how to compare on multiple classes in java?

Question

Right now, I have written at comparator that sorts an array of Integers and Strings. As you can see from the code, if the two classes aren't the same, then the String class takes the greater than value. However, this only allows for two classes. What if I want to add another primitive type to my array, such as Float? I'd have to add more code to to if-else statement. Is there a way to implement compare without having to add a statement for each additional class I want to compare?

import java.util.Arrays;
import java.util.Comparator;

public class SampleComparator implements Comparator<Object> {

public static void main(String[] args) {
    Object[] inputData = { new String("pizza"), new Integer(0),
            new String("apples"), new Integer(5), new String("pizza"),
            new Integer(3), new Integer(7), new Integer(5) };
    Arrays.sort(inputData, new SampleComparator());
    System.out.println(Arrays.asList(inputData));
}

public int compare(Object o1, Object o2) {
    if (o1.getClass().equals(o2.getClass())) {
        return ((Comparable)o1).compareTo((Comparable)o2);
    } else {
        if(o1.getClass().getCanonicalName().equals("java.lang.String")){
            return 1;
        } else {
            return -1;
        }
    }

}

 }

output:

[0, 3, 5, 5, 7, apples, pizza, pizza]

Don't forget to check for `null` before calling `getClass` or `toString`. — searlea, Aug 17 '11 at 12:32

score 3 · Accepted Answer · answered Aug 17 '11 at 12:11

I'd have to add more code to to if-else statement. Is there a way to implement compare without having to add a statement for each additional class I want to compare?

You probably want to compare objects of the same class using their native comparators, and then have some order on the classes (e.g.. all ints before all floats before all strings).

So you could compare the classes first, and then if equal compare the objects.

public int compare(Object o1, Object o2) {
    // maybe some null checks here?

    if (o1.getClass().equals(o2.getClass())) {
        // and what if they are not Comparable ?
        return ((Comparable)o1).compareTo((Comparable)o2);
    } else {
        // for example compare by class name alphabetically
        // another idea would be a map with all supported classes,
        // assigning them an order

        return o1.getClass().getName().compareTo(o2.getClass().getName());
    }

}

It will be difficult to come up with a meaningful comparison of unknown classes. You probably need to make up a list of supported classes and explicit rules how you want them compared.

score 2 · Answer 2 · answered Aug 17 '11 at 12:09

2

If you just want to make sure you group classes together, you could do something like

    ...
} else {
    return o1.getClass().getName().compareTo(o2.getClass().getName());
}

answered Aug 17 '11 at 12:09

aioobe

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score 1 · Answer 3 · answered Aug 17 '11 at 12:10

1

You can always compare the toString() representation:

public int compare(Object o1, Object o2) {
    return o1.toString().compareTo(o2.toString());
}

answered Aug 17 '11 at 12:10

dacwe

43,066
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This is a very simple solution, but may not be desired. If it's sufficient, then I'd *certainly* go with this: less code = less bugs. – Joachim Sauer Aug 17 '11 at 12:12
2

This would sort your Integers alphabetically (meaning 2 will be after 10, for example). Usually not what you want. – Jorn Aug 17 '11 at 12:16

how to compare on multiple classes in java?

3 Answers3

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