如何在Java中按（SQL）执行分组

Question

I have a text file which looks like this:

code    appearance
----------------
j4t8    1
fj89    3
pf6n    1
j4t8    5

And I want to sort by the codes which appear the most. As you can see (and since I want to perform a group by) there are duplicate codes, so using HashMap would be a problem (duplicate keys). Any ideas?

Answer 1

You can use

HashMap map = new HashMap<String, List<Integer>>();

The appearances will be stored in a list associated with every code. Then given a code you just retrieve the list of integers and iterate over it.

Answer 2

You need a Collection of Pair objects. Each pair holds the code and the appearance. You then sort the collection using a Comparator, which only compares the appearance in each Pair object, and disregards the code.

Answer 3

don't know if this is the best solution but you could create a map of a list like this:

Map<String, List<Integer>> map = new HahsMap<String, List<Integer>>();
if(map.contains.(key))
{
  map.get(key).add(new_appearance_value);
}
else
{
  List<Integer> app = new ArrayList<Integer>();
  app.add(new_appearance_value);
  map.put(key, app);
}

Where the map key would be the code and the values of appearance would go into the list.

Note: to determine which code has more appearances just check for the size of the list of each code.

Answer 4

Commons Collections MultiValueMap可用于装饰另一张地图，使其具有多个键值。