sudo update-alternatives --config java

Configures the default for the program "java". That's the Java VM.

sudo update-alternatives --config javac

Configures the default Java compiler.

You can also see that, because the first command lists a lot of "JRE" (Java Runtime Environment) folders and the Program is just called "java".

If I check which version is being used by issuing the command java -version or javac -version, I can see, that each command changes the program being used.

However, using update-java-alternatives with a JDK Version changes both programs for me. Using the first commands, you can use a Java VM and Java Compiler from different JDKs.

update-java-alternatives requires presence of a file with extension .jinfo in directory /usr/lib/jvm. The openjdk package is shipped with a .jinfo file, the jdk of Oracle (formerly Sun) is not. As alternative, you configure alternatives without update-java-alternatives:

For example, to add java from jvm-directory /usr/lib/jvm/jdk-12.0.1 (default directory of Debian package of Oracle) with priority 2082, use the following command:

sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk-12.0.1/bin/java 2082

As for switching for different development environments:

Are you talking about starting the IDE itself with different Java versions or using different versions in the IDE for compilation and running your app?

• For 1.: You can specify which JVM to use in the eclipse.ini, as described here. I don't know how to do that for the Arduino IDE.

• For 2.: In Eclipse you can select the JRE/JDK to be used in Window -> Preferences -> Java -> Installed JREs. And under Java -> Compiler you could choose an older Java compliance if you wish.

EDIT: This DigitalOcean page also has a very nice explanation of everything related to Java on Ubuntu.

Assuming one has installed a JDK in /opt/java/jdk1.8.0_144 then:

1. Install the alternative for javac

   $ sudo update-alternatives --install /usr/bin/javac javac /opt/java/jdk1.8.0_144/bin/javac 1
   

2. Check / update the alternatives config:

   $ sudo update-alternatives --config javac
   

If there is only a single alternative for javac you will get a message saying so, otherwise select the option for the new JDK.

To check everything is setup correctly then:

$ which javac
/usr/bin/javac

$ ls -l /usr/bin/javac
lrwxrwxrwx 1 root root 23 Sep  4 17:10 /usr/bin/javac -> /etc/alternatives/javac

$ ls -l /etc/alternatives/javac
lrwxrwxrwx 1 root root 32 Sep  4 17:10 /etc/alternatives/javac -> /opt/java/jdk1.8.0_144/bin/javac

And finally

$ javac -version
javac 1.8.0_144

Repeat for java, keytool, jar, etc as needed.

java -version is running the wrong version of java.

Diagnostics:

>java -version
java version "1.8.0_66"
Java(TM) SE Runtime Environment (build 1.8.0_66-b18)
Java HotSpot(TM) 64-Bit Server VM (build 25.66-b18, mixed mode)

the following is the Java related contents from the output of PATH:

PATH=C:\ProgramData\Oracle\Java\javapath; ... C:\Program Files\Java\jdk1.6.0_45\bin

Conclusion:

From the above output we can deduce that C:\ProgramData\Oracle\Java\javapath is 1.8.0_66.

You need to change your PATH to put C:\Program Files\Java\jdk1.6.0_45\bin first.

I noticed that after checking the path per your suggestion. Windows 10 does not allow me to edit the path because it says "This environment variable is too large." I know there should be another question to deal with this separately.

You also need to clean up your path. My guess is you have a lot of duplicate entries.

Here's my guide for Windows 10.

Step 1. Go to System Properties. Click on Environment Variables

Step 2. Add new variables, such as JAVA_8_HOME

• JAVA_8_HOME:%ProgramFiles%\Java\jdk1.8.0_152\bin
• JAVA_9_HOME:%ProgramFiles%\Java\jdk-9.0.1\bin
• JAVA_HOME:%JAVA_8_HOME%

In my case, JAVA_8_HOME(JDK8) is pointing to C:\Program Files\Java\jdk1.8.0_152\bin. You can replace this with your own path to javac. %JAVA_HOME% has a default value pointing to JAVA_8_HOME, which is the path for JDK8. That's my preference, feel free to adjust accordingly.

Step 3. Select PATH and click on Edit. PATH

Step 4. Click on New and add %JAVA_HOME%. %JAVA_HOME% will be added to PATH automatically every time you launch a command prompt.

In order to switch JDK version in cmd, here's the trick. Step 5. I created a batch file with

@echo off
:: Switch JDK version
DOSKEY java8=SET PATH=%JAVA_8_HOME%;%PATH%;
DOSKEY java9=SET PATH=%JAVA_9_HOME%;%PATH%

Basically, it disables echo and creates two alias. In batch file any string after :: is the comments. Every time, java8 or java9 is called, it re-exports %PATH% with the new JDK path. Save it as profile.bat. You can name it whatever you want.

Step 6. Search for regedit (Registry Editor). Click on Edit > New > String Value. Give AutoRun as the Value name and %USERPROFILE%\profile.bat as the Value data. Here, please put your actual path value to the profile.bat we just created. So, whenever a command prompt is opened, it automatically loads profile.bat, which creates those two alias in the script.

Step 7. Close any command prompt you're using or just open a new command prompt. This is because your changes will not affect opened cmd window. Environment changes only happens to new CMD.

Step 8. Verify your results here.

If you're using different Python versions, same trick applies, too. Find my python environment settings here.