// Which is best? Is there a more elegant way to do it in Java 6?
// Solution 1 with nested for loops
for(A a : listOfAs){
  for(B b : listOfBs){
    if (b.getId().equals(a.getId())){
      // do stuff
      break;
    }
  }
}

// Solution 2 with a map as a kind of cache
Map<Id, B> cache = new HashMap<>();
for(B b : listOfBs){
  cache.put(b.getId(), b);
}
for(A a : listOfAs){
  B b = cache.get(a.getId());
  // do stuff
}