Java Hashmap Iteration: Look at two values at once?

Question

I have this loop:

 for (Map.Entry<Integer, String> entry: collection.entrySet()) {

 }

I'd like to compare entry and the next one after that. How would I do it?

If you're using HashMap, you need to bear in mind that it doesn't preserve insert-order. If you need the insert-order then you need to use LinkedHashMap. — aberrant80
– aberrant80, Commented Sep 17, 2009 at 1:20
@Tom: sorted maps don't preserve input order per se - they preserve sort order, which is often a different thing, no? — CPerkins
– CPerkins, Commented Sep 17, 2009 at 12:30

cletus · Accepted Answer · 2009-09-17 01:34:21Z

6

You do it on the previous one.

Map.Entry<Integer, String> previous = null;
for (Map.Entry<Integer, String> entry : collection.entrySet()) {
  if (previous != null) {
    // compare entry to rpevious
  }
  previous = entry;
}

edited Sep 17, 2009 at 1:34

answered Sep 17, 2009 at 1:16

cletus

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Comments

joeslice · Accepted Answer · 2009-09-17 01:15:54Z

4

As you loop over you could set a variable called oldEntry for example (initialized to null) and use that in the way you want.

 Map.Entry<Integer,String> oldEntry = null;
 for (Map.Entry<Integer, String> entry: collection.entrySet()) {
   if (oldEntry != null) doStuff(oldEntry, newEntry);
   oldEntry = entry;
 }

answered Sep 17, 2009 at 1:15

joeslice

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2 Comments

Tom Hawtin - tackline Over a year ago

I wonder if that is forcing a use of the posh for loop, and that we should drop to using iterators instead.

Nick Heiner Over a year ago

when would one use an iterator instead of a loop?

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