First read the whole answer. The first part shows you your errors and the second part shows you that alien is not necessary.

First Part

Use -d to create a DEB file

alien -d packagename.rpm

or

alien packagename.rpm

from man alien

-d, --to-deb
    Make debian packages. This is the default.

-g, --generate
    Generate a temporary directory suitable for building a package from,
    but do not actually create the package. This is useful if you want to
    move files around in the package before building it. The package can be
    built from this temporary directory by running "debian/rules binary",
    if you were creating a Debian package, or by running "rpmbuild -bb
    <packagename>.spec" if you were creating a Red Hat package.

Second Part

To install Oracle Java you don't need alien and a RPM file. Simply use the following commands.

sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java8-installer

You can install Oracle's Java Development Kit (Oracle JDK) from the webupd8 PPA by running the commands below from a terminal:

sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java8-installer

This will add their PPA to your software sources, update your software index and then install their currently latest build of the Oracle JDK 8. At the date of writing, this should be version 8u74.

The advantages of this approach over your attempt to convert an rpm package are that the binaries were specifically built for your Ubuntu version and that the package manager will automatically handle updates.

If there's no special reason to use Oracle JDK, you can also just use the open source implementation, the OpenJDK 8 JDK:

sudo apt-get install openjdk-8-jdk

Differences between Oracle JDK and OpenJDK include that OpenJDK might miss some rarely used proprietary multimedia libraries and that it does not come with a web applet launcher by default. This functionality is outsourced and available under the name icedtea. Unfortunately there's no icedtea-8-plugin in the official repositories yet, so if you need to run Java web applets, you must install another PPA to get icedtea-8-plugin if you go for OpenJDK. If you never use Java web applets, there's absolutely no need for that plugin though.

If both install and work for you, it doesn't matter which one you choose. both formats accomplish the same thing, and result in the same software being installed.

Another option in Ubuntu is using apt-get install, which is very simple, and automates the process.

If you know the paths where Oracle packages with Java were installed, you can use update-alternatives to choose which Java you want to use. For example, in my case:

sudo update-alternatives --install "/usr/bin/java" "java" "/usr/lib/jvm/jdk1.8.0_51/bin/java" 1

sudo update-alternatives --install "/usr/bin/javac" "javac" "/usr/lib/jvm/jdk1.8.0_51/bin/javac" 1

sudo update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/lib/jvm/jdk1.8.0_51/bin/javaws" 1

and in the end:

sudo update-alternatives --config java

and choose the the number of your Oracle Java installation.

RPM is Redhat, Fedore, CentOS so unusable.

You need, preferably, a DEB version or otherwise the tar.gz version.

I would advicw to use a personal archive as explained here:http://www.webupd8.org/2012/06/how-to-install-oracle-java-7-in-debian.html

Install method for java 7:

su -
echo "deb http://ppa.launchpad.net/webupd8team/java/ubuntu precise main" | tee -a /etc/apt/sources.list
echo "deb-src http://ppa.launchpad.net/webupd8team/java/ubuntu precise main" | tee -a /etc/apt/sources.list
apt-key adv --keyserver keyserver.ubuntu.com --recv-keys EEA14886
apt-get update
apt-get install oracle-java7-installer
exit

You can then install this...

sudo apt-get install oracle-java7-set-default

"To automatically set up the Java 7 environment variables"

You question is a little bit confusing: you want to know how to patch this Eclipse behaviour or you want to update Java?

The title says 1 thing, the question says another.

If you just want to start eclipse using a specific version of a particular JVM you just have to open the file eclipse.ini in the same folder where the eclipse executable is and add this 2 lines:

-vm
/home/user/jdk/bin/java

This option is pretty self explanatory, you just use the -vm option and specify the full path of your jvm of choice.

If you want to install java from a package downloaded from the internet you need another guide, please be more clear.

In oracle.com there is an option to download a "Linux" version that is a tar.gz file which is the correct option in the case of Ubuntu.

However, it is possible to find 3rd party packages online that have Oracle Java packed as an installable package on Ubuntu, however, since these packages are from 3rd parties, trusting them is your responsibility.

Referring to Ask Ubuntu question How to set JAVA_HOME for OpenJDK?,

How to install Open JDK (Java developement kit) in Ubuntu (Linux)?

1. Open Terminal from Application Dash or press Ctrl+Alt+T

2. Update repository:

   sudo add-apt-repository ppa:openjdk-r/ppa  # only Ubuntu 17.4 and earlier
   sudo apt update
   

3. Optional: To search available distributions of openjdk, use the following command:

   apt search openjdk
   

4. Install the appropriate version with the following command:

   sudo apt install openjdk-8-jdk
   sudo apt install openjdk-8-source #this is optional, the jdk source code
   

5. For JAVA_HOME (Environment Variable) type command as shown below, in "Terminal" using your installation path...

   export JAVA_HOME=/usr/lib/jvm/java-8-openjdk
   

   (Note: /usr/lib/jvm/java-8-openjdk is symbolically used here just for demostration. You should use your path as per your installation.)

6. For PATH (Environment Variable) type command as shown below, in Terminal:

   export PATH=$PATH:$JAVA_HOME/bin

7. To check your installation:

   java -version