java6-sdk software package provides virtual package provided by default-jdk, openjdk-8-jdk, 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 java6-sdk
java6-sdk is installed in your system.
Make ensure the java6-sdk package were installed using the commands given below,
$ sudo dpkg-query -l | grep java6-sdk*
You will get with java6-sdk package name, version, architecture and description in a table
| Distro | Debian 9 stretch |
| Section | Virtual packages |
| Package | java6-sdk |
| Version | |
| Description | virtual package provided by default-jdk, openjdk-8-jdk |
| Sub Section | Virtual |