Re: DefaultFamily and comparing RealTypes

• To: Andrew Donaldson <abcd@xxxxxxxxxx>
• Subject: Re: DefaultFamily and comparing RealTypes
• From: Dave Glowacki <dglo@xxxxxxxxxxxxx>
• Date: Wed, 01 Aug 2001 10:19:22 -0500

Andrew Donaldson wrote:
> I've been comparing RealTypes a bit, and it has just stopped working.
> I'm assuming that it's ok to use == on RealTypes as they seem to be
> a global sort of thing.  I am interested in testing the equality
> of the name and units etc, so I'm not using RealType.equals().
> 
> The problem is that the comparison's behaviour changes after
> I create a DefaultFamily.
> 
> The code below shows this (at least for me!), remove the commented
> out line to change what it prints.
> 
> Is my assumption bad, or is DefaultFamily doing something bad?

The immediate problem is due to the visad.data.netcdf.Plain
creating its own version of the Latitude type based on the
Quantity class, which lets it add a Unit to the Latitude
RealType.  (This is actually a *good* thing, because the
Quantity-based "Latitude" is has more information than the
standard RealType-based "Latitude", but acts the same as the
standard "Latitude" for everything except the == operator.)

However, as Steve points out, if you use the equals()
method rather than ==, things will work as you expect.
This isn't strictly a VisAD requirement, it's a truism
throughout Java.  For example, this program:

  public class I
  {
    public static void main(String[] args)
    {
      Integer i = new Integer(1);
      Integer j = new Integer(1);
      System.out.println("( " + i + " = " + j + " )= " + (i == j));
      System.out.println("( " + i + " eq " + j + " )= " + (i.equals(j)));
    }
  }

prints out

  ( 1 = 1 )= false
  ( 1 eq 1 )= true

The == operator is checking whether the two objects are
exactly the same (i.e. in the same location in memory).
The equals() method typically checks to see if two objects
have the same values.