The Projects window is the main entry point to your project sources. It shows a logical view of important project contents such as Java packages and Web pages. You can open the Projects window by choosing Window > Projects (Ctrl-1).
You can specify how Java source files are displayed in the Projects window by right-clicking inside the white space of the Projects window, choosing View Java Packages As and then choosing one of the following view options in the popup menu.
Whenever you have multiple projects open, one should always be set as the main project. The main project is usually the project that contains an application's main class. The IDE provides shortcut commands to quickly build, run, and debug the main project. In the Projects window, the main project is shown in bold. You can set any project as the main project by choosing Run > Set Main Project from the main menu and selecting a project from the list of open projects in the sub-menu.