CODEWITHSUNDEEP
javacollections
james
james

Reputation: 123

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

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

Upvotes: 12

Views: 20988

Answers (7)

Oleksandr Pyrohov
Oleksandr Pyrohov

Reputation: 16226

Alternative to Collectors.groupingBy solution using Map.computeIfAbsent method, which is also available starting from Java 8:

Map<Integer, List<MyObject>> res = new HashMap<>();
for (MyObject obj : myObjects) {
  res.computeIfAbsent(obj.getId(), k -> new ArrayList<>()).add(obj);
}

Upvotes: 0

Damien O&#39;Reilly
Damien O&#39;Reilly

Reputation: 992

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));

Upvotes: 23

Piotr
Piotr

Reputation: 649

Using JDK 1.8:

List<MyObject> objects= new ArrayList();
Map<Integer, List<MyObject>> obejctMap = new HashMap();
objects.stream().map(MyObject::getId).distinct().forEach(id -> obejctMap .put(id,
              objects.stream().filter(object -> id.equals(object.getId())).collect(Collectors.toList())));

Upvotes: 2

Kevin Bourrillion
Kevin Bourrillion

Reputation: 40851

Using Guava:

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

Upvotes: 8

Emil
Emil

Reputation: 13789

ArrayList<MyObject> list=new ArrayList<MyObject>();
//fill Objects..
HashMap<Integer,ArrayList<MyObject>> hash=new HashMap<Integer,ArrayList<MyObject>>();
for(MyObject elem:list)//iterate the list
{
ArrayList<MyObject> tmp=null; //temporary variable 
if((tmp=hash.get(elem.getId()))==null) // check if id already present in map
 {
  tmp=new ArrayList<MyObject>();   
  hash.put(elem.getId(),tmp); //if not put a new array list
 }
names.add(elem); //if present add the name to arraylist
}

Upvotes: 1

Mark Elliot
Mark Elliot

Reputation: 77044

// 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);
}

Upvotes: 12

Ian Henry
Ian Henry

Reputation: 22403

Loop through the elements, check their id values, and place them in a Hashtable with id as the key. That's O(N), which is as efficient as you're going to get.

Upvotes: 2

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