split a java collection into sub collections based on a object property

Question

I have a List of MyObjects ... MyObject{ int id,String name}. Now I want to split the the list into sublists that have identical "id" values, can any one suggest an efficient approach for doing this.

Answer 1

If you are using JDK 1.8, you can use an elegant solution like:

Map<Integer, List<MyObject>> myObjectsPerId =
    myObjects.stream().collect(Collectors.groupingBy(MyObject::getId));

Answer 2

// create the thing to store the sub lists
Map<Integer, List<MyObject>> subs = new HashMap<Integer, List<MyObject>>();

// iterate through your objects
for(MyObject o : list){

    // fetch the list for this object's id
    List<MyObject> temp = subs.get(o.getId());

    if(temp == null){
        // if the list is null we haven't seen an
        // object with this id before, so create 
        // a new list
        temp = new ArrayList<MyObject>();

        // and add it to the map
        subs.put(o.getId(), temp);
    }

    // whether we got the list from the map
    // or made a new one we need to add our
    // object.
    temp.add(o);
}

Answer 3

Using Guava:

ListMultimap<Integer, MyObject> myObjectsById = Multimaps.index(myObjects,
    new Function<MyObject, Integer>() {
      public Integer apply(MyObject myObject) {
        return myObject.id;
      }
    });