Why doesn't java.lang.Number implement Comparable?

Does anyone know why java.lang.Number does not implement Comparable? This means that you cannot sort Numbers with Collections.sort which seems to me a little strange.

Post discussion update:

Thanks for all the helpful responses. I ended up doing some more research about this topic.

The simplest explanation for why java.lang.Number does not implement Comparable is rooted in mutability concerns.

For a bit of review, java.lang.Number is the abstract super-type of AtomicInteger, AtomicLong, BigDecimal, BigInteger, Byte, Double, Float, Integer, Long and Short. On that list, AtomicInteger and AtomicLong to do not implement Comparable.

Digging around, I discovered that it is not a good practice to implement Comparable on mutable types because the objects can change during or after comparison rendering the result of the comparison useless. Both AtomicLong and AtomicInteger are mutable. The API designers had the forethought to not have Number implement Comparable because it would have constrained implementation of future subtypes. Indeed, AtomicLong and AtomicInteger were added in Java 1.5 long after java.lang.Number was initially implemented.

Apart from mutability, there are probably other considerations here too. A compareTo implementation in Number would have to promote all numeric values to BigDecimal because it is capable of accommodating all the Number sub-types. The implication of that promotion in terms of mathematics and performance is a bit unclear to me, but my intuition finds that solution kludgy.

It's worth mentioning that the following expression:

new Long(10).equals(new Integer(10))

is always false, which tends to trip everyone up at some point or another. So not only can you not compare arbitrary Numbers but you can't even determine if they're equal or not.

Also, with the real primitive types (float, double), determining if two values are equal is tricky and has to be done within an acceptable margin of error. Try code like:

double d1 = 1.0d;
double d2 = 0.0d;
for (int i=0; i<10; i++) {
  d2 += 0.1d;
}
System.out.println(d2 - d1);

and you'll be left with some small difference.

So back to the issue of making Number Comparable. How would you implement it? Using something like doubleValue() wouldn't do it reliably. Remember the Number subtypes are:

• Byte;
• Short;
• Integer;
• Long;
• AtomicInteger;
• AtomicLong;
• Float;
• Double;
• BigInteger; and
• BigDecimal.

Could you code a reliable compareTo() method that doesn't devolve into a series of if instanceof statements? Number instances only have six methods available to them:

• byteValue();
• shortValue();
• intValue();
• longValue();
• floatValue(); and
• doubleValue().

So I guess Sun made the (reasonable) decision that Numbers were only Comparable to instances of themselves.

For the answer, see Java bugparade http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4414323">bug 4414323. You can also find a discussion from http://coding.derkeiler.com/Archive/Java/comp.lang.java.programmer/2007-07/msg02451.html">comp.lang.java.programmer</a>

To quote from the Sun response to the bug report from 2001:

> All "numbers" are not comparable; > comparable assumes a total ordering of > numbers is possible. This is not even > true of floating-point numbers; NaN > (not a number) is neither less than, > greater than, nor equal to any > floating-point value, even itself. > {Float, Double}.compare impose a total > ordering different from the ordering > of the floating-point "<" and "=" > operators. Additionally, as currently > implemented, the subclasses of Number > are only comparable to other instances > of the same class. There are other > cases, like complex numbers, where no > standard total ordering exists, > although one could be defined. In > short, whether or not a subclass of > Number is comparable should be left as > a decision for that subclass.

in order to implement comparable on number, you would have to write code for every subclass pair. Its easier instead to just allow subclasses to implement comparable.

To try to solve the original problem (sort a list of numbers), an option is to declare the list of a generic type extending Number and implementing Comparable.

Something like:

<N extends Number & Comparable<N>> void processNumbers(List<N> numbers) {
    System.out.println("Unsorted: " + numbers);
    Collections.sort(numbers);
    System.out.println("  Sorted: " + numbers);
    // ...
}

void processIntegers() {
    processNumbers(Arrays.asList(7, 2, 5));
}

void processDoubles() {
    processNumbers(Arrays.asList(7.1, 2.4, 5.2));
}

Very probably because it would be rather inefficient to compare numbers - the only representation into which every Number can fit to allow such comparison would be BigDecimal.

Instead, non-atomic subclasses of Number implements Comparable itself.

Atomic ones are mutable, so can't implement an atomic comparison.

You can use Transmorph to compare numbers using its NumberComparator class.

NumberComparator numberComparator = new NumberComparator();
assertTrue(numberComparator.compare(12, 24) < 0);
assertTrue(numberComparator.compare((byte) 12, (long) 24) < 0);
assertTrue(numberComparator.compare((byte) 12, 24.0) < 0);
assertTrue(numberComparator.compare(25.0, 24.0) > 0);
assertTrue(numberComparator.compare((double) 25.0, (float) 24.0) > 0);
assertTrue(numberComparator.compare(new BigDecimal(25.0), (float) 24.0) > 0);

Write your own Comparator

import java.math.BigDecimal;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;




public class NumberComparator implements Comparator {
@SuppressWarnings("unchecked")
@Override
public int compare(Number number1, Number number2) {
if (((Object) number2).getClass().equals(((Object) number1).getClass())) {
// both numbers are instances of the same type!
if (number1 instanceof Comparable) {
// and they implement the Comparable interface
return ((Comparable) number1).compareTo(number2);
}
}
// for all different Number types, let's check there double values
if (number1.doubleValue() < number2.doubleValue())
return -1;
if (number1.doubleValue() > number2.doubleValue())
return 1;
return 0;
}



/**
 * DEMO: How to compare apples and oranges.
 */
public static void main(String[] args) {




ArrayList listToSort = new ArrayList();
listToSort.add(new Long(10));
listToSort.add(new Integer(1));
listToSort.add(new Short((short) 14));
listToSort.add(new Byte((byte) 10));
listToSort.add(new Long(9));
listToSort.add(new AtomicLong(2));
listToSort.add(new Double(9.5));
listToSort.add(new Double(9.0));
listToSort.add(new Double(8.5));
listToSort.add(new AtomicInteger(2));
listToSort.add(new Long(11));
listToSort.add(new Float(9));
listToSort.add(new BigDecimal(3));
listToSort.add(new BigInteger("12"));
listToSort.add(new Long(8));
System.out.println("unsorted: " + listToSort);
Collections.sort(listToSort, new NumberComparator());
System.out.println("sorted:   " + listToSort);
System.out.print("Classes:  ");
for (Number number : listToSort) {
System.out.print(number.getClass().getSimpleName() + ", ");
}
}
}
ArrayList listToSort = new ArrayList();
listToSort.add(new Long(10));
listToSort.add(new Integer(1));
listToSort.add(new Short((short) 14));
listToSort.add(new Byte((byte) 10));
listToSort.add(new Long(9));
listToSort.add(new AtomicLong(2));
listToSort.add(new Double(9.5));
listToSort.add(new Double(9.0));
listToSort.add(new Double(8.5));
listToSort.add(new AtomicInteger(2));
listToSort.add(new Long(11));
listToSort.add(new Float(9));
listToSort.add(new BigDecimal(3));
listToSort.add(new BigInteger("12"));
listToSort.add(new Long(8));
System.out.println("unsorted: " + listToSort);
Collections.sort(listToSort, new NumberComparator());
System.out.println("sorted:   " + listToSort);
System.out.print("Classes:  ");
for (Number number : listToSort) {
System.out.print(number.getClass().getSimpleName() + ", ");
}
}
}

there is no stardard comparison for Numbers of different types. However you can write your own Comparator and use it to create a TreeMap<Number, Object>, TreeSet<Number> or Collections.sort(List<Number>, Comparator) or Arrays.sort(Number[], Comparator);

why this would have been bad idea? :

abstract class ImmutableNumber extends Number implements Comparable {
    // do NOT implement compareTo method; allowed because class is abstract
}
class Integer extends ImmutableNumber {
    // implement compareTo here
}
class Long extends ImmutableNumber {
    // implement compareTo here
}

another option may have been to declare class Number implements Comparable, omit compareTo implementation, and implement it in some classes like Integer while throw UnsupportedException in others like AtomicInteger.

My guess is that by not implementing Comparable, it give more flexibility to implementing classes to implement it or not. All the common numbers (Integer, Long, Double, etc) do implement Comparable. You can still call Collections.sort as long as the elements themselves implement Comparable.

Looking at the class hierarchy. Wrapper classes like Long, Integer, etc, implement Comparable, i.e. an Integer is comparable to an integer, and a long is comparable to a long, but you can't mix them. At least with this generics paradigm. Which I guess answers your question 'why'.

byte (primitive) is a int (primitive). Primitives have only one value at a time.
Language design rules allows this.

int i = 255

// down cast primitive
(byte) i == -1

A Byte is not an Integer. Byte is a Number and an Integer is a Number. Number objects can have more than one value at the same time.

Integer iObject = new Integer(255);
System.out.println(iObject.intValue());   // 255
System.out.println(iObject.byteValue());  // -1

If a Byte is an Integer and an Integer is a Number, Which one value will you use in the compareTo(Number number1, Number number2) method?