Running Java in VS Code and IntelliJ

Where you are: Week 0 review > Running Java in VS Code and IntelliJ

Try This First

You wrote Hello.java and want to see its output. Two paths exist: type javac Hello.java then java Hello in a terminal, or press a Run button in an editor. Decide what the Run button does that the two terminal commands do not.

Before You Start

Check each box you can do from memory. A box you cannot check yet is not a problem; it points you to a quick refresher, not a grade.

• I can name the two terminal commands that compile and then run a Java program.
• I can explain what the compile-and-run pipeline does before any output appears.
• I can state why pressing a Run button in an editor is not different from typing those two commands by hand.

What You Need To Walk In With

Walk into the next class able to state these:

• An editor does not replace the language. VS Code and IntelliJ both run the same javac and java underneath; either one produces the same .class and the same output.
• VS Code (used in CSCD 210) is a light editor: open the folder, press Run, read the Terminal panel.
• IntelliJ (used in CSCD 211) manages a whole project: src/ holds source, the green Run arrow uses a run configuration, and CSCD 211 projects build with Gradle.
• Turn off the editor’s AI code completion while learning, so that the code you write is your own.

You should be able to: open and run a simple program in either editor, and turn off AI completion.

How It Works

VS Code (CSCD 210)

VS Code with the Extension Pack for Java (from Microsoft) adds syntax highlighting, a Run button, and a terminal.

1. Open the folder that holds your .java file with File then Open Folder.
2. Press the Run triangle at the top right, or use the keyboard shortcut for Run.
3. Output appears in the Terminal panel at the bottom.

The same two commands work by hand in any terminal:

javac Hello.java
java Hello

IntelliJ IDEA (CSCD 211)

IntelliJ manages a project rather than single files. On opening one you see four areas:

The first time you open a class that has a main method, press the green arrow in the left margin next to main. IntelliJ creates a run configuration, and after that the top toolbar arrow runs the same class.

Turn off AI completion while learning

IntelliJ and many editors ship with inline AI suggestions that finish your code as you type. Turn this off while you are learning, so that every line is your own:

1. Open Settings.
2. Find the AI Assistant or inline completion setting.
3. Switch it off, and confirm.

You can switch it back on later. While learning, typing each line yourself builds the recall you need for written exams.

A Common Mistake

A common belief is that the editor is the language, so a program that runs in IntelliJ might not run in VS Code. Both editors call the same javac and java. A program that compiles in one compiles in the other; the editor only changes the convenience around the same pipeline. (Source: BJP (Reges and Stepp), Ch 1.)

Go Deeper (optional)

For the curious: the reason CSCD 211 adds Gradle is that real projects have many files and outside libraries, and typing javac by hand stops scaling. Gradle records what to compile, what libraries to fetch, and how to run tests, so one command builds the whole project the same way on any machine.

Check Yourself

Close the notes and answer each one from memory, then reveal it. Pulling an idea back from memory is one of the strongest ways to make it stick.