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.

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.

On most Linux distributions you can use update-alternatives like this:

sudo update-alternatives --config java

It will list all packages that provide java command and will let you change it. If you don't want to change it, simply Ctrl-C from it.

There is only one catch - if you installed some java not using official package manager (dpkg/apt-get, rpm/yum), but simply extracted it, update-alternatives will not show it.

Confusing error messages: Clojure's error messages more often than not are very confusing. They usually involve stack traces that do not thoroughly explain where the error was caused or what caused it. | Immutability is the default: Clojure programmers are highly encouraged to use immutable data in their code. Therefore, most data will be immutable by default. · State change is handled by functions (for transformations) and atoms (an abstraction that encapsulates the idea of some entity having an identity). | Minimal syntax: Being a LISP, programs are simple: they're just functions and data. That it doesn't get bogged down with syntax or the loftier FP concepts like monads makes it one of most approachable functional languages for beginners. | Tries to solve problems as simply as possible: Simplicity is one of the pillars on which Clojure is built. Clojure tries to solve many problems in software development as simply as possible. Instead of building complex interfaces, objects or factories, it uses immutability and simple data structures. | Good for writing concurrent programs: Since Clojure is designed for concurrency, it offers things like Software Transaction Memory, functional programming without side-effects and immutable data structures right out of the box. This means that the development team can focus their energies on developing features instead of concurrency details. | Huge ecosystem of libraries to work with: There's a very large ecosystem of high-quality Clojure libraries which developers can use. One example is Incanter. It's a great data analytics library and a very powerful tool for dealing with matrices, datasets and csv files. | Code tends to be nightmare to maintain for non-authors: Tendency to devolve into difficult to manage mess of styles. Not recommended for professional use. | Too cult-ish: Way too niche and in-group behavior, while trying to trash other languages. Only pays for select few, who run the "game" on others. | Cross platform: Clojure compiles to JVM bytecode and runs inside the JVM. This means that applications written in Clojure are cross-platform out of the box. | Rich Hickey: The creator is so awesome, he's a feature. Just look up his talks and see why. | Dynamic language: A superb data processing language. While rich type and specification systems are available they are optional. | Tied to the JVM and it's limitations: Some language constructs were obviously created as workarounds for JVM limitations. This makes the language much less elegant than it could have been. · Also, the JVM has a very cumbersome FFI. | Syntax can be alien / jarring for those used to other Lisps: Perhaps some may consider this attribute an advantage, but I do not. Clojure does not attempt to maintain significant compatibility with other Lisps. So, if you already know a Lisp or are used to the way Lisp works in general, you'll probably be confused if you take a look at Clojure. See these resources for more details on this subject: · - https://clojure.org/reference/lisps · - http://stackoverflow.com/q/6008313/2636454 · - http://gilesbowkett.blogspot.com/2015/01/one-major-difference-between-clojure.html · - http://softwareengineering.stackexchange.com/q/153128/166211 | Extensible: Clojure has an elegant macro system which enables language additions, Domain-specific languages (DSLs), to be created much easier than most other languages (with the exception of Racket, perhaps). | Great tool used in automating, configuring and managing dependencies available: Leiningen is a very useful tool for Clojure developers. It helps wiht automation, configuration and dependency management. It's basically a must for every Clojure project. | No C/Java syntax: Refreshing, BTW! | Game is available with which you can learn Clojure: Nightmod is a tool used to make "live-moddable" games. It displays the game's code while you are playing and allows you to inject new code using Clojure. This can be a fun and useful experience for people trying to learn Clojure. | Dynamic types: You can put anything in. This makes reasoning about code after a time has passed very hard.