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Re: Is there a better way to group and sort objects?

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On 8/7/2018 9:44 AM, Eric Sosman wrote:
> On 8/7/2018 9:00 AM, Eric Douglas wrote:
>> I want to combine and sort objects by key properties and merge their 
>> secondary properties.  Is there a better way?  This is what I came up 
>> with..
>>
>> public abstract class TestStatic {
>>     private static class TestClass extends Object implements 
>> Comparable<Object> {
>>         private int var1;
>>         private int var2;
>>
>>         @Override
>>         public int compareTo(Object compareMe) {
>>             if (compareMe == null || compareMe.getClass() != 
>> getClass()) {
>>                 return 1;
>>             }
>>             return var1 - ((TestClass) compareMe).getVar1();
>>         }
> 
>      Note that this method violates the Comparable contract.  If you use
> this compareTo() method with Arrays.sort() or Collections.sort() or a
> SortedMap or whatever, and the objects being compared are a mixture of
> different TestClass subclasses, there's no telling what might happen.

     Now that I think of it, the method violates Comparable in at least
two other ways, too.

     Violation #1:

	TestClass test = new TestClass();
	Integer that = 42;
	int one = test.compareTo(that);
	int two = that.compareTo(test);

Comparable requires that `one' and `two' have opposite signs or are
both zero, but the final line won't even compile, much less yield an
acceptable result.  (Because of the antisymmetry requirement, I think
it is impossible for *any* class to implement Comparable<Object>.)

     Violation #2:

	TestClass small = new TestClass();
	TestClass medium = new TestClass();
	TestClass large = new TestClass();
	small.setVar1(Integer.MIN_VALUE);
	large.setVar1(Integer.MAX_VALUE);
	int smallMedium = small.compareTo(medium);  // small < medium
	int mediumLarge = medium.compareTo(large);  // medium < large
	int smallLarge = small.compareTo(large);    // small > large !!!

Comparable must define a /total order/, which must be transitive: If
X is less than Y and Y is less than Z, X must be less than Z.

-- 
esosman@comcast-dot-net.invalid
Eight hundred ninety-seven days to go.

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Is there a better way to group and sort objects? Eric Douglas <e.d.programmer@gmail.com> - 2018-08-07 06:00 -0700
  Re: Is there a better way to group and sort objects? Eric Sosman <esosman@comcast-dot-net.invalid> - 2018-08-07 09:44 -0400
    Re: Is there a better way to group and sort objects? Eric Sosman <esosman@comcast-dot-net.invalid> - 2018-08-07 11:33 -0400
    Re: Is there a better way to group and sort objects? Eric Douglas <e.d.programmer@gmail.com> - 2018-08-07 10:22 -0700
      Re: Is there a better way to group and sort objects? Eric Sosman <esosman@comcast-dot-net.invalid> - 2018-08-07 14:16 -0400
        Re: Is there a better way to group and sort objects? Eric Sosman <esosman@comcast-dot-net.invalid> - 2018-08-07 14:36 -0400
        Re: Is there a better way to group and sort objects? Eric Douglas <e.d.programmer@gmail.com> - 2018-08-07 12:13 -0700
          Re: Is there a better way to group and sort objects? Eric Sosman <esosman@comcast-dot-net.invalid> - 2018-08-07 16:00 -0400
  Re: Is there a better way to group and sort objects? Martin Gregorie <martin@mydomain.invalid> - 2018-08-08 08:20 +0000
    Re: Is there a better way to group and sort objects? Eric Sosman <esosman@comcast-dot-net.invalid> - 2018-08-08 06:57 -0400
      Re: Is there a better way to group and sort objects? Martin Gregorie <martin@mydomain.invalid> - 2018-08-08 11:08 +0000
      Re: Is there a better way to group and sort objects? Martin Gregorie <martin@mydomain.invalid> - 2018-08-08 11:22 +0000
        Re: Is there a better way to group and sort objects? Eric Sosman <esosman@comcast-dot-net.invalid> - 2018-08-08 07:45 -0400
          Re: Is there a better way to group and sort objects? Martin Gregorie <martin@mydomain.invalid> - 2018-08-08 14:12 +0000
        Re: Is there a better way to group and sort objects? Eric Douglas <e.d.programmer@gmail.com> - 2018-08-08 05:50 -0700
          Re: Is there a better way to group and sort objects? Eric Sosman <esosman@comcast-dot-net.invalid> - 2018-08-08 09:23 -0400
            Re: Is there a better way to group and sort objects? Eric Douglas <e.d.programmer@gmail.com> - 2018-08-08 06:47 -0700
              Re: Is there a better way to group and sort objects? Eric Sosman <esosman@comcast-dot-net.invalid> - 2018-08-08 10:31 -0400
                Re: Is there a better way to group and sort objects? Eric Douglas <e.d.programmer@gmail.com> - 2018-08-08 08:24 -0700
          Re: Is there a better way to group and sort objects? Robert Klemme <shortcutter@googlemail.com> - 2018-08-14 14:06 +0200
            Re: Is there a better way to group and sort objects? Eric Douglas <e.d.programmer@gmail.com> - 2018-08-14 05:49 -0700
  Re: Is there a better way to group and sort objects? Daniele Futtorovic <da.futt.news@laposte-dot-net.invalid> - 2018-08-09 16:06 +0200
    Re: Is there a better way to group and sort objects? Eric Douglas <e.d.programmer@gmail.com> - 2018-08-14 05:47 -0700

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