Writing Your First Java Class in Eclipse

Before you can write a single line of Java in Eclipse, you have to decide where that code will live. Eclipse does not treat a loose source file as a starting point. Instead, all of your work is organized inside a container called a project. A project gathers together the source files, the library settings, and the other resources that a program needs, so the IDE always knows what belongs to which program.

This article follows one path: opening the wizard that builds a new Java project, deciding what to fill in, and then examining what Eclipse actually puts on your screen. The starting point is the File menu. Inside it, open the New submenu, then choose the Java Project command. This opens a window in which you describe the project before Eclipse creates it.

Read this sequence with a real Eclipse window open so that each step can be checked as you go. The wizard is a planning stage. Nothing is built yet while the window is open; you are only supplying the choices that Eclipse will use later.

The easiest way to understand the wizard is to ask, at each moment, what Eclipse actually holds. Before Finish, Eclipse holds only your answers. After Finish, it holds a real project with its own contents. The table below traces that change.

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.

The wizard has two sections beyond the name that need your attention. One controls which Java environment the project runs against. For this course, the execution environment should be set to JavaSE 17 or later. This setting is the JRE choice, and it matters because it determines which JRE the project is connected to.

The diagram shows the chain. You choose the execution environment in the wizard, and that choice is what the JRE System Library reflects inside the project. You will see the library itself later, when you examine the new project's contents.

The other section of the wizard controls whether the project is treated as a modular program. It contains the option labeled 'Create module-info.java file'. For non-modular textbook programs, which is the case this course expects, that option should be unchecked. The decision is a simple one: find out whether the program is modular, then set the option to match.

The source states the unchecked case explicitly for non-modular textbook programs. The other branch in the diagram simply reflects the wizard's purpose: the option controls modular treatment, so a program that is not being written as a modular program should not have it switched on for this course.

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 that you can expand. Click the small triangle or plus sign beside its name to see what it contains. Assuming the default settings, two items matter most at this point.

The src directory is where your Java source code files go. 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.