Java : Sort a list objects based on a list of strings as its property

Question

I am having a scenario where I have a list of objects say List<Vehicle>. Each of the Vehicle object has a list of another object say Group. So the Vehicle object has a property called groups which is a List<Groups>.

Basically, the structure is like this :

public class Vehicle {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Integer wuid;
    
    private String description;

    @ManyToMany
    @JoinTable(
        name="wuclassification"
        , joinColumns={
            @JoinColumn(name="wuid")
            }
        , inverseJoinColumns={
            @JoinColumn(name="classid")
            }
        )
    private List<Group> groups;
}

And the Group class is like :

public class Group {

    @Id
    @GeneratedValue(strategy=GenerationType.IDENTITY)
    private Integer groupId;

    private String name;
}

So now I have a list of Vehicle objects with its list of Group objects. Is there a way of sorting the work unit objects based on the list of group objects with their name attribute?

For example, say I have a list of Vehicle objects which has a Bike, Car and Truck. And then the list of groups with their names for this vehicles are :

Bike : ['short distance','bike taxi']
Car : ['goods career','long distance','short distance','taxi']
Truck : ['goods carrier','long distance']

So sorting this based on the list of groups should look like :

Truck : ['goods carrier','long distance']
Car : ['goods career','long distance','short distance','taxi']
Bike : ['short distance','bike taxi']

I followed this link of sorting List<List> from this link : Java Sort List of Lists

But I am not able apply a similar approach in this scenario. How can I can implement the sorting? Would gladly appreciate your help and thanks in advance.

What is the logic? Why should a truck be sorted before a car or a bike? — Eritrean
– Eritrean, Commented Apr 16, 2021 at 15:18
Yes, I do, but I just wanted to show the structure of the class at a very basic level here. — Aravind S
– Aravind S, Commented Apr 16, 2021 at 15:19
@Eritrean the requirement is such that we want to sort the results in alphabetical order of the group names for a list of vehicles. — Aravind S
– Aravind S, Commented Apr 16, 2021 at 15:22
So two vehicles having the same list of groups but in different order are considered as not equal? — Eritrean
– Eritrean, Commented Apr 16, 2021 at 15:29

Unmitigated · Accepted Answer · 2021-04-16 15:19:40Z

4

You can create a custom Comparator to compare elements at corresponding indexes in the Lists.

vehicles.sort((a,b)->{
   List<Group> aGroups = a.getGroups(), bGroups = b.getGroups();
   for(int i = 0; i < Math.min(aGroups.size(), bGroups.size()); i++){
      int cmp = aGroups.get(i).getName().compareTo(bGroups.get(i).getName());
      if(cmp != 0) return cmp;
   }
   return Integer.compare(aGroups.size(), bGroups.size());
});

answered Apr 16, 2021 at 15:19

Unmitigated

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Eritrean · Accepted Answer · 2021-04-16 16:26:46Z

To add another alternative: I would create a function which takes a list of groups and returns a string by joininig the names of the groups, then i would create a comparator which compares the strings returned by the mentioned function. Something like:

Function<List<Group>,String> extractGroupNames = list -> list.stream()
                                                             .map(Group::getName)
                                                             .collect(Collectors.joining());

Comparator<Vehicle> byGroupNames = (v1,v2) -> extractGroupNames.apply(v1.getGroups())
                                                  .compareTo(extractGroupNames.apply(v2.getGroups()));

Then sort like:

vehicles.sort(byGroupNames);

or if you need the reversed order

 vehicles.sort(byGroupNames.reversed());

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