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Re: mollifying generics

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On 6/29/16 1:43 PM, Daniel Pitts wrote:
> On 6/29/16 1:11 AM, Roedy Green wrote:
>>
>> Prior to generics, I could say something like this:
>>
>> Comparable a = "abc";
>> Comparable b = "def";
>>
>> or Comparable a = new Long( 2 );
>> Comparable b = new Long ( 4 );
>>
>>
>> int diff = a.compareTo(b);
>>
>> The run time would use the corresponding compareTo method
>>
>> Now generics complains about an unchecked operation.
>>
>> Is there a way to mollify it?  I have tried what I thought were the
>> obvious means.
>>
>
> Let's say you used to have this:
>
>
> public void doSomethingWithDiff(Comparable a, Comparable b)
> {
>    int diff = a.compareTo(b);
>    if (diff < 0) {
>       handleLess();
>    } else if (diff > 0) {
>       handleMore();
>    } else {
>       handleEqual();
>    }
> }
>
> You would want to replace it with:
>     public <T> void doSomethingWithDiff(Comparable<T> a, T b)
>
> Or:
>   public <T extends Comparable<? super T>> void doSomethingWithDiff(T a,
> T b)
>
> Either one of those will work as a type safe alternative.
>
> You'll have to keep track of the type with generics *or* specifics up to
> that point to have the compiler satisfied.
>
>

You could also more easily not keep track of the type in your client 
code, and move that into a separate class:

public class ComparablePair<T extends Comparable<? super T>> {
     private final T left;
     private final T right;

     public ComparablePair(T left, T right) {
         this.left = left;
         this.right = right;
     }

     public int compare() {
         return left.compareTo(right);
     }
}

You could use it then:

ComparablePair<?> pair
     = new ComparablePair<>("abc", "def");

ComparablePair<?> anotherPair
     = new ComparablePair<>(new Long(2), new Long(4));

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mollifying generics Roedy Green <inquiry@mindprod.com> - 2016-06-29 01:11 -0700
  Re: mollifying generics Andreas Leitgeb <avl@logic.at> - 2016-06-29 08:33 +0000
  Re: mollifying generics Daniel Pitts <usenet@coloraura.com> - 2016-06-29 13:43 -0700
    Re: mollifying generics Daniel Pitts <usenet@coloraura.com> - 2016-06-29 13:47 -0700
  Re: mollifying generics Arne Vajhøj <arne@vajhoej.dk> - 2016-07-05 22:28 -0400

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