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How PyCharm runs without JRE although it is made using Swing?

Is PyCharm made using Java-Swing?
If it is, then how does it runs on a PC where JDK or JRE hasn't been set up yet?

When I look at my own PyCharm installation directory structure, I see a jre64 directory, which contains a Java installation.
And that seems to be a normal JRE:
$ ./pycharm-community-2018.2.4/jre64/bin/java -version
openjdk version "1.8.0_152-release"
OpenJDK Runtime Environment (build 1.8.0_152-release-1248-b8)
OpenJDK 64-Bit Server VM (build 25.152-b8, mixed mode)
So PyCharm comes with a Java distribution which it uses (even instead of an already available Java installation). Instead of my pre-installed and configured java 1.8.0_191, it uses the version it comes bundled with:

Can't run java applications

I have trouble when trying to open my jar files with the following command:
java -jar software.jar
They load until a certain point and then nothing happens.
I deinstalled default-jdk and OpenJDK with synaptic and re-installed it with apt-get, but nothing changed. I had a similar problem one month ago and solved it (apparently only temporarly) by deleting the package gcj-5-jre-lib (5.4.0-6ubuntu1~16.04.4) (Java runtime library for use with gcj (jar files))
The following outputs will help:
java -version
openjdk version "1.8.0_131"
OpenJDK Runtime Environment (build 1.8.0_131-8u131-b11-2ubuntu1.16.04.3-b11)
OpenJDK 64-Bit Server VM (build 25.131-b11, mixed mode)
javac -version
1.8.0_131
My variables $JAVA_HOME and $JRE_HOME were correctly added to my $PATH.
I'm on Ubuntu 16.04
EDIT
Here is an example of what I get when I try to start PhyDE.jar
java -jar PhyDE.jar
Welcome to PhyDE 0.9971!
Reading preferences.
Creating PhyDE-Application window
Checking for updates...
You are using the latest version of PhyDE
Done with checking for updates
When checking the core use with the command top I see that the process is active, but the user interface does not open as it usually does. I am located in the folder where the jar file is, and same happens with other jar files.

Eclipse Neon installation fails because of missing JRE/JDK

it opens a web browser and a page tells me that I have to install either JRE or JDK version 1.7.0 or higher. I have installed JRE 1.8.0, in fact in command line I can see:
d:>java -version
java version "1.8.0_121"
Java(TM) SE Runtime Environment (build 1.8.0_121-b13)
Java HotSpot(TM) Client VM (build 25.121-b13, mixed mode)
I also setup JAVA_HOME and PATH accordingly but no luck. Is there something else I can try?
Thanks
I found the issue: I installed JRE X86 while Eclispe wanted the X64 version. I have installed it now and the installation process goes on. I didn't install JDK because I want Eclipse for C++ I don't need Java things. Thanks anyway, I hope this will help others!

You need the JDK installed to run Eclipse, or even better, to develope java apps at all...
verify that by doing in a terminal:
>javac -version

Failed to install jdk 1.7 on mac

I'm trying to install JDK 1.7 update 21 on Mac OSX 10.7 .
I downloaded the installer from oracle site (as dmg file). ran the installer which completed successfully.
Now I can't find the java SDK anywhere on the file system.
The output of /usr/libexec/java_home -V is :
***/System/Library/Java/JavaVirtualMachines/1.6.0.jdk/Contents/Home***
which is the previous version installed.
also running java -version executes the same old version.
***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)***
In the java control panel I do see that the JRE has updated to Java 1.7 for the browser but there aren't any details regarding the JDK there.
Where can I find the new JDK on the file system and how do I set it as default ?
Is there any log file for the installation that I can use to debug ?

Try adding this to your shell startup script (.bash_profile off of ~ if you are using bash):
export JAVA_HOME=`/usr/libexec/java_home -v '1.7*'`
The changeover from Apple providing the JDK to Oracle has been a complete disaster.

GWAN not recognising Sun Java 7

OS:
Ubuntu 12.04 Desktop
Java installed:
java version "1.7.0_05"
Java(TM) SE Runtime Environment (build 1.7.0_05-b05)
Java HotSpot(TM) 64-Bit Server VM (build 23.1-b03, mixed mode)
Running sudo ./gwan gives following error:-
Please install OpenJDK or SUN Java
.
'report.java': to use Java (*.java) scripts, install 'javac' (sudo apt-get install javac).
'loan.java': to use Java (*.java) scripts, install 'javac' (sudo apt-get install javac).
'argv.java': to use Java (*.java) scripts, install 'javac' (sudo apt-get install javac)..
'hello.mm': to use Objective-C++ (*.mm) scripts, install 'gobjc++' (sudo apt-get install gobjc++)
'all.java': to use Java (*.java) scripts, install 'javac' (sudo apt-get install javac)
Javac, Java both in sys path, both available in terminal
I have not touched any configuration files just went through
http://gwan.com/download
then
tried heading over to
localhost:8080/?hello.java
note: all C files work.
Have not tried openJDK though I would like to stay away from this option.

Try to use the JAVA_HOME environment variable to tell which JVM should be used. The G-WAN site has a dedicated Java FAQs entry for that question.
Reporting the same kind of problems in the past I have been told that the next release will be more JVM-agnostic.