Prototype your Java with JShell | Java for Beginners

Aug 15, 2023

Prototype your Java with JShell | Java for Beginners

Microsoft Principal Architect for Java and Java Champion Ed Burns introduces the Java Shell tool (JShell), an interactive command line tool for working with Java code.

Check out the rest of the Java for Beginners series at https://aka.ms/javaforbeginners
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Content

0 -> [MUSIC]

9.33 -> >> Greetings, everyone. Welcome back to Java for Beginners.

13.185 -> I'm joined by Ed Burns,

15.45 -> who's going to talk to us about JShell.

18.15 -> Ed, do you want to introduce yourself

19.95 -> and tell us a little bit about our topic today.

22.815 -> >> Thanks, Brian. I'm really excited to be in the Java for

25.86 -> Beginners series and especially talking about JShell,

29.43 -> which is a technology that's really great

31.41 -> for beginners because it strips out

33.66 -> all of the other noise that you don't have to

35.76 -> worry about with Java and lets you focus just on the code.

38.475 -> It's even more lean than the leanest IDE you can possibly imagine.

44.01 -> It's a read-evaluate-print loop for Java.

46.56 -> It has full access to all of

48.12 -> the classes that you would load and unload,

51.405 -> being able to access the class loader in

54.54 -> a command line fashion into a shell environment.

57.555 -> For developers that are just coming to Java,

59.94 -> it's a great way to just copy, paste, code in,

62.425 -> type it in, edit the code and see what it does.

66 -> It's a very good beginner's tool.

69.74 -> In this brief video,

71.89 -> we're going to talk about JShell,

73.28 -> the shell for Java.

75.01 -> It's a command line tool and it's a part of the JDK.

78.565 -> If you have Java 9 or newer,

80.62 -> you already have it installed.

82.925 -> It's useful because it's a command line

86.37 -> read-evaluate-print loop for Java.

88.53 -> Think the approachability of

90.18 -> basic programming with the power of Java.

93.15 -> This means it's a really great tool

94.98 -> for exploring new language features.

96.69 -> There's no compilation step,

98.43 -> no build process, no Maven or Ant.

100.92 -> You can really just get in and get things started very quickly.

106.69 -> It was developed like everything else in

109.37 -> core Java with community governance and transparency.

112.79 -> If you want to see the original Java enhancement process request,

116.57 -> you can follow this link here and in the description below.

120.57 -> Everything done in the core JDK is done with the JEP process.

124.915 -> Everything done in Java EE is done with the JCP process.

129.49 -> A five minute video like this is not intended to

132.07 -> teach everything there is to know about anything.

134.925 -> But JShell on self has

136.57 -> very excellent context aware in product help.

139.69 -> If you want to look for the definitive guide,

141.7 -> you can follow the documentation below.

144.065 -> Let's start it up. Here's a pro tip.

147 -> We are using -v printing arguments,

150.165 -> -v is for verbose feedback.

152.305 -> Printing is a shorthand to cause JShell to import

155.5 -> the most common language features

156.91 -> related to printing on the console.

159.025 -> Let's take a look at that context

160.84 -> sensitive help that I mentioned earlier.

162.775 -> Everything you type in JShell is treated as a snippet of Java,

166.465 -> except if you start with a slash.

168.57 -> If you start with a slash,

170.05 -> you're talking to the JShell interpreter.

172.105 -> These are called slash commands and it's a tip of the hat

175.66 -> to the late 90s IRC text chat tool,

179.58 -> or as us olds like to call it slack without emojis.

183.085 -> Here's another pro tip.

184.9 -> There is full tab completion.

186.49 -> Pressing the tab key will cause the possible valid inputs,

189.4 -> giving the current context to be presented,

192.04 -> prompting you to choose the right one.

194.005 -> Let's talk about some foundation concepts.

196.42 -> Aside from slash commands,

198.01 -> everything input into JShell is a snippet.

200.5 -> There are three kinds of snippets.

203.23 -> Generally, you don't need the closing semicolon.

205.58 -> JShell can usually tell when

207.01 -> one is required and automatically add it.

209.23 -> A snippet is either an expression,

212.44 -> a statement, or a declaration.

214.57 -> Declarations can be either non-nested.

217.305 -> That is top level declarations that stand on their own or nested.

221.215 -> That is declarations that make sense within other declarations.

224.725 -> I'll explain each one of these now.

227.285 -> Let's type an expression.

232.13 -> 2 plus 2 is an expression.

235.28 -> When you type an expression,

236.77 -> JShell will assign a scratch variable to it,

239.83 -> so you can refer to it later.

241.81 -> You can see all of the currently known variables by doing /vars.

249.88 -> You can see we have an assigned

252.52 -> $22 to the expression we just typed 2 plus 2.

257.465 -> Let's talk about statements.

259.59 -> A statement is a Java statement,

262.54 -> just like what you'd expect.

264.175 -> Hello world is a statement.

266.44 -> You can say print "Hello world"

268.285 -> or if we're coming from a C background,

270.775 -> we might say printf, percent s, then exclamation point

274.44 -> "Hello world, " and it will

276.57 -> give us the same output with the exclamation point there.

279.105 -> Those are statements.

280.98 -> Let's talk about declarations.

283.525 -> In Java, you declare something like

286.14 -> a class so you can use it later.

288.6 -> The complete list of declarations in Java is import, method,

293.16 -> class, field, and interface.

296.91 -> First, let's review nested versus non-nested declarations.

303.35 -> There are some rules for

305.89 -> non-nested declarations which you can read here.

308.5 -> Generally these special rules don't get in the way,

311.555 -> but they are necessary to allow JShell to do its job where you're

317.18 -> basically typing in your statements

320.105 -> at the top level of the Java state.

324.17 -> Let's do an import.

326.14 -> The first kind of declaration is an import.

328.6 -> We're going to import scanner, java.util.Scanner.

332.425 -> If you do /imports, you can see all of

335.05 -> the imports that we have currently in the state.

337.87 -> The ones I've shown above are imported by

340.72 -> either the -v printing or the default load up startup ones.

348.7 -> That's imports.

350.785 -> Let's talk about methods.

352.81 -> Here's a simple method declaration.

355.405 -> This is a loan calculator.

357.25 -> These sample methods and classes are part

359.65 -> of Java Champion and educator Ken Fogel's

362.17 -> Java in Education Resources.

364.15 -> The link to these and other classes is in the description below.

368.375 -> Let's go ahead and paste that in and declare created method loan.

374.02 -> We can invoke the method just like we would in Java by

376.87 -> doing a parentheses after the method name.

379.75 -> We'll type in the term,

381.61 -> the interest rate and the amount of the loan.

387.99 -> We see that our monthly payment is 94.36.

392.41 -> If you'd like to see the method,

394.67 -> you can do /list,

396.43 -> and all of the things that we've typed in so

398.96 -> far will be printed out.

401.9 -> You can also do /methods and see the methods that we can call,

405.695 -> including the ones that were defined by -printing. That's methods.

410.6 -> We can put the same method inside of a class.

414.31 -> Here we have the same thing but is inside of a class and we

417.65 -> call the loan method from the main of that class.

421.295 -> We're going to declare the class right there,

423.98 -> paste it in, or you could type it if you like,

427.33 -> and now that it says created class JavaCalculator01,

431.925 -> you can do /types

435.1 -> and see that we have defined a new class there.

439.09 -> Let us invoke the loan method via

445.23 -> the main method and we're just going to say new

448.47 -> JavaCalculator.main and we're passing

451.48 -> the string array we need for the main method,

453.73 -> so we get prompted for the same set of values as before.

458.89 -> The next type of declaration is a field.

461.86 -> Now, fields can be defined at the top level,

464.38 -> the non-nested or, of course, as

467.065 -> member variables or static variables inside of a class.

471.205 -> Now, we can see we've got

472.885 -> i as the one that we've defined and also the

475.72 -> implicit one that we did with 2 plus 2 earlier, and that's fields.

481.04 -> Let's take a look at interfaces.

483.01 -> We will define a new interface that we can

485.77 -> slap on to our existing loan calculator class.

489.185 -> We'll define an interface called Banker,

491.05 -> which has a method called loan,

492.55 -> which is a coincidence since we've already defined loan.

496 -> Let us define the interface.

499.835 -> If we do types,

501.34 -> we can see we have interface Banker and class JavaCalculator01.

505.645 -> If we do list,

507.175 -> we can see the class does not implement the interface.

510.08 -> See, there's no implements there, so how can we fix that?

513.205 -> Well, we can modify the state

516.085 -> and this is one of the really impressive things about JShell.

520.45 -> All of the state is completely mutable,

522.46 -> so we're going to edit our class using tab completion there,

526.865 -> and we can see because I've specified the editor value of Emacs,

531.65 -> it loads up Emacs and loads the class there and we're going

536.48 -> to specify implements Banker and save the file.

542.71 -> If we do /list again,

546.325 -> you'll see that it now implements Banker.

550.395 -> That's the intro to JShell.

552.56 -> Stay tuned for the second part where

554.59 -> I show a practical application

556.72 -> of interacting with a Java EE plain old Java object using JShell.

562.95 -> Thanks for watching. Please take a look

565.72 -> at the resources for JShell in

567.61 -> the show notes and enjoy many of

570.04 -> the other episodes we have in the Java for Beginners series.

573.55 -> >> Thank you, Ed.

574.78 -> [MUSIC]

Source: https://www.youtube.com/watch?v=201IpTSRIGs