Java - Class Data Type - Sorting Problems

Question

Assume that I have implemented a class Edge which has 4 attributes, all of which are of type int: from to quality length.

In my program , I have created an Edge[] array.

I want to Implement 2 Sorting Parameters -

One of them will sort the Edge array in descending order of the qualities,

The other will sort based on increasing order of lengths.

I will be needing these two orderings in distinct parts of my code.

I will be using the library function Arrays.sort() for sorting.

The only way I know of sorting Class Data Type Arrays to implement compareTo() within the class Edge but this only works for one parameter(quality or length but not both).

How can I implement two sorting functions(2 compareTo() functions ?) and decide which one to be called during sorting? In C++ , we can make many compare functions and simply state the function to go through.How to achieve this in Java?

Note: My goal is to sort an array of DataType Edge using Arrays.sort() , and use two different parameters for sorting and DECIDE which one is to be used at which point.

MadConan · Accepted Answer · 2015-04-09 19:18:51Z

2

You create two separate Comparator implementations that perform the algorithms you describe. Then you can pass the appropriate instance to Collections.sort.

public class EdgeCompareLength implements Comparator<Edge>{
    @Override
    public int compare(Edge o1, Edge o2) {
        // your implementation here
        return 0;
    }
}

public class EdgeCompareProperties implements Comparator<Edge>{
    @Override
    public int compare(Edge o1, Edge o2) {
        // your implementation here
        return 0;
    }
}

So someplace you would have

List<Edge> edges = getEdges();

Collections.sort(edges,new EdgeCompareLength());
List<Edge> moreEdges = getMoreEdges();
Collections.sort(edges,new EdgeCompareProperties();

edited Apr 9, 2015 at 19:18

answered Apr 9, 2015 at 19:14

MadConan

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4 Comments

Animesh Fatehpuria Over a year ago

Could you provide some code to implement this? How would we select which one to use?

MadConan Over a year ago

You would select the appropriate comparator implementation based on whatever you wanted. How did you plan on selected when to sort?

Animesh Fatehpuria Over a year ago

Last Question: What if I wanted to sort a range in the array using these comparators. Arrays.sort(arr,N,M) sorts array arr from N to M(exc) according the properties.Can i do sth like Collections.sort(edges,new EdgeCompareLength(),N,M); or sth?

MadConan Over a year ago

Collections doesn't have a ranged sort. You'd have to use Arrays.sort(T[] a, int fromIndex, int toIndex, Comparator<? super T> c) and pass the appropriate comparator implementation.

rgettman · Accepted Answer · 2015-04-09 19:19:49Z

1

In Java, you can create 2 classes that implements Comparator, defining your compare method, one for each of your sort orders you desire.

Then you can pass an instance of one of those Comparators to Arrays.sort.

If you're using Java 8, you can pass a method reference to Comparator.comparing to construct a Comparator based on a getter method.

Comparator<Edge> lengthAscEdgeComp = Comparator.comparing(Edge::getLength);

To make a descending sort, you can call reversed.

Comparator<Edge> qualityDescEdgeComp = Comparator.comparing(Edge::getQuality).reversed();

answered Apr 9, 2015 at 19:19

rgettman

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Comments

John Bollinger · Accepted Answer · 2015-04-09 19:20:21Z

0

You want to implement the two sort orders (or at least one of them) by implementing Comparator<Edge>. You then sort via the version of Arrays.sort() that accepts a Comparator as a second parameter.

Do not confuse that with having Edge itself implement the Comparable interface. The Comparator implementations will be different classes (from Edge and from each other).

Example:

public class DescendingByQuality implements Comparator<Edge> {
    public int compare(Edge e1, Edge e2) {
        if (e1.getQuality() < e2.getQuality()) return 1;
        if (e1.getQuality() > e2.getQuality()) return -1;
        return 0;
    }
}

...

Edge[] edges = ...;

Arrays.sort(edges, new DescendingByQuality());

edited Apr 9, 2015 at 19:20

answered Apr 9, 2015 at 19:14

John Bollinger

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