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Arrays and Command Line Arguments
5:34We will wrap up exploring how to pass arguments into your Java application as the program starts. We will have a well deserved introduction to the basics of arrays and how to access their different elements.
It's hard to believe we've made it this
far without actually directly talking
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about arrays.
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We've definitely seen them, and
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in fact as we saw briefly strings
are just an array of characters.
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Arrays are basically ways to store
multiple items in a single variable.
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You can get access to the different
elements in the array and
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you can use other properties
that help you loop through them.
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You can plan to learn more about these and
other data structures in upcoming courses.
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I'm introducing this to you now however
because it's the last part of the Java
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boilerplate code that we have to go over.
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So we want to pass in the puzzle phrase
when we call our hangman program.
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This is done by passing in an additional
argument to the Java program,
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here let me show you how to do it.
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All right so let me show you how
to make one of these arrays.
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So we'll make a single variable so I'll
do string, and then we're gonna do these
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brackets, open bracket, close bracket and
we'll call that fruits.
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And an array literal starts with
a curly brace on some languages it
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uses the indexing thing there but
Java we use the curly braces.
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So just be careful there,
banana and cherry.
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And now we'll have this thing,
this object.
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It's a single variable but
we can access it by different indices.
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So arrays are zero index based,
so the first one is zero.
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So if I do fruits[0] I'm
gonna get back "apple".
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Cool, and I can also have, there is a
property on these called fruits.link that,
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show me how long my array is.
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So there's three entries there, now, for
more on arrays, check the teacher's notes.
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But what I want you to see right now is,
look at how this is declared.
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And see this is declaring
an array of strings.
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And if we look at our main method here,
it's an array of strings.
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So what we wanna do is we wanna make
sure that somebody passes in a value.
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So, let's do that, so we'll say,
if args.length, right.
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So we know that it's an array that
has something if it's equal to == 0,
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means they didn't pass something in,
right.
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And what we'll do is we'll warn them,
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we'll say System.out.println("Usage:
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java Hangman, and
then we'll just format like this.
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This is kinda standard command line CLI
sort of things so we'll say system.
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There's another thing we,
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have out and in, we also have an error and
this'll be a little bit louder.
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So ("answer is required") is
the error that we're gonna send.
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And then, I don't know if you
remember this or not, but
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we can exit out of our program
by saying system.exit(1).
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And now none of the rest
of this will matter.
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So now we know that we have,
a value in args, if we get here we know.
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So we're gonna,
instead of treehouse let's go ahead and
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let's use args[ 0], cuz we know
that there's at least one there.
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So now if I try to start this program and
I don't specify it,
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I'm gonna go ahead and get out of here.
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So let's go ahead and we'll say clear and
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Hangman.java, and java Hangman and
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I didn't specify anything
there at the end.
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So is gonna tell me that I made a mistake.
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Oop, so there is it is so Usage: java
Hangman is required answer is required.
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Awesome, so we block it, so
let's apply a parameter.
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So I'm gonna use the up arrow and
here we go.
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And now we have to guess
what you actually are
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P-R-O-G- A-M-E.
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Programmer, you did it, one more
thing I wanted to show is, look how
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the class here is declared as public, this
hangman class is declared as public and
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you've probably seen that in all
the classes that we've done.
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Now typically this is
something that you wanna do.
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If you refer back to the access level
modifier table that I shared in the very
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beginning of this course you'll see that
classes actually fall under the same
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access that fields and methods do.
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So if we wanted a class to be used
outside of it's package, right.
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So right now it doesn't have any
declaration or other classes here, right,
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or prompter.
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And our Java just says class, anything
outside the package couldn't access it.
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So we should really be professional and
put public in here.
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Now I did that because
there's a little trick with
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Jshell that lets you import the classes
if you don't put public there.
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So that's why we did that awesome,
so now it's labeled properly.
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You know how arrays work,
you can play this.
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Glad we finally got to go over arrays.
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You'll end up using these a lot and
we'll talk about those and
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more handy ways to structure
your data in upcoming courses.
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You've learned a ton and I love that we're
now at a place in our learnings where we
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don't need to ignore things in
the boilerplate code anymore.
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Let's look at it one more time just to
show you what a good grasp you have on it.
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So again when we run the Java
program we pass it a class name.
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What that does is it looks for
a public method named main in that class.
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Because it is a class method
not a instance method
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it is marked with the keyword static.
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The method doesn't return anything at all,
so its return type is void.
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It then takes any arguments it was passed,
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you can pass multiple
arguments with spaces.
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It puts together an array of strings and
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then passes that as an argument into that
method which you can access through this
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parameter here called
args in your program.
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Pretty cool right.
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