Java: Best/elegant way to iterate over 2 lists

Question

Assume I have a List<SomeObject> a.
Now also assume that I have another List<SomeProcessor> b.
Each SomeProcessor uses a for its processing.

Besides:

int idx = 0;   
for(SomeProcessor o:b){  
    o2 = a.get(idx);  
    o.doSomething(o2);  
    idx++;   
}

Is there a more elegant way to handle this?

No.It is just a custom object.How would that help?I mean should there be one? — Jim, Feb 21 '12 at 07:42
Check out this answer: http://stackoverflow.com/a/8104205/1029272 Easy to implement, and works a treat. I got it up and running within the last 5 minutes to try it out. — Deco, Feb 21 '12 at 07:56
possible duplicate of [Best way to iterate over two lists simultaneously?](http://stackoverflow.com/questions/3137944/best-way-to-iterate-over-two-lists-simultaneously) — skaffman, Feb 21 '12 at 09:20

Alex D · Accepted Answer · 2012-02-21T07:47:04.893

7

public interface Processor<T> {
    public void process(T object);
}

And then a helper method:

public static <T> void processAll(Collection<T> items, Collection<? extends Processor<T>> processors) {
    Iterator<T> i = items.iterator();
    Iterator<? extends Processor<T>> p = processors.iterator();
    while(i.hasNext() && p.hasNext())
        p.next().process(i.next()); 
}

You could put that helper method on the class which uses it, if there is only one (and make it private), or put it a utility class which is shared by the entire program.

Of course there are other ways to code processAll; for example, you could use a for loop on one of the collections. But in any case, breaking this low-level code out into a helper method will make your higher-level code cleaner and less "noisy". And if you are doing something similar in multiple parts of the program, they can share the helper method.

edited Feb 21 '12 at 07:47

answered Feb 21 '12 at 07:34

Alex D

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I don't want to process it like that.I need to process each object once by 1 processor – Jim Feb 21 '12 at 07:35
@narek.gevorgyan:Yes they are always same size – Jim Feb 21 '12 at 07:41
@Alex D:So you suggest essentially to use the 2 iterators of the `lists` simultaneously, right? – Jim Feb 21 '12 at 07:50
@Jim, you could use a `for` loop and an iterator, or 2 iterators. In either case, the biggest thing you can do to make your code cleaner, is to move this detailed, low-level code into a helper method with a meaningful name. – Alex D Feb 21 '12 at 08:23
It's unfortunate that Java doesn't have closures... you could write a very nice, general-purpose method for "iterating over two lists and doing something to each pair" if it did. – Alex D Feb 21 '12 at 08:26

score 1 · Answer 2 · answered Feb 21 '12 at 07:41

1

If you can generate a Map<SomeObject, SomeProcessor> instead of two lists, then it'll be elegant. This may not apply, but I'll just give it a shot.

answered Feb 21 '12 at 07:41

Bob Wang

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score 1 · Answer 3 · answered Feb 21 '12 at 09:15

1

Maybe you can try with this simpler way :

int idx = 0;
while(idx<a.size() && idx<b.size()){   
    b.get(idx).doSomething(a.get(idx++));
}

answered Feb 21 '12 at 09:15

alain.janinm

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Java: Best/elegant way to iterate over 2 lists

3 Answers3