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From: Eric Sosman <esosman@comcast-dot-net.invalid>
Newsgroups: comp.lang.java.programmer
Subject: Re: Is there a better way to group and sort objects?
Date: Tue, 7 Aug 2018 11:33:32 -0400
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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.