Java compile and run project

Compiling and Running

Use IntelliJ IDEA to run your Hello World application.

Now you have written your ‘HelloWorld.java’ class, you need to compile and run it. IntelliJ IDEA can do this for you. There are lots of ways you can run an application but to start with, you can click the green arrows to the left of the method. These are known as gutter icons.

You’ll notice that there are two green arrows. The top one is adjacent to the class, and it will run the class. Our class only contains Java’s main method so that is all that it will run. However, if you’re working with classes with multiple tests in for example, using this green run arrow will run all the tests in the class.

The second green arrow is adjacent to Java’s main method. Clicking this will run Java’s main method. For the purpose of our application, both the green arrows do the same thing. When you click on the green arrow you’ll get different options including debug, but we will just run it for now so select that option.

IntelliJ IDEA will now compile the file into a class file and then run it. The output of the run is shown in the Run tool window at the bottom of your screen.

The Run Window

Let’s take a look at your Run Window in more detail.

The first line shows the command that was used to run the program. We don’t usually need to worry about what this was, but it’s useful to know in case you want to see exactly what was run and how. For example, if you scroll to the right you will see com.example.helloworld.HelloWorld . This tells you that it was your HelloWorld class that was run. You can also see exactly which JDK was used, which can be useful if you have multiple versions of Java on the same machine.

The second line in this window is the output of your program — the «Hello World» statement that you told it to print.

The last line, which says Process finished with exit code 0 shows the program ran without an error.

What IntelliJ IDEA Created

Let’s take a quick look at what happened when you ran the application. IntelliJ IDEA compiled your HelloWorld.java file into a class file. By default, the IDE created a folder called out . Production code, meaning code that isn’t test code, is put into the production folder within the out folder. IntelliJ IDEA creates a folder for our HelloWorld project and then the directory structure for our package. The compiled class file HelloWorld.class can be found at the end of this directory hierarchy.

Run Configurations and Shortcuts

IntelliJ IDEA also created a run configuration for the application we ran. If you want to, you can run or debug any run configurations from the navigation bar. We will take a more detailed look at run configurations later in this tutorial.

If you want to go back to the Run Window, you can use Cmd+4 on macOS, or Alt+4 on Windows to open it and the same shortcut again to close it and return the focus back on the editor.

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Create your first Java application

In this tutorial, you will learn how to create, run, and package a simple Java application that prints Hello World! to the system output. Along the way, you will get familiar with IntelliJ IDEA features for boosting your productivity as a developer: coding assistance and supplementary tools.

Prepare a project

Create a new Java project

In IntelliJ IDEA, a project helps you organize your source code, tests, libraries that you use, build instructions, and your personal settings in a single unit.

1. Launch IntelliJ IDEA. If the Welcome screen opens, click New Project . Otherwise, from the main menu, select File | New Project .
2. In the New Project wizard, select New Project from the list on the left.
3. Name the project (for example HelloWorld ) and change the default location if necessary.
4. We’re not going to work with version control systems in this tutorial, so leave the Create Git repository option disabled.
5. Make sure that Java is selected in Language , and IntelliJ is selected in Build system .
6. To develop Java applications in IntelliJ IDEA, you need the Java SDK ( JDK ). If the necessary JDK is already defined in IntelliJ IDEA, select it from the JDK list. If the JDK is installed on your computer, but not defined in the IDE, select Add JDK and specify the path to the JDK home directory (for example, /Library/Java/JavaVirtualMachines/jdk-20.0.1.jdk ). If you don’t have the necessary JDK on your computer, select Download JDK . In the next dialog, specify the JDK vendor (for example, OpenJDK), version, change the installation path if required, and click Download .
7. Leave the Add sample code option disabled as we’re going to do everything from scratch in this tutorial. Click Create .

After that, the IDE will create and load the new project for you.

Create a package and a class

Packages are used for grouping together classes that belong to the same category or provide similar functionality, for structuring and organizing large applications with hundreds of classes.

1. In the Project tool window, right-click the src folder, select New (or press Alt+Insert ), and then select Java Class .
2. In the Name field, type com.example.helloworld.HelloWorld and click OK . IntelliJ IDEA creates the com.example.helloworld package and the HelloWorld class.

Together with the file, IntelliJ IDEA has automatically generated some contents for your class. In this case, the IDE has inserted the package statement and the class declaration.

This is done by means of file templates. Depending on the type of the file that you create, the IDE inserts initial code and formatting that is expected to be in all files of that type. For more information on how to use and configure templates, refer to File templates.

The Project tool window Alt+1 displays the structure of your application and helps you browse the project.

In Java, there’s a naming convention that you should follow when you name packages and classes.

Write the code

Add the main() method using live templates

Live templates are code snippets that you can insert into your code. main is one of such snippets. Usually, live templates contain blocks of code that you use most often. Using them can save you some time as you don’t have to type the same code over and over again.

For more information on where to find predefined live templates and how to create your own, refer to Live templates.

Call the println() method using code completion

After the main() method declaration, IntelliJ IDEA automatically places the caret at the next line. Let’s call a method that prints some text to the standard system output.

1. Type Sy and select the System class from the list of code completion suggestions (it’s from the standard java.lang package). Press Control+. to insert the selection with a trailing period.
2. Type o , select out , and press Control+. again.
3. Type p , select the println(String x) method, and press Enter . IntelliJ IDEA shows you the types of parameters that can be used in the current context. This information is for your reference.
4. Type » . The second quotation mark is inserted automatically, and the caret is placed between the quotation marks. Type Hello World!

Basic code completion analyzes the context around the current caret position and provides suggestions as you type. You can open the completion list manually by pressing Control+Space .

For information on different completion modes, refer to Code completion.

Call the println() method using a live template

You can call the println() method much quicker using the sout live template.

After the main() method declaration, IntelliJ IDEA automatically places the caret at the next line. Let’s call a method that prints some text to the standard system output.

1. Type sout and press Enter .
2. Type » . The second quotation mark is inserted automatically, and the caret is placed between the quotation marks. Type Hello World! .

Build and run the application

Valid Java classes can be compiled into bytecode. You can compile and run classes with the main() method right from the editor using the green arrow icon in the gutter.

1. Click in the gutter and select Run ‘HelloWorld.main()’ in the popup. The IDE starts compiling your code.
2. When the compilation is complete, the Run tool window opens at the bottom of the screen. The first line shows the command that IntelliJ IDEA used to run the compiled class. The second line shows the program output: Hello World! . And the last line shows the exit code 0 , which indicates that it exited successfully. If your code is not correct, and the IDE can’t compile it, the Run tool window will display the corresponding exit code.

When you click Run , IntelliJ IDEA creates a special run configuration that performs a series of actions. First, it builds your application. On this stage, javac compiles your source code into JVM bytecode.

Once javac finishes compilation, it places the compiled bytecode to the out directory, which is highlighted with yellow in the Project tool window.

After that, the JVM runs the bytecode.

Automatically created run configurations are temporary, but you can modify and save them.

If you want to reopen the Run tool window, press Alt+4 .

IntelliJ IDEA automatically analyzes the file that is currently opened in the editor and searches for different types of problems: from syntax errors to typos. The Inspections widget in the top-right corner of the editor allows you to quickly see all the detected problems and look at each problem in detail. For more information, refer to Current file.

Package the application in a JAR

When the code is ready, you can package your application in a Java archive (JAR) so that you can share it with other developers. A built Java archive is called an artifact .

Create an artifact configuration for the JAR

1. From the main menu, select File | Project Structure ( Control+Alt+Shift+S ) and click Artifacts .
2. Click , point to JAR and select From modules with dependencies .
3. To the right of the Main Class field, click and select HelloWorld (com.example.helloworld) in the dialog that opens. IntelliJ IDEA creates the artifact configuration and shows its settings in the right-hand part of the Project Structure dialog.
4. Apply the changes and close the dialog.

Build the JAR artifact

1. From the main menu, select Build | Build Artifacts .
2. Point to HelloWorld:jar and select Build . If you now look at the out/artifacts folder, you’ll find your JAR there.

Run the packaged application

To make sure that the JAR artifact is created correctly, you can run it.

Use Find Action Control+Shift+A to search for actions and settings across the entire IDE.

Create a run configuration for the packaged application

To run a Java application packaged in a JAR, IntelliJ IDEA allows you to create a dedicated run configuration.

1. Press Control+Shift+A , find and run the Edit Configurations action.
2. In the Run/Debug Configurations dialog, click and select JAR Application .
3. Name the new configuration: HelloWorldJar .

Run configurations allow you to define how you want to run your application, with which arguments and options. You can have multiple run configurations for the same application, each with its own settings.

Execute the run configuration

• On the toolbar, select the HelloWorldJar configuration and click to the right of the run configuration selector. Alternatively, press Shift+F10 if you prefer shortcuts. As before, the Run tool window opens and shows you the application output.

The process has exited successfully, which means that the application is packaged correctly.

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