重组名单<MyType>列出<List<MyType> &gt;

Question

I have an ArrayList which looks like this:

final List<MyType> myList = new ArrayList<>();

MyType looks like this:

public class MyType {
   ...
   private DateTime startTime;
   private DateTime endTime;
   private DateTime dateOfContainer; // I use @JSONSerializer here to only serialize date
   ....
}

The JSON looks like this:

[
 {
...
"endTime": "16:00",
"startTime": "13:00",
"dateOfContainer": "27.11.2015"
},
{
...
"endTime": "12:00",
"startTime": "08:00",
"dateOfContainer": "27.11.2015"
},
{
... 
"endTime": "16:00",
"startTime": "13:00",
"dateOfContainer": "25.11.2015"
}, 
{
...
"endTime": "19:00",
"startTime": "18:00",
"dateOfContainer": "21.11.2015"
},
...

Now I would a list like this:

List<List<MyType>>

where MyType's with the same dateOfContainer are together, eg the first two.

Is there a simple way with Java-8 to do this? Thanks a lot!

Answer 1

myList.stream().collect (Collectors.groupingBy(MyType::getDateOfContainer)) will give you a Map<DateTime,List<MyType>> where your objects are grouped by the date property.

To get a List<List<MyType>> from this Map , you can create an ArrayList and initialize it with the values of that Map :

List<List<MyType>> list = new ArrayList<> (
    myList.stream().collect (Collectors.groupingBy(MyType::getDateOfContainer)).values());

Note that in order for groupingBy to function as expected, the DateTime class must override equals . I'm not sure if that's the case here.