Oracle has special mechanism to download files from them. I wrote just wget and download link. It did not download the full file.

Solution is to click through their site with accepting licence or to write wget like this:

 wget --no-cookies --no-check-certificate --header "Cookie: gpw_e24=http%3A%2F%2Fwww.oracle.com%2F; oraclelicense=accept-securebackup-cookie" "http://download.oracle.com/otn-pub/java/jdk/8u112-b15/jdk-8u112-linux-x64.rpm"

After that you can install java with yum:

 yum install jdk-8u112-linux-x64.rpm

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.

Try rpm -ql jdk-2000 to see the list of files provided by the package jdk-2000 (hint: the rest of the package name given is the version and the architecture, so it's optional to provide it).

If it doesn't link java or javac to /usr/bin/, you can just create the symbolic links from /usr/<wherever>/.../java/ to /usr/bin/ using the ln command with the -s (symbolic link) flag.

It's probably already installed because you tried to install it once, tried the command, it didn't work, so you tried to install it again. Either that, or some other package you installed along the way (or some script you copy and pasted from the internet) installed the package for you. If you want to install the specific version of Java you have in that RPM, first remove the existing version with rpm -e jdk-2000 (-e stands for "erase").

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.

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

Try to run it using sh itself, i.e

sh jdk-6u24-linux-i586-rpm.bin