Question

I have a Java collection:

Collection<CustomObject> list = new ArrayList<CustomObject>();

CustomObject has an id field now before display list I want to sort this collection by that id.

Is there any way I could that do that?

Answer 1

Use a Comparator:

List<CustomObject> list = new ArrayList<CustomObject>();
Comparator<CustomObject> comparator = new Comparator<CustomObject>() {
    @Override
    public int compare(CustomObject left, CustomObject right) {
        return left.getId() - right.getId(); // use your logic
    }
};

Collections.sort(list, comparator); // use the comparator as much as u want
System.out.println(list);

Additionally, if CustomObjectimplements Comparable, then just use Collections.sort(list)

With JDK 8 the syntax is much simpler.

List<CustomObject> list = getCustomObjectList();
Collections.sort(list, (left, right) -> left.getId() - right.getId());
System.out.println(list);

Much simplier

List<CustomObject> list = getCustomObjectList();
list.sort((left, right) -> left.getId() - right.getId());
System.out.println(list);

Simplest

List<CustomObject> list = getCustomObjectList();
list.sort(Comparator.comparing(CustomObject::getId));
System.out.println(list);

Obviously the initial code can be used for JDK 8 too.