Compact source files were introduced in Java 25. To create a compact source file, right-click the Project tool window (or press ⌘N (macOS) / Alt+Ins (Windows/Linux)) and select New | Java Class. In the New Java Class popup, select the option Compact source file.

This compact source file is created in the root directory of your project, even if you create it from another package. IntelliJ IDEA automatically adds an instance main method void main() to the file.