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How can I install libgfortran3 in Ubuntu/Lubuntu 20.04?

I'm using jBlas and it require libgfortran3 and tell me to install it from:
sudo apt-get install libgfortran3
But I'm using Lubuntu 20.04, which use the same packages as Ubuntu 20.04. I can't find the package here
https://packages.ubuntu.com/search?keywords=libgfortran3
So what should I do?

In short, libgfortran3 is no longer supported in Ubuntu 20.04. It depends on an older version of gcc which is no longer maintained in this version of the distribution.
As an alternative, you might want to try installing its successor package with:
sudo apt-get install libgfortran5
It will provide these files for your Ubuntu installation:
/usr/lib/x86_64-linux-gnu/libgfortran.so.5
/usr/lib/x86_64-linux-gnu/libgfortran.so.5.0.0
/usr/share/doc/libgfortran5
This way, you have an option of dynamically loading/linking against libgfortran at runtime.
Hope it helps.

If you have to use libgfortran3, you can just copy a libgfortran.so.3 in /usr/lib/x86_64-linux-gnu/.
It works for me.

Getting Java to work with Windows 10 Ubuntu

I installed the Windows 10 Anniversary Update so that I could try Bash on Ubuntu on Windows. See how to install it. I wanted to see how performant it was. I then installed the Oracle Java JDK as per this article, so I could compare compilation times with the native Windows Java compiler.
I then tried to run Maven on my project, but it didn't work. Maven just hung starting. I noticed in Task Manager that the "java" process was hovering around 92% CPU.
So I tried something simple. I created a dummy Java Hello-world-like class and attempted to compile it with javac. Similarly, the "javac" process sat around 92% CPU.
I thought maybe it had something to do with running headless, but the -Djava.awt.headless=trueoption didn't make it any better.
Has anyone got this to work or have anything to try?
Update:
I also tried installing the Open JDK instead of the Oracle JDK (via sudo apt-get install openjdk-7-jdk) with the same result.

I figured it out. I couldn't use the download directly from Oracle. Instead I had to install it as per this link. Here are the steps:
sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java7-installer

I had the same experience with the Windows 10 Anniversary Update. The first Fast Track Insider Build after the Anniversary Update and later support both the oracle Java 8 installer and the open JDK 7 installer. I just verified the java -version command still works on Insider Preview Build 14951 161014-1700 from the Ubuntu subsystem's bash shell. I have run other JVM based tools from bash shell as well but not extensively.
Here is a link to an issue, Cannot install Oracle JDK 8 #315, describing both installation and JVM hang issues tracked by the BashOnWindows project.

How can I install Eclipse and Java SDK on Ubuntu?

I have just downloaded Ubuntu. It runs much faster than my Windows XP. But, I don't know how I can set up Eclipse v4.2 (Juno), or the Java SDK. I am currently developing in the Lightweight Java Game Library, so I might need some help with that too.

Execute this in your terminal:
sudo apt-get install eclipse
sudo apt-get install openjdk-6-jdk openjdk-6-source openjdk-6-demo openjdk-6-doc openjdk-6-jre-headless openjdk-6-jre-lib
The first one installs Eclipse (you will be asked to enter your password), and the second one installs all the Java stuff you may need!

Don't install Eclipse from the Linux repositories. These repositories may not be up-to-date with the last versions released by Eclipse. A better approach is to follow the instructions in this answer from Ask Ubuntu. For Java programming, make sure you have a JDK and a JRE installed
apt-get install openjdk-7-jdk openjdk-7-jre
You can download whatever Eclipse Java IDE version suits you best from the Eclipse website (link to the Eclipse v4.4 (Luna) release).

You might want to consider installing Sun Java 6 for your Java programming needs; correct me if I'm wrong, but I feel that OpenJDK may still be lacking in some libraries and compatibilities.
If you want to install Java 6 on your machine, you can follow this guide on how to do so: https://codingforme.wordpress.com/2012/05/14/installing-oracle-java-jdk-6-or-7-on-ubuntu-12-04/

Java installation on Ubuntu:
sudo add-apt-repository ppa:webupd8team/java
sudo apt-get update
sudo apt-get install oracle-java8-installer
Eclipse IDE installation on Ubuntu:
Since it requires a detailed list of steps, I would suggest you to visit the blog post Install Eclipse IDE Mars in Linux Ubuntu to check out steps to install Eclipse on Linux Ubuntu.

You don't really install Eclipse - you just download the folder and run the program - just like in windows. Just make sure you download a Linux version.
To install JDK, type:
sudo apt-get install openjdk-6-jdk
You will be asked to type in your password before the installation will begin. You could also install JDK through the software manager.

Just remove all Eclipse packages typing:
sudo apt-get autoremove eclipse
And with tab find all installed Eclipse packages.
Do the same with Java.
I just reinstalled Eclipse with the command
sudo apt-get install eclipse
which also contains Java.

Is it mandatory to use open jdk in linux distribution instead of jdk versions available in oracle website

I am new to Linux and installed Ubuntu. Fine. I gone through the basics of file system and basic things to know.
I installed Jdk1.7. by downloading the related .rpm file from oracle website. I installed Eclipse. I gave the the installed jdk1.7 jre folder in the eclipse project classpath.
But it is not identifying that and a simple java code is not getting compiled.
In one of the forums I found that we should use openjdk for linux distributions.
could someone explain me Why is that? and am I doing anything wrong by installing the jdk7 from oracle website in these linux distributions?
What difference they make?
Thanks

No, you don't have to use openjdk, any jdk should work fine. You say that you gave path to the jre in the jdk, not jdk itself? JRE has no compiler, JDK has.

Try
sudo apt-get install openjdk-6-jdk eclipse
and see if it helps.

The Openjdk or icedtea or ... are JRE or JDK which try to be as open as possible. Oracles JDK/JRE is not Open enough in respect to the open source community, which is something the usual Linux user doesn't want in its Linux distribution as default implementation.
For a to releasing product you should specify the supported/tested Java implementation. There can be difference for example in performance or in tooling.
For developing with eclipse you don't necessarily need a JDK, because eclipse has its own Java Compiler. But it is recommended to use a JDK. It has nice tools and for some projects you need a JDK.

(The oracle jdk (and jre) are working just fine (at the current u05 release)).
Given that I've understood where you unpacked/put/installed the jdk (check the path), the following commands should fix a lot for you.
sudo update-alternatives --install "/usr/bin/java" "java" "/usr/java/jdk1.7/bin/java" 1
sudo update-alternatives --install "/usr/bin/javac" "javac" "/usr/java/jdk1.7/bin/javac" 1
(
sudo update-alternatives --install "/usr/bin/javaws" "javaws" "/usr/java/jdk1.7/bin/javaws" 1
)
sudo update-alternatives --config java
sudo update-alternatives --config javac
Hopefully this should make ubuntu take care of your java specific env. variables.

Getting started Java with Linux OS

I know this is very basic question, but these days I have done all my Java programs on Windows OS. But now I like to go with Linux.
How JDK and all be installed in Linux OS?
I am using Ubuntu 10.04 LTS.

That is dependent on the distribution you are using -- provide more details so people can be more helpful.
In most distributions however this is as simple as installing a package from the official repositories.
For example, in Debian/Ubuntu etc. you can do:
sudo apt-get install sun-java6-jdk
which will get you both the JDK and the JRE which is a dependency.

On Ubuntu:
$ sudo apt-get install openjdk-6-jdk openjdk-6-demo openjdk-6-doc
gets you the SDK, its demos and documentation. You should then be able to run the first set of examples.

It depends on your distribution. In Ubuntu you would do something like this (as root)...
# aptitude search jdk
After your choice, you install it.
# aptitude install sun-java6-jdk
Don't bother about downloading anything from any website. The package manager (apt-get, aptitude) is doing that for you.

Many Linux distributions package the OpenJDK into their repositories so you can install Java like any other application. If you want the Sun version of Java then you can download a bin or rpm file directly from Sun/Oracle that you can install manually.
The Sun/Oracle version can be found here: http://java.sun.com/javase/downloads/widget/jdk6.jsp Just select Linux as your platform in the form.

In addition to others' answers: If you need Java packages beyond what your distro provides, then check out the JPackage project. JPackage provides RPMs (so it won't work on Debian or Ubuntu, unless you use Alien) of a large number of Java libraries, frameworks, and tools.

As people have noted, you can use apt-get to install the JDK:
sudo apt-get install sun-java6-jdk
Also, if you're looking for a graphical editor, Eclipse is a good one, especially when you're not very familiar with Unix operating systems. It also should come with the JDK included with the default install. You can download that with:
sudo apt-get install eclipse
As Tedil mentioned, you can also get this without the console work via Ubuntu's Software Center in the Applications menu from the top bar.