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Hi there this is they don't want to look too before looking at genetic methods.

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Let's look at this property called invariance we mentioned about it earlier too.

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It's a very simple concept and it is responsible for ensuring compile time type safety.

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So let's go ahead and look at what it is.

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We know that the promise of genetics is typesafe be at compile time.

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Right.

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Invariance is a property that also helps with this.

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For instance let's consider a familiar hierarchy where book is it some type of bookmark.

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Hello.

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With ingredients we have this that is a list of book is not a subtype off list stuff bookmark.

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So even the book is it's a type of bookmark list of book is not a pipe off the stuff bookmark and similarly

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go to invariance.

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Start off book is also not a supply off store or bookmark.

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What it means is you cannot assign an instance of store or book store or bookmark all the book and be

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assigned to bookmark.

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And this is for ensuring compile time types if be not an example let's see why invariance is needed

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to ensure compile time types.

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If here let's consider this method go whose parameter is an idealist or bookmark.

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Now for the sake of this discussion let's assume that genetics does not how invariant property and so

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do with that here we are able to invoke the motet go which has an ideal list of bookmark estimate that

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parameter bit and idlest logbook.

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So the method argument is a list of book while the meathook barometer is it is not a bookmark on VTOC

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invariants that should be possible.

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Now that double up one would assume that the input argument which is an analyst or book would all of

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this storm need book instances right.

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That's the promise of genetics.

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But putting them at that.

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Go a movie object can be added to the input ideas.

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And this is possible as the input of a list is declared as an idealist not a bookmark.

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And we know that movie is a subclass of bookmark kits or movie object can be added to the list.

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So essentially what is happening is a movie object is being added to a list that must contain only books

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.

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So if this is allowed then in the calling code the moronic class caste exception at one time as a movie

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cannot be trusted with a book it sort of the court is extracting an object from the artist.

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Then there will be an implicit cast on that would generate a cost kust exception as a movie cannot be

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strictly a book.

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So compile time types if he is lost hence invariance properties needed for ensuring compile time type

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CFT with invariants.

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This would be a compilation error in the method invocation statement itself.

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And in this particular example the method go would have to be in more than idlest off bookmark.

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So we know that immediacy does not allow this.

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However for the same type argument inheritance of genetic types or so if that type argument is bookmarked

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then an idealist or bookmark can be assigned to a list of bookmark as idlest is a subtype awfullest

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can be a little bit confusing.

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But you should keep that in mind.

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So JENNETT exciting variant.

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However arrays are something called Koiari.

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What it means is if book is a subtype of book maake then a book or B is a subtype off.

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Bookmark it.

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And because of that we can assign a book ID to a book marketing.

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And here is the same example as before but now I'm at that parameter is a bookmark for.

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And in the invocation meant a book or is being passed so the body must contain only book instances there

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will not be any competition at first.

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Google Corleone's property but within the matter a movie object may be added to the input.

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No this would not give a compilation error but at runtime they would end up with an arrest or exception

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.

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The exception would be thrown if a wrong type of object is being stored and then not it.

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This was also demonstrated in one of the demotes earlier in the chapter.

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So at no point we have compassion error here.

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We only have runtime exception which is not good with generics.

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The issue would be highlighted at compile time itself.

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So book arrays as well as generics and shirt type safety.

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It is just that generics insured type safety at compile time itself while R is and sure it on the runtime

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and we know that compile time type c of D is prefered over types if we done.

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And you could get this item from Effective Java recommends using less or worse.

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I guess that doesn't mean that arrays are not important.

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They have their place but if you feel typesafe needs to be ensured at compile time then you should go

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Burtless non-attorney want to eclipse for a very short demo.

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OK here we are in the genetics demo class and we are going to test both invariants as well as covariance

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.

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So we have these methods go for invariance and another go for covariance.

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OK so the methods have been named as Tcl.

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So let's just look at invariants and it just accepts an astronomer.

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OK I know it's an empty body and we're going to invoke it from here from the main method.

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So here we are passing.

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And I don't list off integer.

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OK here it's a list off number but here it is a list of integer integer we know is a subtype of number

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but are a list of integer will not be a subtype of list of number.

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All right so that's in millions.

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And so we get a compilation error here.

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OK.

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Because we are compromising type safety.

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So that's it.

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So there is nothing to test here.

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Next test go method here.

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It accepts an ID off numbers OK.

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And here we are passing an array of integer.

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So this should store only integers but within this method we are storing a double case so there is no

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compression order but we get a runtime exception.

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If you run it.

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OK so but we know that integer is just a pipe of number so an integer adic and all is also a subtype

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off number and or F number.

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So that's covariance property.

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So you filter on it as you can see it says here is or it's invoking another method here.

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Let me just come on this out

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artist uniters side gives an artist or exception right.

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OK and that's what is happening here.

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Because we are trying to store it wrong type of object here.

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It's supposed to be an integer but we are starting a double.

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So you are getting in out store.

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Exception at runtime which is not good.

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So the item from effective Java.

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It recommends using lists over this so that we get compile time type safety advantage.

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So that's about it.

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Thank you.

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The code is available in the resource section.

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Happy coding