ExtendJ print-assign operator extension

This project is an extension to the Java compiler ExtendJ in which the print-assign (.=) and spread-dot (*.) operators are implemented.

The print-assign operator is a decorator of the normal assign, such that it also prints the name and new value of the assigned variable.

The spread-dot operator is a simplification of the one existing in Groovy. The one implemented in this project only supports operations on arrays, and no chaining of dots and spread-dots. See the demo code for what is implemented.

Cloning this Project

To clone this project you will need Git installed.

Use this command to clone the project using Git:

git clone --recursive

The --recursive flag makes Git also clone the ExtendJ submodule while cloning the extension-base repository.

If you forgot the --recursive flag you can manually clone the ExtendJ submodule using these commands:

cd extension-base
git submodule init
git submodule update

This should download the ExtendJ Git repository into a local directory named extendj.

Build and Run

If you have Gradle installed you can issue the following commands to build and test the extension:

gradle jar

java -jar compiler.jar example/
java -cp example/ Demo

Additional information about the examples and instructions on how to run them can be found in the "example" directory.

If you do not have Gradle installed you can use the gradlew.bat (on Windows) or gradlew (Mac/Linux) script instead. For example to build on Windows run the following in a command prompt:


The gradlew scripts are wrapper scripts that will download Gradle locally and run it.

File Overview

Here is a short explanation of the purpose of each file in the project:

  • build.gradle - the main Gradle build script. There is more info about this below.
  • jastadd_modules - this file contains module definitions for the JastAdd build tool. This defines things such as which ExtendJ modules to include in the build, and where additional JastAdd source files are located. Also defines where the parser and scanner specifications can be found.
  • - this file.
  • gradlew.bat - Windows Gradle wrapper script (explained above)
  • gradlew - Unix Gradle wrapper script
  • src/parser/print-assign.beaver - parser specification for the extension
  • src/scanner/print-assign.flex - scanner specification for the extension
  • src/jastadd/print-assign.ast - this file extends the abstract grammar
  • src/jastadd/*.jrag - aspect files for inter-type declarations. Contains the static analysis and code generation for the extension.
  • testfiles/ - this directory contains the compilation and runtime tests

How this Extension Works

This extension builds a compiler that generates calls to "System.out.print" whenever the print-assign operators is used, in addition to assigning.

Primitives and regular objects are passed unproccessed to print, while 1d arrays are called as an argument to Arrays.utils.toString, and arrays of higher dimensions are called as an argument to Arrays.utils.deepToString.

The compiler performs the same static analysis for print-assign as it would for regular assign. Any syntatic violations will prevent class files from being produced.

Additional Resources

More examples on how to build ExtendJ-like projects with the JastAdd Gradle plugin can be found here:


All the files in the repository EXCEPT the following originate from the repository extension-base which includes ExtendJ and the base framework for building extensions, licensed under BSD 2-clause:

  • testfiles/
  • src/jastadd/
  • src/parser/
  • src/scanner/
  • src/test/org/extendj/
  • src/test/org/extendj/


This repository is covered by the license BSD 2-clause, see file LICENSE.