Comparing the values of two generic Numbers

Question

I want to compare to variables, both of type T extends Number. Now I want to know which of the two variables is greater than the other or equal. Unfortunately I

Answer 1

A working (but brittle) solution is something like this:

class NumberComparator implements Comparator<Number> {

    public int compare(Number a, Number b){
        return new BigDecimal(a.toString()).compareTo(new BigDecimal(b.toString()));
    }

}

It's still not great, though, since it counts on toString returning a value parsable by BigDecimal (which the standard Java Number classes do, but which the Number contract doesn't demand).

Edit, seven years later: As pointed out in the comments, there are (at least?) three special cases toString can produce that you need to take into regard:

• Infinity, which is greater than everything, except itself to which it is equal
• -Infinity, which is less than everything, except itself to which it is equal
• NaN, which is extremely hairy/impossible to compare since all comparisons with NaN result in false, including checking equality with itself.

Answer 2

You can simply use Number's doubleValue() method to compare them; however you may find the results are not accurate enough for your needs.

Answer 3

If your Number instances are never Atomic (ie AtomicInteger) then you can do something like:

private Integer compare(Number n1, Number n2) throws SecurityException, NoSuchMethodException, IllegalArgumentException, IllegalAccessException, InvocationTargetException {

 Class<? extends Number> n1Class = n1.getClass();
 if (n1Class.isInstance(n2)) {
  Method compareTo = n1Class.getMethod("compareTo", n1Class);
  return (Integer) compareTo.invoke(n1, n2);
 }

 return -23;
}

This is since all non-Atomic Numbers implement Comparable

EDIT:

This is costly due to reflection: I know

EDIT 2:

This of course does not take of a case in which you want to compare decimals to ints or some such...

EDIT 3:

This assumes that there are no custom-defined descendants of Number that do not implement Comparable (thanks @DJClayworth)

Answer 4

This should work for all classes that extend Number, and are Comparable to themselves. By adding the & Comparable you allow to remove all the type checks and provides runtime type checks and error throwing for free when compared to Sarmun answer.

class NumberComparator<T extends Number & Comparable> implements Comparator<T> {

    public int compare( T a, T b ) throws ClassCastException {
        return a.compareTo( b );
    }
}

Answer 5

What about this one? Definitely not nice, but it deals with all necessary cases mentioned.

public class SimpleNumberComparator implements Comparator<Number>
    {
        @Override
        public int compare(Number o1, Number o2)
        {
            if(o1 instanceof Short && o2 instanceof Short)
            {
                return ((Short) o1).compareTo((Short) o2);
            }
            else if(o1 instanceof Long && o2 instanceof Long)
            {
                return ((Long) o1).compareTo((Long) o2);
            }
            else if(o1 instanceof Integer && o2 instanceof Integer)
            {
                return ((Integer) o1).compareTo((Integer) o2);
            }
            else if(o1 instanceof Float && o2 instanceof Float)
            {
                return ((Float) o1).compareTo((Float) o2);
            }
            else if(o1 instanceof Double && o2 instanceof Double)
            {
                return ((Double) o1).compareTo((Double) o2);
            }
            else if(o1 instanceof Byte && o2 instanceof Byte)
            {
                return ((Byte) o1).compareTo((Byte) o2);
            }
            else if(o1 instanceof BigInteger && o2 instanceof BigInteger)
            {
                return ((BigInteger) o1).compareTo((BigInteger) o2);
            }
            else if(o1 instanceof BigDecimal && o2 instanceof BigDecimal)
            {
                return ((BigDecimal) o1).compareTo((BigDecimal) o2);
            }
            else
            {
                throw new RuntimeException("Ooopps!");
            }

        }

    }

Answer 6

Let's assume that you have some method like:

public <T extends Number> T max (T a, T b) {
   ...
   //return maximum of a and b
}

If you know that there are only integers, longs and doubles can be passed as parameters then you can change method signature to:

public <T extends Number> T max(double a, double b) {
   return (T)Math.max (a, b);
}

This will work for byte, short, integer, long and double.

If you presume that BigInteger's or BigDecimal's or mix of floats and doubles can be passed then you cannot create one common method to compare all these types of parameters.