I understand the motivation behind your question. update-alternatives elegantly solves the problem of managing several installed versions of a program (for example Java) or different ports of a program (vim vs. elvis vs. vile) by making one of them the default one.

update-alternatives, found in many Linux distros, creates and manages a set of symlinks from /bin, /usr/bin (and the like) to /etc/alternatives, and from there to the location where the program is installed.

For example, in openSUSE java is symlinked to /etc/alternatives/java, which is symlinked to a "private" path where java resides:

/usr/bin/java -> /etc/alternatives/java
/etc/alternatives/java -> /usr/lib64/jvm/java-1.5.0-sun-1.5.0/jre/bin/java

Unfortunately, and although some think it would be necessary, at least for MacPorts, there is no such thing in OS X. I have personally solved the lack of update-alternatives with aliases in my .bashrc:

alias ls='/usr/local/bin/ls'

or simply changing $PATH:

PATH=/usr/local/bin:$PATH

If you can't solve it like this and are weighing the fact of porting it to OS X be aware that although update-alternatives' first incarnation was a Perl script, it was rewritten in C for Debian, and some other distros adopted it later (for instance openSUSE 12.1).

openSUSE 11.4 provides the Perl version as an RPM package. If you are looking for the C version, go for any recent Ubuntu or Debian release, or openSUSE 12.1 or later.

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.

First run /usr/libexec/java_home -V which will output something like the following:

Matching Java Virtual Machines (3):
1.8.0_05, x86_64:   "Java SE 8" /Library/Java/JavaVirtualMachines/jdk1.8.0_05.jdk/Contents/Home
1.6.0_65-b14-462, x86_64:   "Java SE 6" /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home
1.6.0_65-b14-462, i386: "Java SE 6" /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home

/Library/Java/JavaVirtualMachines/jdk1.8.0_05.jdk/Contents/Home

Pick the version you want to be the default (1.6.0_65-b14-462 for arguments sake) then:

export JAVA_HOME=`/usr/libexec/java_home -v 1.6.0_65-b14-462`

or you can specify just the major version, like:

export JAVA_HOME=`/usr/libexec/java_home -v 1.8`

Now when you run java -version you will see:

java version "1.6.0_65"
Java(TM) SE Runtime Environment (build 1.6.0_65-b14-462-11M4609)
Java HotSpot(TM) 64-Bit Server VM (build 20.65-b04-462, mixed mode)

Add the export JAVA_HOME… line to your shell’s init file.

For Bash (as stated by antonyh):

export JAVA_HOME=$(/usr/libexec/java_home -v 1.8)

For Fish (as stated by ormurin)

set -x JAVA_HOME (/usr/libexec/java_home -d64 -v1.8)

Updating the .zshrc file should work:

nano ~/.zshrc

export JAVA_HOME=$(/usr/libexec/java_home -v 1.8.0)

Press CTRL+X to exit the editor Press Y to save your changes

source ~/.zshrc
echo $JAVA_HOME
java -version

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.

Oracle Java 7 and Apple Java 6 are completely different and they coexist on the same machine as they inhabit totally separate locations.

Java 7, if installed, lives in:

/Library/"Internet Plug-Ins"/JavaAppletPlugin.plugin/Contents/Home

Java 6, if installed, lives in:

/System/Library/Frameworks/JavaVM.framework/Versions/A/

(And the more traditional Java 6 JDK is at: /System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home)

When you type java, you're running /usr/bin/java, which is a symbolic link to Java 6. In fact, if you type ls -l /usr/bin | grep -i java you will see a bunch of symbolic links for the typical JDK/JRE executables.

So if you have installed Java 7, and that's what you want to use from the command line, you can change into its directory and run its specific binaries in bin. To avoid that, you can add its bin directory to your Bash search path, so its contents are invoked instead of the Java 6 symlinks in /usr/bin. To do this, alter /etc/paths to add the bin directory before the first line:

{ echo "/Library/Internet Plug-Ins/JavaAppletPlugin.plugin/Contents/Home/bin"; \
  cat /etc/paths; } | sudo tee /etc/paths > /dev/null

Then set the JAVA_HOME environment variable, so supporting software knows where to find Java 7:

{ echo -n "export JAVA_HOME=";
echo "/Library/Internet\ Plug-Ins/JavaAppletPlugin.plugin/Contents/Home"; } \
| sudo tee -a /etc/bashrc > /dev/null

Now, in any new Terminal window, when you type java -version, you'll see java version "1.7.0_51". (And if you still want to be able to run the Java 6 binaries, you can call them with /usr/bin/java, /usr/bin/javac, etc.)