Given the class:
public class CategoryValuePair
{
String category;
String value;
}
And a method:
public Map<String,List<String>> convert(CategoryValuePair[] values);
Given that in values we can receive many entries with the same category, I want to convert these into a Map grouped on category.
Is there a quick / efficient way to perform this conversion?
As far as I know there is not easier way than iterating on values, and then putting the values in the map (like some predefined method).
Map<String, List<String>> map = new HashMap<String, List<String>>();
if (values != null) {
for (CategoryValuePair cvp : values) {
List<String> vals = map.get(cvp.category);
if (vals == null) {
vals = new ArrayList<String>();
map.put(cvp.category, vals);
}
vals.add(cvp.value);
}
}
I changed the map values from String[] to List<String> since it seems easier to me to use that so you don't have to hassle with array resizing.
if (vals == null) { vals = new ArrayList<String>(); map.put(cvp.category, vals); }
To make it in fewer lines of code, use Google Collections:
public Map<String, Collection<String>> convert(CategoryValuePair[] values) {
Multimap<String, String> mmap = ArrayListMultimap.create();
for (CategoryValuePair value : values) {
mmap.put(value.category, value.value);
}
return mmap.asMap();
}
If you don't want to allow duplicate values, replace ArrayListMultimap with HashMultimap.
With lambdaj you just need one line of code to achieve that result as it follows:
group(values, by(on(CategoryValuePair.class).getCategory()));
Just for the sake of implementation... The method returns Map and also checks for duplicates in the arrays... though performance wise its heavy ...
public Map<String,String[]> convert(CategoryValuePair[] values)
{
Map<String, String[]> map = new HashMap<String, String[]>();
for (int i = 0; i < values.length; i++) {
if(map.containsKey(values[i].category)){
Set<String> set = new HashSet<String>(Arrays.asList(map.get(values[i].category)));
set.add(values[i].value);
map.put(values[i].category, set.toArray(new String[set.size()]));
}else {
map.put(values[i].category, new String[]{values[i].value});
}
}
return map;
}
String[]instead ofList<String>in your map? I can't think of very many reasons why the array would be better, and with regards to your question, the way I'm thinking, the list would make it a bit easier.