If you are happy to use external libraries, then Guava's Ordering.explicit generally lets you provide a list of values in the desired order and make a Comparator out of that.

Otherwise, if you have a small list of characters in order, you could probably get away with

Comparator<Character> cmp = Comparator.comparingInt("BACDF..."::indexOf);

If you want to compare strings lexicographically with this order...it's a little harder and you'll probably have to write it more explicitly.

Comparator<String> cmp = (a, b) -> {
  for (int i = 0; i < a.length() && i < b.length(); i++) {
    if (a.charAt(i) != b.charAt(i)) {
      String ordering = "BACDF...";
      return ordering.indexOf(a.charAt(i)) - ordering.indexOf(b.charAt(i));
    }
  }
  return a.length() - b.length();
}

Comparator#compare

What is the goal of the Comparator#compare method?

Compares its two arguments for order. Returns a negative integer, zero, or a positive integer as the first argument is less than, equal to, or greater than the second.

Given your formula of ( numOfWins2 - numOfWins1 ):

• For the case of numOfWins1 = 30 and numOfWins2 = 20, ( 20 - 30 ) is -10, a negative number. So the first should be sorted after the second.
• For the case of numOfWins1 = 20 and numOfWins2 = 30, ( 30 - 20 ) is 10, a positive number. So the first should be sorted above the second.
• For the case of numOfWins1 = 25 and numOfWins2 = 25, ( 25 - 25 ) is 0, zero. So the two items are tied for sorting.

Complete example

Here is some example code. You can tweak the constructors of Alice and Carol to see the sorting in action.

This code uses the new compact record definition of a class, in Java 16+. But using record is beside the point of this Answer. You can just as well use a conventional class definition.

Copypackage work.basil.example;

import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

public class Comparing
{
    public static void main ( String[] args )
    {
        Comparing app = new Comparing();
        app.demo();
    }

    private void demo ( )
    {
        List < FootballClub > clubs =
                List.of(
                        new FootballClub( "Alice" , 25 ) ,
                        new FootballClub( "Bob" , 7 ),
                        new FootballClub( "Carol" , 25 )
                );

        Comparator < FootballClub > sortByWinsDescending = new Comparator < FootballClub >()
        {

            public int compare ( FootballClub footballClub1 , FootballClub footballClub2 )
            {

                int numOfWins1 = footballClub1.wins();
                int numOfWins2 = footballClub2.wins();

                /* Returns the number of goals scored in  descending order */
                return numOfWins2 - numOfWins1;
            }
        };

        List < FootballClub > sorted = new ArrayList <>( clubs );
        sorted.sort( sortByWinsDescending );

        System.out.println( "clubs = " + clubs );
        System.out.println( "sorted = " + sorted );
    }

    record FootballClub(String name , Integer wins)
    {
    }
}

Not recommended

This code return ( numOfWins2 - numOfWins1 ) ; is the kind of “clever” programming that should be avoided. This code is confusing and troublesome. I even confused myself while writing this Answer.

Better would be letting objects compare themselves. In my example code, we use the class Integer. Well, Integer objects already know how to compare themselves. So call Integer#compareTo.

Copyreturn footballClub1.wins().compareTo( footballClub2.wins() ) ;

Or in the case of int primitives as seen in your code, call the static utility method Integer.compare( int x , int y ):

Copy        int numOfWins1 = footballClub1.getNumOfWins();
        int numOfWins2 = footballClub2.getNumOfWins();

        /* Returns the number of goals scored in  descending order */
        return Integer.compare ( numOfWins1 , numOfWins2 ) ;

By the way, we can look at the OpenJDK source code to see how this Integer.compare( int x , int y ) method is implemented. They use a nested pair of ternary operators. A ?: ternary works this way: If the predicate test is true, use item following the ?, else if false use the item following the :.

Copy// Copyright (c) 1994, 2020, Oracle and/or its affiliates. All rights reserved.
// Excerpt from https://github.com/openjdk/jdk/blob/master/src/java.base/share/classes/java/lang/Integer.java
// See licensing terms in that original file.
return (x < y) ? -1 : ((x == y) ? 0 : 1);

Make a habit of writing code this is as plain and simple as possible. Humans spend much more time reading code than writing code. On a job, when you have a bug in one of your report-generating methods, and you are debugging in the middle of the night, you do not want to be trying to figure out if return numOfWins2 - numOfWins1 is for ascending or descending.

As a bonus, plain and simple code that is readable by humans is often more likely to be an opportunity for the Java compiler and runtime to optimize for performance.

For fun, add this line before your return statement to see the individual comparisons.

CopySystem.out.println( "numOfWins1: " + numOfWins1 + " | numOfWins2: " + numOfWins2 + " | numOfWins2 - numOfWins1: " + ( numOfWins2 - numOfWins1 ) );

Ascending order

Tip: If you wanted ascending order rather than descending, just multiply the result of that call Integer#compareTo or Integer.compare by negative 1 (-1). Doing so flips the sign of the result, negative becomes positive, positive becomes negative, and zero remains zero.

Copyreturn ( -1 * Integer.compare ( numOfWins1 , numOfWins2 ) ) ;  // Ascending rather than descending sort order.

Method reference

As Ole V.V. commented, if you are comfortable with lambdas and method references, you can collapse your entire Comparator implementation to a single-line solution for ascending sort by calling Comparator.comparingInt.

CopyComparator < FootballClub > sortByWinsDescending = Comparator.comparingInt( FootballClub :: wins );

Our example above becomes quite shorter.

Copypackage work.basil.example;


import java.util.ArrayList;
import java.util.Comparator;
import java.util.List;

public class Comparing2
{
    public static void main ( String[] args )
    {
        Comparing2 app = new Comparing2();
        app.demo();
    }

    private void demo ( )
    {
        List < FootballClub > clubs =
                List.of(
                        new FootballClub( "Alice" , 20 ) ,
                        new FootballClub( "Bob" , 7 ) ,
                        new FootballClub( "Carol" , 30 )
                );

        Comparator < FootballClub > sortByWinsDescending = Comparator.comparingInt( FootballClub :: wins );

        List < FootballClub > sorted = new ArrayList <>( clubs );
        sorted.sort( sortByWinsDescending );

        System.out.println( "clubs = " + clubs );
        System.out.println( "sorted = " + sorted );
    }

    record FootballClub(String name , Integer wins)
    {
    }
}

There are a couple of awkward things with your example class:

• it's called People while it has a price and info (more something for objects, not people);
• when naming a class as a plural of something, it suggests it is an abstraction of more than one thing.

Anyway, here's a demo of how to use a Comparator<T>:

public class ComparatorDemo {

    public static void main(String[] args) {
        List<Person> people = Arrays.asList(
                new Person("Joe", 24),
                new Person("Pete", 18),
                new Person("Chris", 21)
        );
        Collections.sort(people, new LexicographicComparator());
        System.out.println(people);
        Collections.sort(people, new AgeComparator());
        System.out.println(people);
    }
}

class LexicographicComparator implements Comparator<Person> {
    @Override
    public int compare(Person a, Person b) {
        return a.name.compareToIgnoreCase(b.name);
    }
}

class AgeComparator implements Comparator<Person> {
    @Override
    public int compare(Person a, Person b) {
        return a.age < b.age ? -1 : a.age == b.age ? 0 : 1;
    }
}

class Person {

    String name;
    int age;

    Person(String n, int a) {
        name = n;
        age = a;
    }

    @Override
    public String toString() {
        return String.format("{name=%s, age=%d}", name, age);
    }
}

EDIT

And an equivalent Java 8 demo would look like this:

public class ComparatorDemo {

    public static void main(String[] args) {
        List<Person> people = Arrays.asList(
                new Person("Joe", 24),
                new Person("Pete", 18),
                new Person("Chris", 21)
        );
        Collections.sort(people, (a, b) -> a.name.compareToIgnoreCase(b.name));
        System.out.println(people);
        Collections.sort(people, (a, b) -> a.age < b.age ? -1 : a.age == b.age ? 0 : 1);
        System.out.println(people);
    }
}

EDIT: First of all, a couple of things:

1. The @Override annotation should not be mandatory. If Eclipse wants you to put it on, don't worry.
2. Don't write your own Comparator interface. Delete that definition NAO and use the one provided by Java. Reinventing the wheel probably violates the Unspoken Code of Computer Programming in about 15 different ways. Use import java.util.Comparator; at the very top of your code (before the public class stuff) to a) use the version given by Java and b) make your code compatible with pretty much everything else that exists in the world.

The Comparator interface is not used to create a class that can put itself in order. This is the Comparable interface.

Both are similar, so I will describe both here.

java.util.Comparator

The Comparator interface, as you already know, has one method: compare. Comparator is generic (uses the angle brackets <>) and takes the type it will compare inside the <>. The thing is that Comparators are used to compare items of other classes. For example, I could create a Comparator for java.lang.Integers that returns the opposite of the "natural order" (how Integers are usually ordered).

Comparators are used mostly to give other objects a way to sort their parameters when they are not in natural order. For example, the java.util.TreeSet class takes a Comparator for its sorting capability.

java.lang.Comparable

Comparable's purpose is to say that an object can be compared. It is also generic and takes the type that it can be compared to. For example, a Comparable<String> can be compared to Strings.

Comparable has one method: compareTo(). Unlike Comparator's compare(), compareTo takes one parameter. It works like compare, except it uses the invoking object as one parameter. So, comparableA.compareTo(comparableB) is the same as comparator.compare(comparableA, comparableB).

Comparable mostly establishes the natural order for objects, and is the default way to compare objects. Comparator's role is to override this natural order when one has different needs for data comparison or sorting.

ArrayList Sorting

To sort a List, you could use the method already available: scroll down to sort on the java.util.Collections class. One method takes a Comparator, the other does not. sort is static; use Collections.sort(...), not Collections c = new Collections(); c.sort(...). (Collections doesn't even have a constructor anyway, so meh.)