You can select the default Java Version by running the following command:

sudo update-alternatives --config java

Select the Java Installation you want to use per default by typing the selection number. Check the default Java Version by running java -version. It should print something similar to the following:

openjdk version "1.8.0_191"
OpenJDK Runtime Environment (build 1.8.0_191-8u191-b12-2ubuntu0.18.04.1-b12)
OpenJDK 64-Bit Server VM (build 25.191-b12, mixed mode)

Try

sudo update-alternatives --install /usr/bin/java java  path_to_java 1

In my case the path was /usr/lib/jvm/java-8-openjdk-amd64

update-java-alternatives has options to update --jre-headless, --jre, and --plugin separately.

Using

sudo update-java-alternatives --jre-headless --jre --set java-1.8.0-openjdk-amd64

worked for me on a Debian Jessie server with no plugin installed.

Those errors are harmless because you're using a JRE; the missing commands are provided by JDKs only. After running update-java-alternatives -s jre-8-oracle-x64, java -version will confirm you're using the Oracle JRE by default:

$ java -version
java version "1.8.0_102"
Java(TM) SE Runtime Environment (build 1.8.0_102-b14)
Java HotSpot(TM) 64-Bit Server VM (build 25.102-b14, mixed mode)

The alternatives are handled by pre- and post-install hooks. There seems to be no hook included in the package you downloaded, but you can easily add an alternative manually:

# update-alternatives --install /usr/bin/java java /path/to/OracleJDKs/java 2
# update-alternatives --install /usr/bin/javac javac /path/to/OracleJDKs/javac 2

Afterwards you should be able to select a Java version by running update-alternatives --config java and update-alternatives --config javac.

However, the java-package package always provided an easy way to install Oracle JDK on a Debian system. Since I have not used it for quiet a while I can not say if it is still compatible with current Java releases.

Just ignore the error(s).

update-java-alternatives has done all it needs to do.
That's most important and what it counts.

Sun-6-java is being phased out by Oracle and is no longer supported. This is why there are no plugins available for your browser anymore.

OpenJDK is the only stack you should be using as sun-java-6 is outdated and deprecated.

http://en.wikipedia.org/wiki/OpenJDK

If you would like to use Proprietary Closed Source features in Java you will need to install the version from java.com (which is also OpenJDK so don't really see any benefit over the Ubuntu Repository version).

OpenJDK is backwards compatible with the old Sun-6-java. What can you not run with OpenJDK?

Oracle Offers Links to Java 7, you have to go out of your way to get Java 6.

The previous edit to my post by "broiyan" is depricated.

I'm somewhat confused. You're directly invoking the java command, which will use the java that's already redirected in in the alternatives configuration. You are literally attempting to set the default java to the java that you've just installed? If you don't change anything, then it should already be pointing to this instance of java.

Secondly, you're assuming that the path to java is the one you're specifying there. It doesn't have to be the case. When I create a minimal docker container with openjdk-8-jdk (and dpkg, so update-alternatives is available), then the path to java is:

root@c330511a7cde:/# update-alternatives --list java
/usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java

So the path that you should have been using was this path.

Now I've manually installed a few more java versions, and because of priorities, java-17 is now the default java, so the output is a bit longer:

root@c330511a7cde:/# update-alternatives --list java
/usr/lib/jvm/java-11-openjdk-amd64/bin/java
/usr/lib/jvm/java-17-openjdk-amd64/bin/java
/usr/lib/jvm/java-8-openjdk-amd64/jre/bin/java

In this case, you're probably better finding the java-8 path from update-alternatives and setting it that way, so something like:

java_8_path=$(update-alternatives --list java | grep 'java-8-openjdk' | head -1)
if [ -n "$java_8_path" ]; then
    sudo update-alternatives --set java "$java_8_path" || echo "darn"
else
    echo "could not find java8 in alternatives list" 1>&2
fi

if you need to install java in Ubuntu, you can try using following command and you will get oracle-java-installer in your synaptic package manager, click it and install it.

sudo add-apt-repository ppa:webupd8team/java

Else try following steps

 $ wget http://download.oracle.com/otn-pub/java/jdk/6u34-b04/jre-6u34-linux-i586.bin
 $ chmod u+x jre-6u34-linux-i586.bin
 $ ./jre-6u34-linux-i586.bin
 $ sudo mkdir -p /usr/lib/jvm
 $ sudo mv jre1.6.0_34 /usr/lib/jvm/
 $ sudo update-alternatives --install "/usr/bin/java" "java" "/usr/lib/jvm/jre1.6.0_34/bin/java" 1
 $ sudo update-alternatives --install "/usr/lib/mozilla/plugins/libjavaplugin.so" "mozilla-javaplugin.so" "/usr/lib/jvm/jre1.6.0_34/lib/i386/libnpjp2.so" 1
 $ sudo update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/lib/jvm/jre1.6.0_34/bin/javaws" 1

To fix it simply run the following commands

sudo update-alternatives --install /usr/bin/java java /usr/lib/jvm/jdk-11.0.1/bin/java 3
sudo update-alternatives --config java

The first correct the errors you made where you had /usr/lib/jvm it should have been /usr/bin/java. The second command gives you the opportunity to pick the default version you want.

The trick is to use update-java-alternatives (from the java-common package).

So to go for OpenJDK 6 to 7, I used update-java-alternatives -l to get a list of Java alternatives and then used sudo update-java-alternatives -s java-1.7.0-openjdk-amd64 to the JDK.

NOTE: The command above threw the following errors,

update-alternatives: error: no alternatives for mozilla-javaplugin.so.
update-java-alternatives: plugin alternative does not exist: /usr/lib/jvm/java-7-openjdk-amd64/jre/lib/amd64/IcedTeaPlugin.so

This is because the openjdk plugin is not installed by default. To fix run

sudo apt-get install icedtea-7-plugin

and rerun update-java-alternatives.

Probably you have made a change in your system that prevent update-alternatives to work.

Expected behavior

As I know, in a typical installation, the java command is a symbolic link in /usr/bin/java. You must exclude the "real" java path from the $PATH variable. If your computer use a java command in a different directory, the update-alternatives will not work.

-- location of the java command
$ type -p java
/usr/bin/java

-- real location of the java command
$ readlink -f /usr/bin/java
/usr/lib/jvm/java-11-openjdk-amd64/bin/java 

If you get a different result for the type -p java command, you must check if the location of the java command is determined by some folder in the $PATH and remove that part.

NOTE: You may define the JAVA_HOME variable processing the /usr/bin/java symbolic link.

-- real location of the JAVA_PATH variable
$ dirname /usr/lib/jvm/java-11-openjdk-amd64/bin/java 
/usr/lib/jvm/java-11-openjdk-amd64/bin/

$ dirname /usr/lib/jvm/java-11-openjdk-amd64/bin/
/usr/lib/jvm/java-11-openjdk-amd64/

-- setting the JAVA_HOME variable
$ export JAVA_HOME=`type -p java|xargs readlink -f|xargs dirname|xargs dirname`