Nifty way to iterate over parallel arrays in Java using foreach

Question

I've inherited a bunch of code that makes extensive use of parallel arrays to store key/value pairs. It actually made sense to do it this way, but it's sort of awkward to write loops that iterate over these values. I really like the new Java foreach construct, but it does not seem like there is a way to iterate over parallel lists using this.

With a normal for loop, I can do this easily:

for (int i = 0; i < list1.length; ++i) {     doStuff(list1[i]);     doStuff(list2[i]); } 

But in my opinion this is not semantically pure, since we are not checking the bounds of list2 during iteration. Is there some clever syntax similar to the for-each that I can use with parallel lists?

Answer 1

I would use a Map myself. But taking you at your word that a pair of arrays makes sense in your case, how about a utility method that takes your two arrays and returns an Iterable wrapper?

Conceptually:

for (Pair<K,V> p : wrap(list1, list2)) {     doStuff(p.getKey());     doStuff(p.getValue()); } 

The Iterable<Pair<K,V>> wrapper would hide the bounds checking.

Answer 2

From the official Oracle page on the enhanced for loop:

Finally, it is not usable for loops that must iterate over multiple collections in parallel. These shortcomings were known by the designers, who made a conscious decision to go with a clean, simple construct that would cover the great majority of cases.

Basically, you're best off using the normal for loop.

If you're using these pairs of arrays to simulate a Map, you could always write a class that implements the Map interface with the two arrays; this could let you abstract away much of the looping.

Without looking at your code, I cannot tell you whether this option is the best way forward, but it is something you could consider.