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(upbeat music)

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(keys clicking)

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Okay, so we've looked at declaring variables

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and using them in calculations.

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We've then looked at using String interpolation

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to print out more descriptive messages

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with our variables.

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Now we've got a couple of things to cover in this video,

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and I'll start with why I've used var

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for some of the variables and val for others.

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And I've actually got an admission to make,

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as this code is at the moment

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I should've used val everywhere.

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But then I wouldn't have had any familiar example code

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to use for this video.

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Alright, so what does this val and var mean,

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and what's the difference between the two?

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Well val stands for value, and var is short for variable.

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Now, we can get a clue as to what the difference is

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by looking at the third warning tick

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that Android Studio is showing in the right margin.

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So if you come down here and just hover over that,

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you can see that it says "variable is never modified

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"and can be declared immutable using val."

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Now immutable here means that it can't be changed,

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which means that once it's been given a value,

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that value can't change.

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Now, that may seem like an odd thing

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to want to do to a variable.

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After all, they're called variables,

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which implies that their values will change.

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And that's true, it's often necessary to assign

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different values to a variable.

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In that case, they'd have to be declared using var

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rather than val.

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Very often, though, you'll assign a value to a variable

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and never change it.

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Now, that might sound surprising, but it's extremely common.

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You'll see examples of that when we look at functions

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and classes later in this section of the course,

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but as an example of this,

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look at the declaration here on line 5 for the tim variable.

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Now I could probably imagine an extreme set of circumstances

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that would result in my changing my name

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and going on the run and going into hiding.

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It's possible, perhaps, but I think I can safely guarantee

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that my name will remain Tim Buchalka,

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I won't change in the time I'm recording these videos,

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at least, if ever.

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So the point is that there's very little point

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storing my name in a var variable.

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Once it's stored, it's not going to change.

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Alright, so that's fair enough,

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but what's the big deal?

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After all, we just have to avoid changing it,

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and everything's fine, right?

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Now this bit's going to be a little bit contrived,

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and we haven't got a lot of code here,

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so it's gonna be obvious where I'm about to go wrong.

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But remember that a real programme will have hundreds

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or thousands of lines of codes

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split over loads of different files and classes.

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So what I'm gonna do here is start by adding

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another variable, Jim,

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and I'm gonna do that above tim on line 5.

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So V-A-R, jim, J-I-M, colon, String.

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Alright, so now that we've done that,

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we're gonna go down to the code

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right down to the bottom here,

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we're gonna give jim a value.

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So right after the last println

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we're going to put tim equals James T Kirk.

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Now when I run the programme,

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you can see that it says "My name is James T Kirk,"

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so I'm no longer Tim Buchalka.

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I've become the captain of the enterprise instead.

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The rest I've said, this is a bit contrived,

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but errors like this are surprisingly easy to make

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when you're dealing with hundred of variables

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and thousands of lines of programming code.

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Now, we can prevent bugs like that from creeping in

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by declaring variables as val

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when we don't expect them to change.

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So I want to go up here into line 6

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and change the var to val.

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And scroll down and have a look,

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the code now would no longer compile a run.

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We're actually getting a warning here,

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an error rather saying val cannot be reassigned.

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So it's an error showing that

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when I incorrectly try to assign

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a new value to tim at the end of the code.

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So the Kotlin compiler is catching the bug for us

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and not allowing the code to compile.

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So as a general rule, always use val

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unless you're sure that you're going to change

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the value of a variable.

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Now if you find that you do need

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to assign a new value to a val,

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you can always go back and make it

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a var instead at that point.

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Alright, so I'm going to correct that rogue line

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so that we assign to jim instead of tim.

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And let's also print something.

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We'll actually replace line 32.

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What I'll do is come down here

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and on line 33, we'll change that to put

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"jim is dollar jim."

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Okay, we'll run that code.

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Quickly now, showing "My name is Tim Buchalka"

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and the jim variable is correctly assigned

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to the value of James T Kirk.

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Alright, so Android Studio is helping to identify

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variables that don't change,

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and all uses of var are actually highlighted.

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We can actually start by clicking on var on line 5,

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over here, and we get the tool tip

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suggesting it can be a val as you can see here.

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"Variable is never modified

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and can be declared immutable using val."

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But even better, though, if I double click it,

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we get this little light bulb up here

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in the left-hand side,

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and we can come down here, we can click on

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make variable immutable, and that changes that,

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as you can see, to a val now.

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And obviously, that warning then disappears.

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Right, so let's do the same for the other vars.

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So I come down here, we can declare that,

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by double clicking it, make variable immutable.

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Same for timsMonthlySalary, double click the var,

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change that to make variable immutable.

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We can come down here to fruit, double click,

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you can also make that variable immutable as you can see.

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And that's all the vars now converted to val.

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In this case we can do that

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because we're only assigning a value to them once.

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Alright, so let's now have a look at some of the warnings

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that are still showing in the right hand margin.

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Now the first one, if you look carefully,

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it looks like it's changed colour slightly.

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It's now a suggestion where as before

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it was showing us a warning.

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Now suggestions aren't as important as warnings,

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but your code will be more readable

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if you take note of them.

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So this one here is saying "Can be joined with assignment."

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And by the way, clicking on one of these warnings

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will take you to the line of code that's applicable

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for that particular warning.

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Now we can come over here, double click it again,

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and we can come down and select the light bulb,

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and we'll click on "Join declaration and assignment."

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And you can see that that's pretty impressive.

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It's pulled the assignment of the jim variable

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from the end of the code and added it to our declaration.

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So jim's now initialised at the point where it's declared,

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and that's good programming practise.

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So I can do the same for tim on the next line,

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just by double clicking on it and selecting the light bulb,

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clicking on "Join declaration and assignment."

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Alright, so let's look at some of the other warnings now.

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"Explicit given type is redundant,"

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and they're actually all the same

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at this point forward, now.

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All these warnings are all the same now,

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"Explicit given type is redundant here."

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Now, I said that you have to tell Kotlin

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what type of value each variable is gonna be given,

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but that's not strictly true.

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Now, every variable has to have a type,

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and once you've made it a String, say,

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you can't go assigning a number to it.

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But we don't have to explicitly tell Kotlin the type.

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Kotlin's very clever and can work out

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what the type should be most of the time.

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Now the first warning is here on line 5,

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I click on over here where we assign

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a String the the variable jim.

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Now if we're making jim equal to the String,

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then double quotes and whatever the name of the String is,

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then jim's obviously a string.

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So in other words, if the value

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is surrounded by double quotes,

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quite clearly, that's a string.

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And that's what the warning's saying here.

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Kotlin can work out that jim is of type String,

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so telling it that it's a string is redundant.

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And again, what we can do here is just to double click,

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click on the light bulb, and we can click on

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"Remove explicit type specification."

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And now that I've done that you can see it just says

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val jim equal double quotes James T Kirk

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and then another double quote.

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So result, I think you'd agree that line 5

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now is a lot easier to read than line 6,

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and we've removed that redundant type

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and we've actually removed that warning as well.

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Now when the code's compiled, the Kotlan compiler

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will infer the type that jim should be.

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Now, you'll come across the term "type inference"

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in the documentation, and that's what it means.

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Kotlan can work out what type something is.

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Right, so I'm gonna use the light bulb now

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to actually remove the remaining redundant type specifiers.

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So I'm gonna start with the next line,

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I'm gonna double click on that.

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I'm gonna change tim, remove explicit type specification.

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And the next one, timsWeeklySalary.

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Gonna remove explicit type specification.

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Next one, timsMonthlySalary,

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remove explicit type specification.

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Now that can also be inferred, but there's no warning,

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but the light bulb did appear when I clicked on

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the variable while type,

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and also appeared by the white,

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and you can, by double clicking on the extra variable top

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that'll produce a light bulb

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or make the light bulb appear as well.

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I can click on "Remove explicit type specification

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for that as well.

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And continuing on down here, I can double click on apples.

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Remove explicit type.

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Oranges, remove explicit type.

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Fruit, remove explicit type.

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Weeks, remove explicit type.

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Years, remove explicit type.

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Okay, so that's basically all the changes there.

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Now that's actually removed all the warnings,

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and our code's more readable without that clutter.

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Notice now that I've done that,

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in the the top right-hand corner here there's a green tick.

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Now that's telling us that there's no other warnings.

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We've got these other things here,

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which aren't warnings as such.

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So this is what you should be aiming for in your code.

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A green tick to indicate that

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there's no outstanding warnings or suggestions

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and obviously, no errors.

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Now it's always worth checking out these

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warnings and suggestions, as they'll often help you

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to write more idiomatic Kotlin code.

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You can ignore them, but paying attention to them

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and acting on them can improve your code.

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Alright, so I'm going to stop this video here.

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In the next one, we're going to look at

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how our code can make decisions.

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See you in the next video.