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‫Welcome back.

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‫In this video, we're going to finally fix the problems that we had in our program here, our virtual

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‫program that we created.

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‫And we're going to also look into defensive programming.

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‫So the idea of defensive programming is to make secure code that really already thinks about what problems

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‫could arise, because in this case, the problems that could arise is that, for example, the list

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‫is empty, or we want to have too many friends, so we want to invite more friends than we have.

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‫And in this case, the list is a super simple one that we created manually.

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‫But imagine it's from a database.

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‫So it comes from a database or it comes from the web, from a cloud database or something like that.

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‫And then it would be a super different story because we would never know how big the list is in this

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‫case.

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‫We can see it in the code, but we might not know it depending on where it comes from.

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‫In this case, we have to anticipate different problems that could arise and we can test them ourselves

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‫when developing.

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‫For example, we already did that, right?

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‫We entered more friends that we have in the list.

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‫So it's a simple one.

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‫We just run this and then we get this argument out of range exception.

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‫But where we are getting it is an interesting one.

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‫So we are getting it at the point of our get party different method even though the problem lies somewhere

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‫else.

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‫Because if we look at it, it says shortest name list zero.

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‫So we're trying to access a list element that is not available because we are past the list, get party

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‫friend and we pass this list over here.

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‫So if we look at where we get this list from, that could be a good starting point because we can see

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‫here we're trying to access something that is not available.

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‫So what is the problem with what is made available?

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‫So we can look at this line here, get party friend.

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‫That's where it's called.

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‫And we can see.

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‫All right, that's the buffer.

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‫Right?

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‫So we get a buffer, which is this list over here.

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‫So what we're getting is a list from the top level.

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‫So again, we can see, all right, we get this list of so so-and-so, so many people, and this is

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‫where we get the value from.

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‫And well, a good point to check if the list is good is actually in our Get Party Friends method because

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‫here we get the list, right.

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‫So at this point we have a list so we can check this list.

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‫We can see if this list is good enough for what we want to do.

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‫And what we want to do is to write something based on the account of elements in that list so we can

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‫check if the count is greater than list dot count.

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‫And this is the list that was passed to us.

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‫So if that's the case, what we can do is we could just print something, don't do that or error or

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‫something like that.

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‫But an even better thing that we can do is we can make the program crash.

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‫So we know that the program has a problem, but at the same time, we stop the program from executing

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‫at a very early stage because imagine we have a connection to a database and now we are creating all

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‫of these different errors or we're just going through the code and this method get party friend is running

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‫and let's say it's connected to database and it write something into the database and then we run the

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‫code again.

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‫And for some reason the database state has changed and now we're mindblowing because the data that is

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‫displayed to us is complete nonsense and we cannot work with it.

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‫So fixing that bug would be a real problem.

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‫All right.

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‫So we can already catch it as early as possible so we can simply throw an argument out of range exception

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‫here.

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‫So throw new argument out of range exception because that's what we were getting right.

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‫And an argument out of exception has multiple overloads.

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‫So we can have this one here with the param name.

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‫We can have a message with the exception or a param name with a message.

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‫So here I could just say something like, okay, the count is the problem and count cannot be greater

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‫than elements in the list or something like that.

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‫So that would be an example for what we could display.

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‫So let's run it again because now we should be able to actually catch this error or at least throw an

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‫argument out of range exception once we get this error.

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‫All right.

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‫So we can see exception on handled it says system argument out of range exception cannot be greater

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‫than elements in the list parameter count.

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‫So that's what is the result of what we have written here.

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‫So this is how the exception will look like.

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‫So now we know what the problem is.

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‫It's a little more expressive.

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‫Right before the problem wasn't actually here.

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‫So let me comment those two lines out.

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‫The problem wasn't thrown at this point, but it was thrown further down.

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‫It was thrown in our get different method.

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‫So it's in line 42.

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‫But the actual problem arose a lot earlier.

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‫Right.

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‫And index was out of range exception.

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‫That's also a very generic statement, right.

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‫It could be anything.

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‫Right.

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‫So catching it earlier helps us to see where the problems are that we have.

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‫So what we could, of course do is we could say, okay, once we enter this value and let's say this

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‫value comes from a user that enters a value, right?

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‫So you can enter any value from zero to the amount of list elements that we have.

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‫So we could catch it even earlier.

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‫We could do a test even here.

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‫So if.

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‫Friends.

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‫Dot counts is.

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‫Less than the value that we enter here, and we could get this value again from the user.

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‫Then we could give them a message, right?

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‫We wouldn't throw our exception.

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‫We wouldn't make our program crash.

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‫We would just ask the user to enter a new value.

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‫But we're not doing it here because we as a programmer can enter the value here and we force these errors

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‫manually right now.

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‫Right.

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‫So that's one way to solve it or actually one way to catch it early on.

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‫Now, let's say.

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‫We enter a stupid value here, such as minus one.

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‫All right, now let's run it again.

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‫Actually, let's run it again.

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‫And our program seems to work, but it doesn't show anything there.

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‫So it didn't show any.

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‫Errors didn't throw any errors, but it doesn't show any friends.

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‫Well, our program seems to work with minus one because we try to get minus one list elements from our

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‫list and well, it doesn't give us any valuable information.

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‫So now what we can, of course, do is we can add another condition here.

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‫The count should be greater equals zero.

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‫If that's not the case, then we could write something like called cannot be greater than the elements

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‫in the list or lower zero.

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‫So that's just one way of expressing it.

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‫And now if we run it again, we will throw this error.

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‫So that's something where the program didn't throw an arrow before, but now we as a developer understood,

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‫okay, this is a state that we don't want to have.

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‫This is a situation that we don't want to work with.

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‫And as I said, we would cut well, we would try to get this value from somewhere and we would try to

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‫catch it even before that.

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‫Now, let's assume we have a situation which is even funnier.

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‫So we have a situation where my friend's list is empty.

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‫So I'm just going to create an empty list here.

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‫Like, so now this will be commented out and now we have this empty list.

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‫Let's run it again and see if our program still works.

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‫And now we still get our argument out of range exception.

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‫So that's pretty cool.

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‫So it seems the exception that we created here fixes even this problem so we don't have to manually

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‫look into what the list is like even though it's empty, right?

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‫So if we look at our friends list, our friends list is empty, but in the end we get a count of zero.

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‫So even though the list is null, the list is still zero.

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‫So the value that we get there is still zero.

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‫But that isn't all we can actually force an actual nul.

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‫So instead of passing France with France which is this empty list, we can actually pass null here.

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‫So let's say for some reason we couldn't create this list because the database connection didn't work

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‫or something like that, and we get a null as a result and now we are passing null and we try to pass

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‫null into a method that actually requires a list.

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‫So let's run it and let's see what kind of error we get this time.

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‫So now we get a null reference exception object reference not set to an instance of an object list was

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‫null.

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‫So the list that we passed now was now.

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‫So we get this error even before we can throw an error.

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‫So what we could do, of course, is we could catch this situation as well.

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‫So we could go ahead and write another if statement.

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‫If list is null, then throw a null pointer.

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‫Exception throw new argument.

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‫NULL exception.

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‫So list.

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‫The list is empty.

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‫All right.

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‫Of course, you can be more descriptive here instead of the list should not be empty.

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‫Please check the Internet connection or something like that.

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‫It could be the Internet connection or wherever you get your data from.

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‫So let's run it again.

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‫And now we should throw our argument now exception.

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‫As you can see, the list is empty parameter list.

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‫Perfect.

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‫All right.

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‫Now, you might wonder, why are we doing all of this?

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‫I don't want my program to crash all the time.

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‫Well, we do this in order to for our program to not get into the state where it would actually change

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‫something in the database.

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‫We're not changing anything in the database, in our little program.

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‫But imagine it would be the case then.

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‫It's a very different situation and we can create some serious problems here.

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‫Now, I want to show you one final thing, because before we had all of this, right?

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‫So before we had this if statement and this one and that one and so forth, and before we had this friends

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‫list.

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‫Yeah, I can actually stop.

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‫Uh, before we had this friends here, and we passed friends over there, and then we try to have ten

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‫friends at our party, but we only have seven friends.

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‫So at this situation, once we run this, we can do is we can force this error that we had and the error

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‫was in our get party friend.

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‫So we get this argument out of range exception index was out of range.

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‫So that's a problem that we had before we fixed it.

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‫Right.

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‫But now the interesting thing that we can look at is the call stack.

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‫So there is something called call stack.

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‫And let me make it a little bigger here.

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‫Actually, let me close all those down.

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‫So now let's look at this call stack.

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‫This call stack here says that we are currently in line 46.

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‫All right.

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‫That's where we are.

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‫So we are in the get party friend method.

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‫Now, if we go one step higher, we can see that this is where to get party friend method was called.

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‫Now we are inside of the get party friends plural method.

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‫And what if you want to know where this method was called?

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‫Well, it was called in line 13.

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‫So with the call stack, we can see from where we were called, from where we were called again and

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‫so forth.

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‫So we can go up all the way to the level at which we what the initial method was called.

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‫And the initial method was get party friends was called.

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‫Then we go into this wild loop, then got get party friend is called, then we get into this method

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‫here and then finally our variable is created based on a list that is empty.

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‫All right, so I hope this gave you a little introduction into debugging.

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‫Of course, there is a lot more to know, but I don't want to bore you with all the details because

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‫some of the things are useful, but many of them will not be required in your day to day life.

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‫Because, I mean, in general, you can, of course, go ahead and go to debug and then check out all

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‫of the different windows you can get here and you can look into the threads that are running and parallel

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‫the processes and so forth.

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‫But I think what you have seen now is really going to be the main things or are going to be the main

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‫things that you're going to require, the main tools that you're going to require in order to debug

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‫your software.

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‫All right.

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‫So I hope you enjoyed this and I hope you can make use of this in your future programs.

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‫And now you can finally see how debugging is solved.

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‫And by the way, thanks for giving this feedback because I got the feedback that a debugging section

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‫was missing, so I decided to create this little section.

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‫So thanks a lot for the feedback.

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‫If you have anything that you think is missing in this course or if you think that something is very

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‫bad, please don't give me a bad rating directly, but let me know what it is so I can improve the course

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‫because my goal is to make this the best C-sharp course on the market.

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‫So thanks a lot for your help and I hope to see you in the next video.