[HD Pg 370, Sec. 12.5.3 - using-predicate-interface]

[raphaelzintec]

hello,

i don't understand anything about predicated, i feel like this chapter miss parts.

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    public static boolean checkList(List list, Predicate<List> p){
        return  p.test(list);
    }

    public static void main(String[] args) {
        boolean b = checkList(new ArrayList<>(), (ArrayList al) -> al.isEmpty());
        System.out.println(b);
    }

and why this code is incorrect? in java like everything accept the subtype, here why it doesnt accept subclass type?

[admin]

I am not sure where are you seeing Predicate<List>. The section that you are referring to uses Predicate<Car>.

However, assuming that you really are interested in Predicate<List> and in knowing why (ArrayList al) -> al.isEmpty() can't implement Predicate<List>:

A lambda expression that implements Predicate<List> must satisfy the contract imposed by Predicate<List>, which means it must be able to transform to a class that implements the abstract method declared by Predicate<List> i.e. boolean test(List ). Something like this:

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class Lambda1 implements Predicate<List>{
  public boolean test(List l){ //correctly implements the method declared by Predicate<List>.
     return l.isEmpty();
  }
}

But the expression (ArrayList al) -> al.isEmpty() translates to:

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class Lambda2 implements Predicate<List>{
  public boolean test(ArrayList l){
     return l.isEmpty();
  }
}

But the Lambda2 class violates the rules of interface implementation. This is not a case of "passing" an ArrayList where a List is required. You are not "calling" the test(List ) method here, you are "implementing" the test(List ) method.

This has nothing to do with lambda and Predicate. It is about implementing an interface (any interface for that matter). For example, if you have an interface :

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interface X{
    void m(List l);
}

You cannot implement it like this:

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class CX implements X{ //will not compile because CX does not have implementation for X's method.
    public void m(ArrayList l){ //this does not implement m(List ) method.
    }
}

Because to implement an interface method, the signature must match exactly as specified by the interface (subject to the rule of covariant returns). You have to define m(List ) in CX for it to correctly implement X.

Please let us know if this addresses your doubt and if it does, we will try to make sure this point is covered in the book.

[raphaelzintec]

hi i understand now predicate

here is the video about code
https://www.youtube.com/watch?v=i0weV7KsLXc

I DIDNT KNOW THAT GENERICS WERENT CONVARIANT

now i know all OCA 8, except Localdate lol, i know only 50% of localdate because bunch of methods to memorize

btw, after i get my OCA 8 with Oracle.

how long will it takes me to learn OCP 17 from mister Hammunt? i will buy his physical book on amazon and send you proof of purchase to get pdf, because its my only way to learn trough pdfs i hate physical books

best regards

[admin]

>here is the video about code https://www.youtube.com/watch?v=i0weV7KsLXc
Ok, I though you were talking about the contents on Section 12.5.3 of the book.

>I DIDNT KNOW THAT GENERICS WERENT CONVARIANT
But this question is not about that at all. This question is about rules of implementing an interface.

>how long will it takes me to learn OCP 17 from mister Hammunt?
It is not possible to guess the time required to prepare because it depends a lot on the candidate.

>i will buy his physical book on amazon and send you proof of purchase to get pdf,
There is no physical book for OCP 17 by Hanumant Deshmukh. There is only a pdf, which you can purchase from enthuware.com/books

[raphaelzintec]

"This question is about rules of implementing an interface."
Still not getting it sorry can you give more explanation plz?

the only thing i got is generics are not convariant that's all

[raphaelzintec]

OCP 17 = OCA & OCP java 17 right cuz there is no more OCA 17?

[admin]

"This question is about rules of implementing an interface."
Still not getting it sorry can you give more explanation plz?

the only thing i got is generics are not convariant that's all

Please go through the detailed explanation given above. There is no mention of generics in there.

[admin]

OCP 17 = OCA & OCP java 17 right cuz there is no more OCA 17?

Right.

[raphaelzintec]

ok thanks