Help comparing float member variables using Comparators

Question

I am able to compare Strings fine, but would like to know how I can rank floating point numbers?

getChange() returns a String. I want to be able to sort descending. How can I do this?

UPDATE:

package org.stocktwits.helper;

import java.util.Comparator;

import org.stocktwits.model.Quote;

public class ChangeComparator implements Comparator<Quote>
{
    public int compare(Quote o1, Quote o2) {
        float change1 = Float.valueOf(o1.getChange());
        float change2 = Float.valueOf(o2.getChange());

        if (change1 < change2) return -1;
        if (change1 == change2) return 0; // Fails on NaN however, not sure what you want
        if (change2 > change2) return 1;
    }
}

I am getting the compile time error:

This method must return a result of type int    ChangeComparator.java   

Solution 1

Read the javadoc of 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.

So, basically:

float change1 = o1.getChange();
float change2 = o2.getChange();
if (change1 < change2) return -1;
if (change1 > change2) return 1;
return 0;

Or if you like conditional operators:

return o1.getChange() < o2.getChange() ? -1 
     : o1.getChange() > o2.getChange() ? 1 
     : 0;

You however need to take account with Float.NaN. I am not sure how you'd like to have them ordered. First? Last? Equally?

Solution 2

How about this:

public class ChangeComparator implements Comparator<Quote>
{
    public int compare(Quote o1, Quote o2) {
        Float change1 = Float.valueOf(o1.getChange());
        Float change2 = Float.valueOf(o2.getChange());
        return change1.compareTo(change2);
    }
}

Note that Java 1.4 introduced Float#compare(float, float) (and an equivalent in Double), which can be pretty much used directly:

public class ChangeComparator implements Comparator<Quote>
{
    public int compare(Quote o1, Quote o2) {
        return Float.compare(o1.getChange(), o2.getChange());
    }
}

(After editing, I notice that @BorislavGizdov has mentioned this in his answer already.)

---

Also worth noting that Java 8 Comparator#comparing(...) and Comparator#comparingDouble(...) provide a straightforward way of constructing these comparators directly.

Comparator<Quote> changeComparator = Comparator.comparing(Quote::getChange);

Will compare using boxed Float values.

Comparator<Quote> changeComparator = Comparator.comparingDouble(Quote::getChange);

Will compare using float values promoted to double values.

Given that there is no Comparator#comparingFloat(...), my preference would be to use the comparingDouble(...) method, as this only involves primitive type conversion, rather than boxing.

Solution 3

You can use Float.compare(float f1, float f2):

  public static int compare(float f1, float f2)

Compares the two specified float values. Returns the value 0 if f1 is numerically equal to f2; a value less than 0 if f1 is numerically less than f2; and a value greater than 0 if f1 is numerically greater than f2.

Comments

[John Gardner]

watch out for Float.NaN! I'm presuming your data doesn't have NaNs, but since NaN on one side of any comparison will always return false (even Float.NaN==Float.NaN is false!), you might want a Float.isNaN(change1) or whatever check. otherwise the sort will be apparently random if there are NaN's involved.

[Sheehan Alam]

I am getting the compile time error (see updated code in my question): This method must return a result of type int ChangeComparator.java

[BalusC]

The compilation error speaks for itself. You need to make sure that it always returns an int. I'll edit the example.

[Steve Kuo]

This is the best answer - delegate the comparison to Float's compareTo.