6

I know there are several ways to iterate through a hashmap, but what is a good way to modify a hashmap as you go along (other than just creating a new hashmap and getting rid of the old one)

I want something like

for (Map.Entry<String, Integer> entry : wordcounts.entrySet()) {
    String key = entry.getKey();
    Integer value = entry.getValue();
    if(blacklist.contains(key))
        //remove key/value for that key from wordcounts
    if(mappings.contains(key))
     //change key in wordcounts from one string to another based on the key's value in a <string,string> map (mappings)
}

Will it be possibly for me to modify my map while I'm going through it? Do i have to use an iterator?

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4
  • You use the Iterator to remove entries as you come upon them. Changing the key value is going to require creating a new Map and either copying the unchanging values or modifying and inserting the changing ones. The cost in time and memory isn't too large if you remove them from the old Map as you add to the new one. Commented Jul 8, 2013 at 17:20
  • fair enough, i will be doing these particular modifications on a small map so it may be simpler to create a new map Commented Jul 8, 2013 at 17:23
  • @LeeMeador I did look at the post at length, though it specified how to remove elements, not modify keys, but if i simply create a new map i guess it isn't an issue Commented Jul 8, 2013 at 17:23
  • i may end up needing to use the iterator to remove words below a certain length Commented Jul 8, 2013 at 17:28

3 Answers 3

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3

Take advantage of the map: go through your other collections first and do your operations.

for(String blacklisted : blacklist) {
    wordcounts.remove(blacklisted);
}
for(String mapping : mappings) {
    String oldKey =    // get old key
    String value = wordcounts.get(oldKey);
    wordcounts.remove(oldKey);
    wordcounts.put(mapping, value);
}
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3 Comments

the first part is smart, does make things much faster. for the second though, does put modify they key or value? because i want to modify the key
For the 2nd one: You could remove it and then add it back modified.
good point, given that i'm dealing with a small map the cost might not be too bad
1

Use Map.Entry.setValue to change the value of the mapping. If you want to remove the mapping, use setValue(null) use an Iterator.

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5 Comments

On second thought, using setValue(null) might not remove the mapping. I'll test this and get back to you.
yeah looking through javadocs looks like it might just set the value to null
(though i think there is a map.remove(key) option)
@Lemonio Yeah, setValue(null) just sets the mapping to null. If you tried to use Map.remove(key) while iterating, you would get a ConcurrentModificaitonException.
yeah that's what i thought from the other threads but vakh's suggestion for removing seems like a good way to go
0

Don't try to remove items during the iteration or you'll get an exception out of the iterator. Best bet is to either

a) clone the map by iterating/copying into a new map or b) keeping track as you go along of items to be removed and removing them after you're done iterating.

If you're changing keys, same thing applies.. keep track as you go and do the remove/add after you're done iterating.

If you're just changing values, just go for it.

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