jpa00 : Getting Started

num ready? description assigned MW lect due MW lect assigned TR lect due TR lect
jpa00 true Getting Started Tue 09/27 05:00PM Wed 10/05 07:59PM

This assignment is jpa00, i.e “Java Programming Assignment 00”.

If you find typos or problems with the lab instructions, please report these via Slack:

• When class is in session (e.g. lecture or discussion) please use #help-lecture-discussion
• At other times, please use #help-jpa00, or if it is a configuration problem, use one of these channels as applicable:
• #help-macos
• #help-windows
• #help-linux-wsl

Goals

This lab checks that you can succesfully edit, compile, run and submit a simple Hello.java to Gradescope for grading.

This lab requires Java 17.

If you want to try to do this lab using CSIL, read this carefully:

• Java 17 was installed on CSIL on 01/02/2022, so we are hoping that you can do this lab on CSIL, but it may require some configuration of your CSIL account; instructions are included in the lab.
• If you do not do the configuration, you might get Java 18 instead. That’s likely not a problem for this particular lab (though I can’t guarantee that!), but it is likely to be a problem at some point. So good to get this sorted now.
• The short version is this: you need to set the environnment variable JAVA_HOME to /usr/lib/jvm/java-17-openjdk, and possibly modify your path.
• If you don’t know what that means, then read and follow the instructions carefully.

We want to encourage you to try to complete this lab on your own machine if possible. Installing a Java 17 environment on your own machine will make everything else in the course a lot easier; while this simple “Hello World” type assignment can be easily done on CSIL, working with full stack webapps on CSIL can be awkward, at best.

How to install Java 17 and Maven on your own machine

Note: these instructions were current as of S22. Things change every quarter—sometimes even from day to day—and the only way we find out is when students try things and tell us. (We don’t have the bandwidth to try all of the instructions on every possible OS version combination.)

• For MacOS, this is fairly straightfoward; see instructions here. If you need help, ask on the #help-macos channel on the Slack.
• For Windows, we recommend installing the Windows Subsystem for Linux (WSL), and then following the instructions for installation of Java 17 from this page.
• For help with installing WSL, you can ask on the #help-windows Slack channel
• For help after you’ve installed WSL, use the #help-linux-wsl Slack channel
• For Linux, there are instructions on this page. that apply to Debian/Ubuntu like systems. If those don’t work for you, ask the staff for help on the #help-linux-wsl Slack channel

Do I really need Java 17?

You may be wondering whether it’s ok if you have Java 19 or Java 18, or Java 11, or some other version of Java instead of Java 17. This short answer is: for this simple Hello lab, it probably won’t make a difference, but eventually it probably will. So if you are going to the trouble of installing Java on your system, try to make sure it’s Java 17, specifically, which (unlike Java 12, 13, 14, 15, 18 or 19) is considered a Long-Term Support release of Java.

Read more here if you are interested: https://ucsb-cs156.github.io/topics/java_versions/

Something for everyone to learn

Note that even if you have done Java programming before, there may be a few things about this Hello.java program that may be unfamiliar to you. These have to do with setting us up for real world programming practices used in large software projects.

• Rather than compiling with command line tools such as javac and running the program with the java command, we’ll be using a package and build manager called Maven. For a small Hello World type program, this will seem like overkill; but it will set us up for being able to manage much larger projects.
• We are setting our code up in a GitHub repo right from the start.
• We are going to be following the directory conventions required by Maven. So it’s important to have the correct directory structure within the GitHub repo.

There a few details, but they are all straightforward. It shouldn’t take very long, and if it goes well, you’ll be able to start on the next lab right away.

That one may actually take quite a bit more work.

Step 0: Getting oriented

Even if you aren’t using CSIL for this assignment, it’s still a good idea to make sure your CSIL account is working and configured.

If you’ve used CSIL before, there continue to be ongoing changes to how we interact with CSIL. So please read this carefully.

1. You should have a College of Engineering (CSIL) Account. If you don’t, you can create one by visiting https://accounts.engr.ucsb.edu

If this is your first Computer Science course at UCSB, you’ll definitely need to do that.

2. Once you have a CSIL account, please know that the way of remotely logging in to CSIL changed in Fall 2020.

Before Fall 2020, you were asked to not use csil.cs.ucsb.edu, but instead to use csil-01.cs.ucsb.edu, csil-01.cs.ucsb.edu, etc.

THE OPPOSITE IS NOW TRUE.

Going forward, until told otherwise please use only csil.cs.ucsb.edu, and do not use csil-01.cs.ucsb.edu, csil-02.cs.ucsb.edu, etc.

As far as how to login, we’ll cover that in in the next item.

3. If you’ve ssh’d to csil.cs.ucsb.edu before, but haven’t logged into that site recently, you might find that you get an error message when trying to login due to stale values in a file ion your system called known_hosts. The old entry in that file for csil.cs.ucsb.edu will need to be deleted, since the hostid value has changed over the summer.

Otherwise, any time you connect to csil.cs.ucsb.edu, the connection may be immediately closed.

The exact location of this file may be different on different systems, but its likely in a directory called .ssh under your home directory. Keep in mind that .ssh may be a hidden file.

So the command you need might be something like this:

• vim ~/.ssh/known_hosts to edit the file and remove the line for csil.cs.ucsb.edu
• emacs ~/.ssh/known_hosts if you prefer that editor
• rm ~/.ssh/known_hosts if you just want to delete the file completely.

Deleting that file will just mean that the first time you connect to any particular system, you’ll be asked whether you really want to connect to that system, and store it’s identifying information in your known_hosts file, and you’ll have to respond yes.

4. If you already have used an SSH client before, you can continue using that same client, but specify csil.cs.ucsb.edu as the hostname.

If you haven’t used an ssh client before, consult these pages for hints:

There are also channels on the course slack, https://ucsb-cs156-f22.slack.com, for #help-windows and #help-macos. Use the #help-linux-wsl if you need help for a system other than Windows or Mac (e.g. Linux, Chromebook, etc.)

5. Ideally, you will already know the following things from previous courses (CMPSC 16, 24, 32). It is possible that if you are joining UCSB for the first time in this course, some of this may be unfamiliar to you. The rest of these instructions will assume you know how to do the items in the list below. If not, then let a member of the course staff know, and we’ll point you to resources where you can come up to speed.

• knowing how to use a basic text editor such as emacs or vim to edit files. (Here, for example, is some basic instruction on vim )
• knowing basic Unix/Linux commands to create directories, change directory, manipulate files, i.e. commands such as: mkdir, cd, pwd, mv, rm, ls. If those topics are new to you, please reach out to the instructor to let them know, and ask for pointers to resources where you can study up on these skills.
6. We strongly encourage the use of VSCode as an editor this course. For this assignment, it will not matter what editor you use, but in future assignments, the use of an IDE will become more important. We’ll discuss this more in lecture.

The rest of the lab: Step-by-Step

Step 1: If you are on CSIL, configure your account for Java 17

If you are not working on CSIL

Otherwise, login to your CSIL account, and start by checking whether JAVA_HOME is already defined. Type echo $JAVA_HOME If it is not already defined, it should look like this: Good: [pconrad@csilvm-03 STARTER-jpa00]$ echo $JAVA_HOME [pconrad@csilvm-03 STARTER-jpa00]$


If instead it looks like this, then you have an old definition of JAVA_HOME associated with your account, and we’ll need to update it.

[pconrad@csilvm-03 STARTER-jpa00]$echo$JAVA_HOME
/usr/lib/jvm/java-11-openjdk
[pconrad@csilvm-03 STARTER-jpa00]$ In either case, what you want is to add these lines to your ~/.bash_profile file: export JAVA_HOME=/usr/lib/jvm/java-17-openjdk export PATH=$JAVA_HOME/bin:$PATH  If you already had a definition of JAVA_HOME it was likely in ~/.bash_profile, but to be sure, check also in ~/.bash_login and ~/.bashrc and remove any old versions of the definition of JAVA_HOME. After making this change, logout and log back in again, and check echo$JAVA_HOME. It should now say: /usr/lib/jvm/java-17-openjdk

The next thing to check is that when you type javac --version for the Java compiler, that you get version 17, like this:

[pconrad@csilvm-03 STARTER-jpa00]$javac --version javac 17.0.1 [pconrad@csilvm-03 STARTER-jpa00]$


Then check that the Java Virtual Machine (the java command) gives you the correct version:

[pconrad@csilvm-03 STARTER-jpa00]$java --version openjdk 17.0.1 2021-10-19 OpenJDK Runtime Environment 21.9 (build 17.0.1+12) OpenJDK 64-Bit Server VM 21.9 (build 17.0.1+12, mixed mode, sharing) [pconrad@csilvm-03 STARTER-jpa00]$


Finally, check that when you run mvn --version which is the command for Maven, that it is pointing to Java 17 as your Java version.

• Notice the line in the output that says Java version: 17.0.1, ...
• If it says something else, that’s a problem that needs to be fixed.
[pconrad@csilvm-03 STARTER-jpa00]$mvn --version Apache Maven 3.6.3 (Red Hat 3.6.3-8) Maven home: /usr/share/maven Java version: 17.0.1, vendor: Red Hat, Inc., runtime: /usr/lib/jvm/java-17-openjdk-17.0.1.0.12-13.rolling.fc34.x86_64 Default locale: en_US, platform encoding: UTF-8 OS name: "linux", version: "5.15.12-100.fc34.x86_64", arch: "amd64", family: "unix" [pconrad@csilvm-03 STARTER-jpa00]$


Step 2: Get setup with github and add yourself to our organization

We will be using github.com in this course. We have created an organization called on github.com where you can create repositories (repos) for your assignments in this course.

The advantage of creating private repos under this organization is that the course staff (your instructors and TAs) will be able to see your code and provide you with help, without you having to do anything special.

To join this organization, you need to do three things.

1. If you don’t already have a github.com account, create one on the “free” plan. Visit https://github.com/

2. If you don’t already have your @ucsb.edu email address associated with your github.com account. go to “settings”, add that email, and confirm that email address. (If you have an @umail.ucsb.edu address registered there, that also works.)

4. There should be a link to the invitation for the GitHub organization for this course (https://github.com/). Click on the invitation link and accept it. You can also go straight to https://github.com/ and see the invitation there (if you’re logged in). Accept the invitation that appears in your browser (from step 3) or log into your account on https://github.com/ to accept the invitation.

Step 4: Set up your local system

If you are doing this lab on CSIL (which you can only do if/when Java 17 is installed there), then you don’t have to do this step for now.

However, we strongly encourage you do to this step. The sooner you get a Java 17 + Maven environment working on your local system, the easier time you will have with the rest of the work in this course.

What you’ll need:

• git
• Java 17 JDK
• Maven

There are instructions here for various platforms.

Step 5: Configure your machine for git/GitHub

Whether you are working on CSIL, or on your own machine, you need to configure the environment you are working in for access to git/GitHub.

We want to be able to use git and GitHub with ssh links, so we need to set up public-key/private-key pairs.

We also want to set up git so that it records our commits properly.

1. git configuration: Detailed Instructions

2. Configure ssh keys for git
3. If you are brand new to git and github, review a few basic facts about git and github.com

Step 6: Finding your jpa00 repo on GitHub

Open a web browser and login to GitHub, then navigate to the course organization page, https://github.com/ucsb-cs156-f22.

You should see that there is a private repo in this organization called jpa00-yourGithubId, where yourGithubId is replaced with your GitHub id. This is the repo that you’ll be using for this assignment.

This is currently an empty repo. In the next step, we’ll clone this empty repo into a directory, either on your CSIL account, or on your local system.

Step 7: Cloning the repo

1. If you are working on CSIL, login to your CSIL account, create a ~/cs156 subdirectory, and change directory into it. Otherwise, create a directory somewhere on your machine for cs156, and cd into that.

mkdir ~/cs156
cd ~/cs156


You can actually use any directory you like, but for consistency, we’ll refer to ~/cs156 throughout the rest of the instructions.

2. Now, go to the github.com web page, and find your jpa00-userid repo. The page should look something like this:

You should see a button for SSH; select that button. Then there is a button to copy the URL shown; click that to copy the URL.

3. Now type this command, replacing url with the url that you copied.

That url should be something like git@github.com:ucsb-cs156-f22/jpa00-cgaucho.git but with your GitHub id in place of cgaucho.

git clone url


You’ll will see a warning message that you are cloning an empty repo; that’s normal.

Cloning into jpa00-cgaucho…
warning: You appear to have cloned an empty repository

4. If you use the ls command, you should now have a subdirectory called jpa00-cgaucho (except cgaucho will be your GitHub username.) Use a cd command to change directory into that directory, e.g.

cd jpa00-cgaucho

An ls -a should reveal an empty directory except for the .git subdirectory indicating that this is a GitHub repo.

% ls -a
.	..	.git
%


We are now ready to pull in some starter code.

Step 8: Locate the starter code.

First, let’s take a look at this remote on GitHub, here:

You should see that the README.md for this repo has an explanation of the contents of the starter code. Read though this explanation to learn more about:

• Maven
• the pom.xml
• the required directory structure

Next, we’ll add this starter code as a second remote for our repo.

Step 9: A remote for starter

If you’ve used git before, you may be familiar with the command:

git pull origin master


The word origin in this case refers to a remote, that is a repo that lives somewhere out there on the network.

The word master refers to the default branch of the repo. The default branch of GitHub repos recently changed from master to main; we’ll be using main throughout this course.

If you type the following command, you’ll see that origin is defined as a remote for the repo that you cloned from. Your output will look similar, except that you’ll have your GitHub in place of cgaucho:

% git remote -v
origin git@github.com:ucsb-cs156-f22/jpa00-cgaucho.git (fetch)
origin git@github.com:ucsb-cs156-f22/jpa00-cgaucho.git (push)
%

Now, we are going to add a second remote. This remote will use the URL for the starter code.

The image below shows how to copy that URL: (1) Click the green Code button. (2) Select SSH to choose that as the network protocol for the URL (3) Click the icon to copy the URL to your clipboard.

Then, use this command to add a remote called starter for the starter code repo:

git remote add starter paste-url-here


After this command, use git remote -v to list all your remotes. Your output should look like this (except your GitHub id in place of cgaucho):

% git remote -v
origin git@github.com:ucsb-cs156-f22/jpa00-cgaucho.git (fetch)
origin git@github.com:ucsb-cs156-f22/jpa00-cgaucho.git (push)
starter git@github.com:ucsb-cs156-f22/STARTER-jpa00.git (fetch)
starter git@github.com:ucsb-cs156-f22/STARTER-jpa00.git (push)
%

Step 10: Pull Starter Code into your Repo

The next step is to pull the starter code into your repo, and then push that code to your origin repo on GitHub.

Here are the three commands:

git checkout -b main
git pull starter main
git push origin main


After these three commands, go look at your repo on GitHub, i.e. the repo at this url (but substituting your GitHub id for cgaucho:)

You should see that instead of an empty repo, you now have a copy of the starter code.

The starter code should compile and run, and can even be submitted to Gradescope for a grade. Of course, it won’t be for full credit, but we can at least make sure that the mechanisms are working. So let’s give it a try.

Step 11: Compile and run the Starter code

To compile the starter code, return to a shell prompt in the directory where your cloned your repo. You should see, when you type ls, that the file pom.xml is in the current directory. For best results, you should always run Maven from this directory.

To compile type mvn compile.

• If you see the message The JAVA_HOME environment variable is not defined correctly... plus a few more lines of output, see this link for a fix.
• Otherwise, you should see no error messages
• There may be warning about missing resources and UTF-8 encoding, but you can safely ignore those for now. If you are curious, see the the section “Warnings you May be able to Ignore” on this page.

Then, type mvn package. You should see a lot of output, but somewhere in that output, something like this:

 [INFO] Building jar: target/hello-1.0.0.jar


That indicates that you have built a .jar (or Java Archive) file. This file is a compressed archive of all of the compiled Java code from your program. You can run it with this command:

java -cp target/hello-1.0.0.jar Hello


You should see output like this:

% java -cp target/hello-1.0.0.jar Hello
This is the wrong output!
%

• Note: If you are working on CSIL and see this instead:
Error: A JNI error has occurred, please check your installation and try again
...


\$JAVA_HOME/bin/java -cp target/hello-1.0.0.jar Hello


This assumes you’ve done the fix described here to define JAVA_HOME to point to Java 17.

The line This is the wrong output! is being produced by the line of code:

        System.out.println("This is the wrong output!");


You should eventually change this line to produce the correct output. But, don’t do that just yet. Let’s first see what happens when you submit a program with errors in it to Gradescope.

Step 12: Submit incorrect Java code to Gradescope

In this step, we’ll see what happens when you submit two incorrect program to Gradescope. We aren’t grading this step, so you could skip it, but we strongly encourage you to do it anyway, because it’s important to be able to understand how the autograders work on a simple case before dealing with a more complex case.

First, we’ll submit the starter code “as is” to Gradescope. Gradescope will be expecting a program that produces, as it’s output Hello, World! (followed by a newline).

Instead, your code currently produces: This is the wrong output! followed by a newline.

We want to see what the Gradescope output looks like in that case.

You should have an account invitation in your email. If you don’t, ask an instructor, TA or mentor for assistance.

To submit your work, you should be able to click on the GitHub link in Gradescope, and locate your repo. The first time you do this, it may take a while; be patient before giving up. If it still doesn’t work after a while, you can either (a) ask the staff for assistance, or submit a zip file as an alternative.

After you submit, it will take some time for Gradescope to process your submission. Once it’s processsed, you should see output similar to this:

The most important part is this:

FAILED:
expected:<[Hello, World]!
> but was:<[This is the wrong output]!
>


Note that it tells you exactly what was different between the expected and actual output (the part in []). The ! is the same in both parts, so it is outside the [].

Once you’ve understood this output, let’s move on and see what happens when you submit code with a syntax error.

Go into the file src/main/java/Hello.java, and remove the semicolon at the end of the statement:

 System.out.println("This is the wrong output!");


  System.out.println("This is the wrong output!")


This, of course, has a syntax error.

Try using mvn compile and see what happens when you submit this.

Then, commit this change to Github.

(Normally, we would NOT push code that has a syntax error, but for purposes of this experiment, we will.)

git add src/main/java/Hello.java
git commit -m "commit code with syntax error to test autograder"
git push origin main


Now, submit to Gradescope again. You should see output like this:

• At the right hand side, you’ll see the following:

This is how you’ll know that the Maven compile failed on Gradescope.

But where can you see what went wrong? Read on:

• Look in the main window, for output with mvn compile failed at the top. It will probably be very long (the whole output is not shown):

This output is really long because it shows every single file that was downloaded by Maven in order to do it’s work (which is quite a few). However, the really useful output is at the bottom, so you have to scroll down a while:

• Look at the bottom of this section, and eventually you should see:

Here, finally, you can see what’s wrong with the compile (the missing semicolon).

Now that you understand what a failed compile looks like, let’s finally fix the code and finish the lab.

Step 13: Submit correct Java code to Gradescope

Now, fix the code so that it produces the correct output. Change the file src/main/java/Hello.java so that the System.out.println method call reads:

        System.out.println("Hello, World!");


Test this locally by compiling and running the code:

mvn compile
mvn package
java -cp target/hello-1.0.0.jar Hello


You should see the correct output, Hello, World!.

Now, commit this change:

git add src/main/java/Hello.java
git commit -m "correct the output"
git push origin main