Software Design and Development :: CS 246

01 Teach : Team Activity

Java Hello World

Objectives

• Install the JDK and the IntelliJ IDE.

• Write a simple Java Hello World program.

Assignment

Part I: Install and Configure the Java JDK

Before continuing, you should have completed this week's preparation assignment.

First, we need to install the Java JDK. The Java JDK contains all the libraries and tools to create Java applications. To install the Java JDK, follow the instructions on the Oracle Java Intallation website.

Once you have the JDK successfully installed, move on to Part II to install an IDE that will use the Java JDK tools that you just installed.

Part II: Install the IntelliJ IDE

While we could now write and compile Java code on the command line, most modern software developers use IDEs (Integrated Development Environments) for these tasks.

There are several Java IDEs available. We'll be using the IntelliJ IDE because it will allow us to create regular Java and Android projects.

To begin, download and install the free, Community edition of IntelliJ.

Part III: Write a Java Hello World program

With your system configured, you're ready to write your first Java program. Here are the guidelines to follow:

1. Launch IntelliJ and select "New Project".

   

2. Select "Java" from the project types on the left-side of the window, make sure the Java SDK version you downloaded is selected at the top, and then click "Next".

   

3. Check the "Create project from template box", select "Command Line App" from the template list, and click "Next".

4. Fill in the project settings. What you enter here isn't too critical, but a good guideline is to name your projects after the assignment number (e.g. this one would be "teach-01").

   You can set your base package name to either be your last name or the name of the project without spaces (e.g. "helloworld"). Note that the package name should be in lowercase, according to the Package Names section of the Java Style Guide.

   Once you've entered the values, click "Next".

   

5. Wait a few moments while the project loads, and if everything went well, you should see a new project loaded with a class called Main that is pre-populated with the main() method.

6. Find the line that says: public class Main {. Rename the class to HelloWorld

   After you do so, you should see a red, squiggly line appear under the class definition. This line indicates there is an error in the code. Hover your mouse over the line to see the error.

   

7. In this case, the error is warning us that a public class must be defined in a file with the same name.

   Click anywhere on the line where the red squiggle appears and press Alt+Enter (or Option+Enter on macOS).

   A list of possible solutions to this error appear. Select the action that says "Rename File". The file will be renamed to HelloWorld.java, and the error indicator should disappear.

   

   This same pattern (hover over squiggly line to see error then click on line and press Alt+Enter to see possible fixes) can often be used in IntelliJ for resolving common problems.

8. Add another method to the the HelloWorld class called sayHello(). This method should print "Hello World" to the console.

9. Have main() create a new instance of the HelloWorld class and call its sayHello() method. This is all that main() should do.

10. Try to compile and execute your code by clicking on the green play button in the upper-right hand corner of the window.

    

    Unfortunately, this will fail with a runtime error stating that it can't find the class called Main. Since we renamed the class that has our main() method, we need to tell the IDE which class it should look in.

    

    To do so, click on the Select Configuration button in the top, right-hand corner of the screen, just to the left of the Run button.

    

    In the menu that appears, select "Edit Configurations...".

    

    You'll see a window appear that contains settings for how to run our program. The setting we're interested in is the one for the Main class.

    

    Change the part in red that says Main to HelloWorld to match the name of the class with the main() function in it. Then click "OK".

    

11. Now click the Run button again, and if all goes well, you should see the text "Hello World" displayed in the output console at the bottom of the window.

    

Once you've finished, or if you get stuck, compare your code to the teacher's solution.

🌟Stretch Challenge🌟

Now that you can write simple Java programs, it's time to make something useful. Write a program that asks the user for the radius of a circle, then have it print the area and circumference rounded to two decimal places.

1. Before you begin, you should read the following:

   • Formatting Numeric Output
   • How to use Java's Scanner class to get user input.

2. Create a public class called CircleCalculator and save it in a .java file with the same name.

3. Add a function to the class called getRadius() that gets the radius from the user as a double.

4. Add a function to the class called displayCircumference() that takes the radius as a parameter, calculates the circumference, and displays it to two decimal places.

5. Add a function to the class called displayArea() that takes the radius as a parameter, calculates the area, and displays it to two decimal places.

6. As before, have main() create an instance of the class. Then call the instance's getRadius(), displayCircumference(), and displayArea() methods.

Congratulations! You now know how to write, compile, and run basic Java programs.

Make sure everyone on your team understands and receives a copy of this code.

Once again, compare your answer to the teacher's solution, then complete the accompanying quiz for this assignment on I-Learn.