Debian's package management system categorizes software into different repositories based on licensing. The main ones are:

• main: Fully free software meeting Debian Free Software Guidelines
• contrib: Free software but depends on non-free components
• non-free: Software that doesn't meet DFSG requirements

To install proprietary packages like Sun Java JDK, you need to enable the non-free repository. Here's how:

sudo nano /etc/apt/sources.list

Find lines starting with deb and add non-free to them. For example:

deb http://deb.debian.org/debian/ stable main contrib non-free
deb-src http://deb.debian.org/debian/ stable main contrib non-free

After editing, update your package lists:

sudo apt update

With non-free repositories enabled, you can now install the JDK:

sudo apt install sun-java6-jdk

Note: The exact package name might vary depending on your Debian version. For newer versions, you might need:

sudo apt install oracle-java8-jdk

Check if Java is properly installed:

java -version
javac -version

If you prefer not to modify sources.list directly, you can use:

sudo add-apt-repository "deb http://archive.canonical.com/ubuntu $(lsb_release -sc) partner"
sudo apt update
sudo apt install sun-java6-jdk

If you have multiple Java versions installed, set the default:

sudo update-alternatives --config java
sudo update-alternatives --config javac

If you encounter "Package not found" errors:

• Ensure you've run sudo apt update
• Check your Debian version codename matches the repository
• Verify the package exists in the non-free repository

Debian maintains strict policies about software freedom, which is why its default repositories are divided into:

main - DFSG-compliant packages (free software)
contrib - DFSG-compliant but depend on non-free software
non-free - Non-DFSG-compliant packages

To enable non-free repositories, you'll need to edit your /etc/apt/sources.list file. Here's how to do it properly:

sudo nano /etc/apt/sources.list

Look for lines starting with deb http:// and add non-free to each relevant line. For example:

deb http://deb.debian.org/debian/ bullseye main contrib non-free
deb-src http://deb.debian.org/debian/ bullseye main contrib non-free
deb http://security.debian.org/debian-security bullseye-security main contrib non-free
deb-src http://security.debian.org/debian-security bullseye-security main contrib non-free

For newer Debian versions, you can use this command to enable non-free repos:

sudo apt-add-repository non-free
sudo apt update

Once non-free repos are enabled, install the JDK with these commands:

sudo apt update
sudo apt install default-jdk

For Oracle's official JDK (when available in non-free):

sudo apt install oracle-java17-jdk

Check your Java version to confirm success:

java -version
javac -version

If you encounter dependency problems, try:

sudo apt --fix-broken install
sudo apt autoremove

For systems with multiple Java versions, use update-alternatives:

sudo update-alternatives --config java

To permanently keep non-free enabled, create this configuration file:

sudo tee /etc/apt/apt.conf.d/90non-free <