Question

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?

Answer 1

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.