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sorted() method of the Stream class in the OCJP 8 book byjeanne boyarsky and scott selikoff.  RSS feed

 
Greenhorn
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Hi,

On page 199, the sorted method uses:



However, we cannot use method reference:



As per my understanding both the implementation do the same thing then how come only the first one works and the other does not? Please help me understand this.
 
Master Rancher
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hi Shahid,

indeed, I found that passage also confusing. But what I always do when I am unsure of a method reference: I replace that reference by the full lambda version, to see what I end up with.
So, let's take a look at Comparator::naturalOrder (or reversedOrder for that matter): we need a Comparator, a Comparator takes two input parameters and returns an int. So, let us replace it with the full version:

And the book rightfuly remarks that this is incorrect, since the method 'Comparator.naturalOrder()' is a method that takes no parameters! And there you have the compiler error!

But you CAN use a methodreference if you want to. Comparator.naturalOrder()  (with one dot!) returns a Comparator, that has the method: compare(a, b). So you can say:

I had to practise this quite a few times before I got the hang of it (and sometimes I still err), but if in doubt, as I said, I try to use the full version.
 
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[Note: this was written without seeing Piet's message.  So, I'm replying to the original poster here, not Piet. - Mike]

They aren't quite the same thing.  

Comparator.reverseOrder() returns a Comparator - when the code is executed, the Comparator actually exists, and a reference to that comparator is passed to... well, whatever method it's being passed to.  (Looks like there's a typo in the code you showed.)

Comparator::reverseOrder gives a reference to a method that could give you a Comparator.  If someone were to invoke that method.  Which they haven't, yet.

The first method can be used like this:


While the second method requires something more to actually get a Comparator from the reference:
 
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