As I've mentioned elsewhere in this newsgroup (but not within this
thread), the concept of equality is context dependent. Are "1.00" and "1.0"
equal? Perhaps yes, if you're talking about pure mathematical numbers.
Perhaps no, if you're talking about engineering measurements with specific
tolerance levels [*]. Definitely not, if you're talking about Strings.

   The Java API was designed so that you could provide a comparator object,
so that you could using different metrics for determining how two objects
compare, depending on which metric you're interested in. I thought it might
be a good idea if they did the same thing for the equals method (so you
could provide a custom object to determine whether two objects are equal,
depending on the context). Unfortunately, the API wasn't designed that way.

   But moving away from idle daydreaming and coming back to practical
matters: recall that in Java, you're free to implement the equals() method
any way you want, as long as some basic requirements (reflexivity,
transitivity, etc.) are met. The people who designed the BigDecimal class
decided to design it in that particular way. Anyway, it's not like you can't
get the above mentioned desired behaviour: All you have to do is use
compareTo().

   - Oliver

[*] An engineer might reject a part that measures "1.0 mm", requiring one
that measures "1.00 mm". Why? Because that engineer might know that if the
part actually measures 1.01 mm (which is considered to be a "1.0 mm" part,
but not a "1.00 mm" part), the device will fail and/or break, whereas if the
device measures 1.004 mm (which is both a "1.0 mm" and "1.00 mm" part), the
device will tolerate this small deviancy, and continue functioning.

Anyone who thinks they are equivalent objects, try running this:

import java.math.BigDecimal;
public class TestBigDecimal {
  public static void main(String[] args) {
    System.out.println(new BigDecimal("1.0").
      divide(new BigDecimal(3),BigDecimal.ROUND_HALF_EVEN));
    System.out.println(new BigDecimal("1.00").
      divide(new BigDecimal(3),BigDecimal.ROUND_HALF_EVEN));
  }
}

If they really were equivalent, I would expect them to do the same for
anything other than the identity related operations, == comparison and
System.identityHashCode().

A BigDecimal has both a decimal value and a scale, not just a decimal
value. The scale affects some operations, such as the rounded divisions
in the example program.

Their decimal values are equal, and decimal value is the natural
ordering, so it makes sense for compareTo to treat them as equal.

Patricia

Not if you are an engineer. 1.00 indicates a higher precision than 1.0.
Depending on the type of rounding used (AFAIK varies in different
countries in engineering), the above numbers e.g. indicate the two
different ranges

1.00:  [0.995, 1.005)
1.0:   [0.95, 1.05)

for an engineer. And these ranges are rather different.

Stichwort "Signifikante Stellen"

/Thomas