Note: This page is about compiling and running the Java lab code, using either the command line or an IDE. For general information on how to do the labs, in particular where to find them on GitLab Chalmers, see the page Doing the labs.

To compile and run the labs, at the very least you need to have a JDK (Java Development Kit) installed on your system. The least version of Java we require is JDK 8. You can work on the lab in several ways:

Using a text editor and the command line. This is the simplest method.
Using an IDE such as Eclipse, IntelliJ IDEA, or NetBeans.

This choice, in particular how you run Java, is entirely up to you. The computers at the department provide a JDK as well as the Eclipse and IntelliJ IDEs. Here, we provide help with the following choices:

The command line
Eclipse
IntelliJ IDEA

The command line

Setting up

Make sure you have set up an SSH key with Chalmers GitLab (see page Doing the labs). Clone the lab project using

git clone <address>

where <address> is the repository SSH URL from the project page on Chalmers GitLab. That’s it if you followed our recommendation not to add a passphrase to your SSH key. Otherwise, you will be asked for this passphrase here.

Note: if you see the prompt git@git.chalmers.se's password:, your SSH key is configured incorrectly.

Navigate the command line to the project directory lab-N.

Git workflow

Follow the standard git workflow explained on the page Doing the labs. Briefly:

git add -u to add all of your changes to git’s index.
git commit to form a commit of all added changes.
git push to send your commit(s) to the shared repository on Chalmers GitLab.
git pull to receive updates from the shared repository on Chalmers GitLab.

Compiling

Suppose there is some Java file Program.java with a so-called main method:

public static void main(String[] args)

Before running this Java program, you need to compile it. Running

javac Program.java

produces a file Program.class containing Java bytecode. (It also makes sure all dependencies of the class Program are compiled). If you are making changes to Java files that Program indirectly depends, you need to recompile those files yourself. If you don’t want to remember which files you changed, you can run

javac *.java

to make sure all bytecode is up-to-date (the prompt will expand *.java to the Java files in the current directory).

Running

Run the program as follows:

java Program

This executes the main method. The program will print its output in the command-line interface. If it expects input, you may type in the command-line interface (following each line by Enter to flush).

Quitting

To abort the execution of a program (e.g. because it takes too long), you press Ctrl-C from the command line. You might get an error message saying that the program was aborted.

When a program expects input, you can also send it the “end-of-input” signal (closing the input stream of the program). It will typically interpret this as a request to exit. This is done by pressing Ctrl-D (on Mac/Linux), or by pressing Ctrl-Z followed by Enter (on Windows).

Arguments

Notice how the main method takes an array of strings as argument. These may be given on the command line, separated by spaces. Running

java Program arg0 arg1 arg2

results in the execution of

String[] args = {"arg0", "arg1", "arg2"};
main(args);

Command-line arguments are typically used to pass metadata to the program, e.g. options or paths to files to process. If an argument contains a special character, e.g. a whitespace character in a file path, it must be escaped, e.g. by enclosing the argument in quotation marks. The details depend on the specific command-line interface.

Enabling assertions

By default, assert statements in the code are ignored when running the program. If you want Java to check that all assertions are true, add the option -ea before the class name:

java -ea Program

or

java -ea Program arg0 arg1 arg2

If the program makes use of assert-statements, this can help you catch mistakes. Note however that it can also change the complexity of methods, depending on how it is used. So you should deactivate it when benchmarking.

Eclipse

Setting up

Clone the lab project manually as described under setting up for the command line. Here is how to turn the folder lab-N into an Eclipse project.

[File → New → Java Project] opens a wizard.
Uncheck “Use default location”. Select as location the folder lab-N:
Click on Finish.

Your project is now be ready to use. The project hierarchy might look as follows:

Note that you have access also to the answer file and data files. The organisation of the folder lab-N has not changed. This makes it convenient to share your work with group members not using an IDE.

Note: If you choose to create the Eclipse project in any other way, make sure that you do not introduce extra packages not found in the folder lab-N. In particular, the top-level Java files, including the ones you are going to submit later, should reside in the so-called default package (the top of the Java package hierarchy). Otherwise, your submission will confuse our automated testing scripts.

Git workflow

The git workflow is the same as for working with the command line: you manually perform the git operations on the directory the Eclipse project is stored in. Alternatively, there are plugins for Eclipse that add version control features directly usable in the IDE.

Running

You run a Java class by opening the corresponding file in editor and clicking on the Run button in the top bar. Recall that the class must have a main method.

Note: Even if you use Eclipse to edit the Java files, you still have the option of using the command line to run them. This can be convenient when you need to pass arguments to the program. Eclipse automatically compiles your source files when you save them (unless you have deactivated the option Build Automatically in the Project menu). Thus, you can skip the compilation step on the command line and proceed straight to running.

Quitting

To abort a running program, press the red square in the Console window of Eclipse. If the program is waiting for input, you can also try sending it the “end-of-input” signal by pressing Ctrl-D (on Mac/Linux) or Ctrl-Z (on Windows). You might have to refocus the Console window for this to work.

Arguments

Running a Java program with arguments is slightly more complicated in the Eclipse IDE than on the command-line. Keep in mind that you may always switch back to the command line for running.

Suppose the class you wish to run with arguments is called Program. Start by running the class (without arguments) at least once as described above. This creates a so-called run configuration. Now, we will edit this run configuration. Open the drop-down menu on the right of the Run button and click on Run Configurations…:

This opens a dialogue focussed on the current run configuration, in this case Program. The second tab Arguments allows us to pass arguments to the program.

Simply enter the arguments separated by spaces or line breaks. Click on Apply to save the changes to the run configuration. Now, whenever you run Program, it will be executed with these arguments. If you wish to change the arguments or go back to no arguments, you need to edit the run configuration again.

Note: Eclipse starts your program with lab-N as working directory. Thus, when you give file paths as arguments to your program, they will be interpreted relative to lab-N. This makes it easy to point to lab data files: simply give the relative path, e.g. data/input1.txt.

Enabling assertions

By default, assert statements in the code are ignored when running the program. If you want Java to check that all assertions are true, add the option -ea as a VM argument in the above dialogue.

IntelliJ IDEA

Setting up

Launch IntelliJ IDEA. If you already had a project open, close it down. A window similar to the following should appear:

Make sure you have set up an SSH key with Chalmers GitLab. Make sure you have set up an SSH key with Chalmers GitLab (see page Doing the labs).

[Recommended] Clone the lab project by clicking Get from VCS and entering the repository SSH URL from the project page on Chalmers GitLab.
Clone the lab project manually as described under setting up for the command line. Then open the project from IDEA.

Note: If IntelliJ highlights lots of strange errors after cloning, one of the below steps may resolve the problem:

Go to File → Project stucture… and specify a Project SDK version.
Go to File → Invalidate caches… and click on Invalidate and restart.

Let us know what worked!

Git workflow

The git workflow is integrated directly into IntelliJ IDEA. It is accessible via the Git menu. You can also use the following taskbar:

– pull
– commit
– push

Compiling and running

You can compile and run code via the menu, context menu, or the toolbar.

Quitting

To abort a running program, press the red square in the Console window of IntelliJ. If the program is waiting for input, you can also send it the “end-of-input” signal by pressing Ctrl-D.

Run Configurations

Some labs requires you to enter command line arguments and/or decide a main executable class. In order to tinker with such options a Run Configuration must be added. A Run Configuration is added in the following manner.

Click on the button next to the Hammer icon. The text on it might vary.
Click on the Plus sign and select Application in the drop down menu.
Fill in the fields with your configuration. The field Main class refers to the class that contains the main method that the program will be run from. You can add command-line arguments by filling in the field Program arguments.

You can edit your run configuration later on, for example to change the program arguments, by clicking on Edit configuration… in the drop-down list that now appears when you click on the button from before next to the Hammer icon.

Enabling assertions

By default, assert statements in the code are ignored when running the program. If you want Java to check that all assertions are true, add -ea to the field VM Options in the above dialogue.