J is an extension specific to files that store source code written in the multi-paradigm Java programming language. Technically speaking, J is a plain text file and can be handled by any text editor. The program code contained in the J files is compiled and the result is machine code understandable for the Java runtime.

To edit J files, developers use code editors that optimize work with its syntax. Larger projects are created with the use of integrated programming environments, eg IntelliJ.