Running a Java Program in Eclipse

Before you can write a single line of Java in Eclipse, you have to decide where that code will live. This article follows one path: opening the wizard that builds a new Java project, deciding what to fill in, and then examining what Eclipse puts on your screen. Read it with a real Eclipse window open so you can check each step as you go.

Think of the project as the single place where one program lives. Your source files, the settings that say which Java environment the program runs against, and any other resources are all held together in that one container. When you later look at a project, you are looking at everything Eclipse needs to know about that program.

At this stage nothing has been built yet. You are only supplying the choices that Eclipse will use. The project does not appear in the Package Explorer while this window is still open; it appears only after Finish is clicked.

The first field you must fill in is the Project Name. Eclipse is flexible here: the name can be anything you like. Choose something that identifies the program, because that name is what you will see in the project list later. Beyond the name, the wizard has two sections that need your attention. One controls which Java environment the project runs against, and the other controls whether the project is treated as a modular program. The table below summarizes the settings this course expects.

The JRE setting in the wizard should use an execution environment of JavaSE 17 or later. The JRE choice you make in the wizard is the part of the project that shows up as the JRE System Library, which holds the standard built-in classes that come with Java. In other words, the execution environment you pick determines which set of built-in classes your program is compiled and run against.

The source for this course asks for JavaSE 17 or later as the expected setting, so when you create a project, look at the JRE field and confirm that the version chosen is 17 or newer. If an older version is selected, change it before clicking Finish so that the project's library matches the course's expectation.

The second setting that needs attention is the 'Create module-info.java file' option. The course expects this option to be unchecked for non-modular textbook programs. Unchecking it tells Eclipse that the project is not a modular program. The diagram below shows the decision you face at this point in the wizard.

After you click Finish, the new project appears in the Package Explorer view on the left side of the Eclipse window. The project is a folder-like entry. Click the small triangle or plus sign beside its name to expand it and see what it contains. Assuming the default settings, two items matter most: the src directory and the JRE System Library.

The src directory is where your Java source code files go, and when you later add a class, it belongs inside src. The JRE System Library is different: it is not something you write. It is the collection of standard built-in classes that come with Java, and it is the part of the project that the JRE setting controls. This is why the JRE choice made in the wizard shows up in the project's contents.