Compile and Run
Note: the video's "Run" menu reference is obsolete.
Now that we’ve written our first Java program, we must compile and run the program to see the fruits of our labors. There are many different ways to do so. We’ll discuss each of them in detail here.
This textbook was written for the Codio learning environment, so many of the steps below will reference features in Codio. However, most integrated development environments (IDEs) also include features to compile and run your code, and you can always do so manually using commands in the terminal for your operating system. If you aren’t sure how to get it to work, ask for help!
Terminal
Codio includes a built-in Linux terminal, which allows us to perform actions directly on a command-line interface just like we would on an actual computer running Linux. We can access the Terminal in many ways:
- Selecting the Tools menu, then choosing the Terminal option
- Pressing SHIFT + ALT + T in any Codio window (you can customize this shortcut in your Codio user preferences)
- Pressing the Open Terminal icon in the file tree
Additionally, some pages may already open a terminal window for us in the left-hand pane, as this page so helpfully does. As we can see, we’re never very far away from a terminal.
No worries! We’ll give you everything you need to know to compile and run your Java programs in this course.
If you’d like to learn a bit more about the Linux terminal and some of the basic commands, feel free to check out this great video on YouTube:
Let’s go to the terminal window and navigate to our program. When we first open the Terminal window, it should show us a prompt that looks somewhat like this one:
There is quite a bit of information there, but we’re interested in the last little bit of the last line, where it says ~/workspace. That is the current directory, or folder, our terminal is looking at, also known as our working directory. We can always find the full location of our working directory by typing the pwd command, short for “Print Working Directory,” in the terminal. Let’s try it now!
Enter this command in the terminal:
pwdand we should see output similar to this:
In that output, we’ll see that the full path to our working directory is /home/codio/workspace. This is the default location for all of our content in Codio, and its where everything shown in the file tree to the far left is stored. When working in Codio, we’ll always want to store our work in this directory.
Next, let’s use the ls command, short for “LiSt,” to see a list of all of the items in that directory:
~/workspace$ lsjava README.mdWe should see a short list of items appear in the terminal.
We can use the cd command, short for “Change Directory,” to change the working directory. To change to the java directory, type cd into the terminal window, followed by the name of that directory:
~/workspace$ cd java~/workspace/java$We are now in the java directory, as we can see by observing the ~/workspace/java on the current line in the terminal. Finally, we can do the ls command again to see the files in that directory:
~/workspace/java$ lsHelloWorld.javaWe should see our HelloWorld.java file! If it doesn’t appear, try using this command to get to the correct directory: cd /home/codio/workspace/java.
Once we’re at the point where we can see the HelloWorld.java file, we can move on to actually compiling and running the program.
Compiling in Terminal
To compile a Java program in the terminal, we’ll use the javac command, short for Java Compiler, followed by the name of the Java file we’d like to compile. So, in our case, we’ll do the following:
javac HelloWorld.javaIf it works correctly, we shouldn’t get any additional output. The compiler will look through our Java file and create a new file containing the Java bytecode for our program, called HelloWorld.class. We can use the ls command to see it:
ls
If the javac command gives you any output, or doesn’t create a HelloWorld.class file, that most likely means that your code has an error in it. Go back to the previous page and double-check that the contents of HelloWorld.java exactly match what is shown at the bottom of the page. You can also read the error message output by javac to determine what might be going wrong in your file.
We’ll cover information about simple debugging steps on the next page as well. If you get stuck, now is a great time request help via your course’s help system. You aren’t in this alone!
Running in Terminal
Finally, we can now run our program! Once it is compiled, just type the following in the terminal to run it:
java HelloWorld
That’s all there is to it! We’ve now successfully compiled and run our first Java program. Of course, we can run the program as many times as we want by repeating the previous java command. If we make changes to the HelloWorld.java file, we’ll need to recompile it using the previous javac command first. Then, if those changes instruct the computer to do something different, we should see those changes when we run the program after compiling it.
See if you can change the HelloWorld.java file to print out a different message. Once you’ve changed it, use the javac and java commands to compile and run the updated program. Make sure you see the correct output!
Codio Assessments
Last, but not least, many of the Codio tutorials and projects in this program will include assessments that we must solve by writing code. Codio can then automatically run the program and check for specific things, such as the correct output, in order to give us a grade. For most of these questions, we’ll be able to make changes to our code as many times as we’d like to get the correct answer.