基于类别拆分arraylist的最佳方法

Question

I have an ArrayList, where DataType is a class:

class DataType {
  String id;
  String dType;
  String description;
  // setters and getters follow
}

With the given ArrayList<Datatype> , I want to create sub lists, such that every sublist has the same value for dType . ie all DataType Objects having same value for dType should come in one sublist and so on.

Further, all sublists will be added to an ArrayList<ArrayList<Datatype>> as created.

Could someone please suggest the most appropriate approach for this.

Answer 1

The way I'd do this is to create a Map:

Map<String, List<DataType>> data;

Then loop through your original array, for each value insert them into the Map using dType as the key.

for (DataType dt: toSort) {
    List<DataType> list = data.get(dt.dType);
    if (list == null) {
        list = new ArrayList<>();
        data.put(dt.dType, list);
    }
    list.add(dt);
}

Now you have them all sorted nicely.

Answer 2

I would create a hash map: HashMap<DataType, List<Data>> . Make sure you override equals() and hashCode() for your DataType class.

Answer 3

One option is to create a HashMap<String, List<DataType>> by manually iterating over the list. In addition to that, if you can use a 3rd party library, then consider using Guava's Multimaps.index method:

Function<DataType, String> groupFunction = new Function<DataType, String>() {
        @Override
        public String apply(DataType dataType) {
            return dataType.getDType();
        }
};

ImmutableListMultimap<String, DataType> grouped = Multimaps.index(persons, groupFunction);

System.out.println(grouped);