How to sort an ArrayList using a Map key as list object and value as the order?

Question

I need to keep the order of my list of objects as it can be changed and reordered anytime in my webapp.

In my model I have a array list of objects example

objectList = [object1, object2, object3, object4]

and I save the order of each object belonging to this instance to a Map example: order

{
   "object1":4,
   "object2":2,
   "object3":1,
   "object4":3
}

Therefore i want to sort my array according to its value in the map retrieved by the objects Id as the key:

Expected result:

[object3, object2, object4, object1]

How would I go about doing this?

Answer 1

Assuming your objects have a getter for the id getId() , then you can make use of the Comparator.comparing() helper method, which creates a comparator for the provided lambda:

objectList.sort(Comparator.comparing(o -> yourMap.get(o.getId()));

This effectively the same as writing your own Comparator or writing your own compare method.

Answer 2

Something that may look overkilling but it could be more natural to define a new class

class IndexedElement implments Comparable<Integer> {
    private final int index;
    private final YourObject element;

    public IndexedElement(int index, YourObject element) {
       this.index = index;
       this.element = element;
    }

    @Override
    public int compareTo(Integer o) {
       return this.getIndex().compareTo(o.getIndex()) ;
    }
}

storing objects like that in a TreeSet<IndexedElement> you can automatically achieve the sorting and retrieving (TreeSet.ceiling(new IndexedElement(yourIndex, null)), just remember to Override the equals/hashCode in IndexedElement)