Writing a comparator that sorts based on multiple fields

EDIT: I have misunderstood the problem. This solution sorts on Field 1 followed by Field 2 followed by Field 3. This is not what OP is looking for.

I suspect that you have a Java object that contains these three fields. I'll assume that they can be accessed via getters. I will also assume that your objects are stored in a List of some kind.

You have not specified which version of Java you are using so I'll go with a Java 8 solution. This could be used with earlier version of Java but it would be more verbose.

    List<MyObject> myObjects = Arrays.asList(new MyObject(1, 2, 3),
                                             new MyObject(0, 1, 2),
                                             new MyObject(1, 1, 1),
                                             new MyObject(1, 1, 0),
                                             new MyObject(1, 2, 1));
    List<MyObject> sortedList = myObjects.stream()
                                         .sorted(Comparator.comparing(MyObject::getField1)
                                                           .thenComparing(MyObject::getField2)
                                                           .thenComparing(MyObject::getField3))
                                         .collect(Collectors.toList());

    System.out.println(sortedList);

This program outputs

[0-1-2, 1-1-0, 1-1-1, 1-2-1, 1-2-3]

This solution uses Java 8 Stream API with the method sorted() which allows you to easily sort a Stream. The sort is achieved by using a Comparator which is a simple class that can determine which of two instances is "larger" than another.

Creating a Comparator that compares instances based on a field is very simple thanks to the Comparator.comparing() method. Comparing based on multiple fields is a simple process of chaining the returned Comparator using .thenComparing().

Java 8 method references are used to refer to field's getters.

Full code:

import java.util.Arrays;
import java.util.Comparator;
import java.util.List;
import java.util.stream.Collectors;

public class MyObject
{
    private int field1;
    private int field2;
    private int field3;

    public MyObject(int field1,
                    int field2,
                    int field3)
    {
        this.field1 = field1;
        this.field2 = field2;
        this.field3 = field3;
    }

    public int getField1()
    {
        return field1;
    }

    public int getField2()
    {
        return field2;
    }

    public int getField3()
    {
        return field3;
    }

    @Override
    public String toString()
    {
        return field1 + "-" + field2 + "-" + field3;
    }

    public static void main(String[] args)
    {
        List<MyObject> myObjects = Arrays.asList(new MyObject(1, 2, 3),
                                                 new MyObject(0, 1, 2),
                                                 new MyObject(1, 1, 1),
                                                 new MyObject(1, 1, 0),
                                                 new MyObject(1, 2, 1));
        List<MyObject> sortedList = myObjects.stream()
                                             .sorted(Comparator.comparing(MyObject::getField1)
                                                               .thenComparing(MyObject::getField2)
                                                               .thenComparing(MyObject::getField3))
                                             .collect(Collectors.toList());

        System.out.println(sortedList);
    }
}

I would do this in two parts. Part 1 would be to sort based on Field 1.

Part 2 would involve creating a map (HashMap) with the values of Field 2 as the keys mapping to a binary heap (PriorityQueue) of the Field 3 values, sorted in reverse order. I would then iterate though the original array and replace Field 3 with the element take form the top of the Field 2 heap.

Ok after reading your reply to the earlier answer, I think you are after a two pass approach. First you want to sort the list of objects by the first field. Then you want to go back through the collection and if two items have the same field two, we sort them by field three.

So this is easiest to do by sorting the collection twice using two different comparators, the first to sort by field one:

@Override
public int compare(MyObject o1, MyObject o2) {
    return Integer.compare(o1.getFieldOne(), o2.getFieldOne());
}

And the second that sorts by field three only when field two is equal

    @Override
    public int compare(MyObject o1, MyObject o2) {
        int secondFieldComparison = Integer.compare(o1.getFieldTwo(), o2.getFieldTwo());
        if (secondFieldComparison == 0) {
            return Integer.compare(o1.getFieldThree(), o2.getFieldThree());
        } else {
            return 0;
        }
    }