How to Sort a Java Collection of Custom Objects by ID

How can I sort a Java collection of custom objects based on their ID field?

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

Sorting a Java collection of custom objects can be effortlessly achieved by implementing either the `Comparable` interface or using a `Comparator`. This process involves defining how the sorting logic should be applied based on the specific field, in this case, the ID field of the `CustomObject`.

import java.util.*;

class CustomObject implements Comparable<CustomObject> {
    private int id;
    // constructor
    public CustomObject(int id) {
        this.id = id;
    }
    public int getId() {
        return id;
    }
    @Override
    public int compareTo(CustomObject other) {
        return Integer.compare(this.id, other.id);
    }
}

public class SortExample {
    public static void main(String[] args) {
        Collection<CustomObject> list = new ArrayList<>();
        list.add(new CustomObject(3));
        list.add(new CustomObject(1));
        list.add(new CustomObject(2));

        // Sorting using Comparable
        Collections.sort((List<CustomObject>) list);

        // Display sorted list
        for (CustomObject obj : list) {
            System.out.println(obj.getId());
        }
    }
}

Causes

• The collection is not sorted by default when created.
• Custom objects require a comparator or a comparable interface for sorting.

Solutions

• Implement the `Comparable` interface in the `CustomObject` class to define the natural order.
• Use a `Comparator` to sort the collection explicitly without modifying the `CustomObject` class.

Mistake: Not implementing the Comparable interface in the CustomObject class.

Solution: Implement Comparable in your CustomObject class and define the compareTo method.

Mistake: Using a custom comparator without casting the collection type.

Solution: Ensure that the collection is cast to List when using Collections.sort.