How Can I Correctly Sort a List of Custom Objects in Java Using Comparators?

Using Comparators to Sort Objects in Java

Understanding Comparators

In Java, comparators are interfaces that define how objects should be compared for sorting purposes. Implementing a comparator allows you to define custom sorting behavior that deviates from the natural ordering provided by the Comparable interface.

Error Analysis

Your code encounters an exception because you're trying to sort an ArrayList containing People objects without implementing the Comparable or Comparator interface. The compare method in the People class is incorrect as it should compare instances of the People class, not the Integer wrapper objects.

Solution

To resolve the issue, you can implement a Comparator that compares People objects based on their IDs, as follows:

import java.util.Comparator;

public class PeopleComparator implements Comparator<People> {

    @Override
    public int compare(People p1, People p2) {
        return p1.getId() - p2.getId();
    }
}

In your TestPeople class, use the PeopleComparator to sort the peps list:

import java.util.ArrayList;
import java.util.Collections;

public class TestPeople {

    public static void main(String[] args) {
        ArrayList<People> peps = new ArrayList<>();

        peps.add(new People(123, "M", 14.25));
        // ...

        Collections.sort(peps, new PeopleComparator());

        for (People person : peps) {
            System.out.println(person);
        }
    }
}

Alternative Approaches

Java provides an alternative way to implement custom sorting using the Comparator class, which can be assigned directly to the sort method without needing a separate class. For example:

Collections.sort(peps, new Comparator<People>() {

    @Override
    public int compare(People p1, People p2) {
        return p1.getId() - p2.getId();
    }
});

Additionally, you can use the new lambda expression syntax (Java 8 ) to simplify the comparator implementation:

Collections.sort(peps, (p1, p2) -> p1.getId() - p2.getId());

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