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in the last video we saw how composition

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is a powerful technique for combining

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objects to create something more complex

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and we built up a HTML document from

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the three classes that it's composed of

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the doctype a head element and a body now

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aggregation is very similar and is often

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described as a weak form of composition

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now with composition the objects that another

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object is composed of don't exist

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outside of their container so in our

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example looking down here on line 50

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the init method for HTML doc there

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instances of a doctype which is created

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in a HTML doc init method on line 51 as i

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mentioned and a head and a body created

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on the next two lines so instances of

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all three classes exist but they only

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exist within the context of our HTML

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doc object my_page now if we

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delete my_page then those

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three instances will no longer exist

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either now in our program we've

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indicated at the three instance

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variables self dot underscored doc _

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type self._head and

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self dot _ body are quote unqote

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private

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now as we learned previously this is python

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and so they're not really private as

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such but there should be treated as if

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they are now this program was written in

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java for example there'd be no way to

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change those fields from outside the

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HTML dot class now I mention that

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because we can change this program to

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use aggregation without changing the

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HTML dot class in Java for example the

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code will have to be written a bit

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differently so in Python the distinction

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between aggregation and composition can

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blur slightly in Python focus more on

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the intent rather than what's possible

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with language that's because in Python

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just about anything is possible

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alright so let's use aggregation now

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to get our documents a new content then

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will go ahead and rewrite the class so

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that it uses aggregation rather than

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composition

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and then we're going to add some more

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code at the end of the program here so

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right down to the bottom and let's

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start here on line 74 we're going to type

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new....

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....

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...

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...

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....

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...

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...

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....

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...

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...

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alright so you can see what we've done

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there we created a new instance of the

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body class called new_body

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then we added to several tags to it we

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then assign the new_body object to our

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documents_body attributed

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directly if you run this program now

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you've see we got a test 2 . HTML showing

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their now and if I right click that now

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and open in browser and select a

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chrome in my case you select whichever

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browsers relevant you can see that we've

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got the new output showing their

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let's just go back to the code now so

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the point here is that if the my_

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page object is deleted then the

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new _ body object continues to

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exist and could actually be used in

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another document if we wanted to do that

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now as I said in python the

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distinction can get a little bit blurred

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so what I'm going to do is change the HTML dot

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class so that it's written to use

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aggregation rather than composition

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which is what you'd have to do in

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languages like Java if you wanted an

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aggregate class rather than a composite

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one we go back up to the HTML . class

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we defined here and what we're

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going to do first is we're going to

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remove the title and we're going to add

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some parameters going to add the

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doctype the head and the body and then

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we're going to initialize them so we're

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going to put...

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....

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...

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....

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...

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...

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....so now that we've done that in our HTML

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. class we need to provide instances

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of all three classes when we create a

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new html doc object now my

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original code that we have defined down

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here for my page my _ page

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won't work now so I'm going to actually

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comment that out will comment out the

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entire bit of code there and by doing

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that you can see these errors further on

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down in the code but we can ignore

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them for now because i'm going to go

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once we've created a new document

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so below that now i'm going to type in...

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....

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..

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...

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...

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...so the are only going to be

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executed if the if that we defined

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on line 66 returns true but what we do need to

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do after the new body the adding of the

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three tags is we need to assign new doc

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type so let's go ahead and do that so....

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...

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...

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...so what we should probably should

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have done with all these code is have

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it indented anyway so I'm going to just go

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ahead and do that so we're going to

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indent to that level and

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will delete the comment because we no longer

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need that anymore

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delete the my page that's because we're

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going to change the definition of that

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my page will become...

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...

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...

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we're going to leave the width open

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there because that's now valid because

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we've recreated our my_page

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variable but we will do is just change

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the filename so its test three .

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HTML

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it's not trying to override the previous one

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we've created right so let's just open

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the project pane and run to

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confirm that it does work works ok and

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got test three dot HTML so let's open in

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browser open in chrome and we've got

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working as you can see again we

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don't see the title bar but we just go

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back in there

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view page source you can see that the

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title is actually showing in the HTML

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and depending on your browser you may

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see that title appearing for you but for

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me the way that I've configured

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chrome its not showing ok so the

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difference now is that the objects are

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created independently of my _ page

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but then passed to the constructor to

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create the new document now when it's

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done that way

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in other words when the class is

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rewritten to use aggregation rather than

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composition then the difference between

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aggregation and Composition becomes more

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obvious note that in both cases we

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still got has a relationships our new

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my_page object still has a

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doctype has a head element and has a

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body it's just what owns those objects

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that's changed now if we delete the

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my_page variable the other

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three objects obviously continue to

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exist and can be used elsewhere in the

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program and that's obviously because

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we've actually defined them as

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separately here on line 74 line 81 and line 82 so

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that's aggregation compared to

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composition so the question now is which

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one should you use and when and the

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answer to that is that it depends

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so in this example you might want the

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ability to swap out the documents

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contents that can be useful and web

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pages often swap out content in response

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to selections for menus for example now

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admittedly they tend to work on div tags

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rather than swapping the entire body

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section but the principles the same

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now it's very unlikely that you want to

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change the doctype so there's no real

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reason that you couldn't use composition

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for the doc type and maybe even the head

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element while using aggregation for the

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body design a structure of the classes

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that make up a program is arguably the

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hardest part of object-oriented

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programming and you have to think

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carefully about how the various classes

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will work together and also what sort of

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things you want to do with them now

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sometimes the relationship is obvious

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thinking back to our player class in the

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game program we probably want to give

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the players weapons of some sort

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now there's an obvious has a

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relationship there a player will have a

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weapon now the weapons will probably be

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strung across the dungeons waiting to be

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found by the players so they need to exist

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independently of the players the players

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will also exchanged their weapons when

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they stumble across better ones

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so in that scenario you'd probably be

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better off choosing aggregation rather

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than composition but other times such as

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our document here the relationship is less

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clear you could make a case for either

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aggregation or composition when

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considering the body of a document it

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really does depend on what you want to

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do with it as we saw changing the

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composition relationship to an

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aggregation one isn't really that

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difficult

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we created the test 2 HTML document by

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accessing the quote-unquote private body

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attribute and replacing it if you found

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your class was used like that a lot

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then you could add a property so that

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the body attribute was no longer hidden

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and we saw how to do that a little bit

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earlier in this section

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alright so let's move on to the next

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video