

Often you will find that creating an extra result variable, just to hold the return value, and then making sure no other part of the function overwrites the result, when you already know you can just return, just creates noise which makes it less readable not more. The reader has to deal with cognitive overload to see the result is not modified further down. During debugging this increases the pain even more.

I don't think your example is premature optimisation. It is a logical and critical part of your search algorithm. That is why you can break from loops, or in your case, just return the value. I don't think the JIT could realise that easily it should break out the loop. It doesn't know if you want to change the variable back to false if you find something else in the collection. (I don't think it is that smart to realise that valueFound doesn't change back to false ).

In my opinion, your second example is not only more readable (the valueFound variable is just extra noise) but also faster, because it just returns when it does its job. The first example would be as fast if you put a break after setting valueFound = true . If you don't do this, and you have a million items to check, and the item you need is the first, you will be comparing all the others just for nothing.

Answer 2

Java compiler cannot do an optimization like that, because doing so in a general case would change the logic of the program.

Specifically, adding an early return would change the number of invocations of thingIterator.hasNext() , because your first code block continues iterating the collection to the end.

Java could potentially replace a break with an early return, but that would have any effect on the timing of the program.