ecj software package provides standalone version of the Eclipse Java compiler, you can install in your Debian 9 (Stretch) by running the commands given below on the terminal,
$ sudo apt-get update $ sudo apt-get install ecj
ecj is installed in your system.
Make ensure the ecj package were installed using the commands given below,
$ sudo dpkg-query -l | grep ecj *
You will get with ecj package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Java |
| Package | ecj |
| Version | 3.11.0-7 |
| Description | standalone version of the Eclipse Java compiler |
| Sub Section | Java |