Develop Reference

Has every program that is installed a default complier with itself?

I am not native English user and I am sorry for bad English but very interested in programming.
My question is: Has every program that is installed on a specific OS a separate compiler or all programs that are installed on that OS use from a default OS compiler?
Thanks.

Your question is marked with 'java'. Regarding Java
you must have JRE to run a java program. JRE stands for java runtime environment, which includes JVM (java virtual machine) and another runtime units. Java feature is to have single JRE to run every java program (let alone versions etc). JRE will execute java program.
if you want to create a java program on your computer, you should have have Java SDK (software development kit). It includes compiter and huge bunch af another necessary and useful tools. SDK can turn your java source code to an executable java program.

If the question is general, like, what happens when you install a desktop app, it depends of a app type and it's installer type.
There are some, that are distributed with sources and are to be compiled at the installation phase right at your machine. Most often they have anything they need to run the installation successfully, including all necessary compilters. In vast majority of cases you are not supposed to have something preinstalled on you OS prior to the program installation.
But mainly a program is distributed as a bundle of binary artifacts. They are already compiled and packed and are delived in this state. So no, here you are not expected to have a compiler on you machine.

Can we install two versions of Java JDK on Windows?

I have created an executable JAR file developed on Java version 8. The JAR file was opening on double click. But as the Oracle applications support only Java 6, I had to install JRE 6, but then after the JRE 6 installation, my executable JAR file is not opening.
I have set the JDK 8 bin path in Path environment variables. Is there a solution for this problem? Why is the JAR file not opening after two Java versions in the system?
JAR should open even if two versions 6 and 8 of Java are installed in the system.

You are facing a backward compatibility problem. Backwards compatibility means that you can run a Java 6 program on a Java 8 runtime, but not the other way around.
You can run a lower configuration on a higher configuration, not vice-versa
There are several reasons for that:
Bytecode is versioned and the JVM checks if it supports the version it finds in .class files.
Some language constructs cannot be expressed in previous versions of bytecode.
There are new classes and methods in newer JREs which won't work with older ones.
If you really, really want (tip: you don't), you can force the compiler to treat the source as one version of Java and emit bytecode for another, using something like this:
javac -source 1.8 -target 1.6 MyClass.java
You can compile your code to Java 1.6 bytecode using JDK 1.8. Just take care of the following:
-source=1.8 and -target=1.6 compiler options
If you use Maven, consider having two pom.xml files, with an optional parent file.
Source: Can program developed with Java 8 be run on Java 7?

I am not sure if this solution going to work or not.
Try to run command java -version and look if it returns java 6 or 8 path. Also try to give path of JDK 8 as JAVA_HOME variable and add that into path like this path=%JAVA_HOME%/bin and see if it works. If you get the java 6 as java version try to use above method and then install JRE 6

Hi All Thank you for your response. I kept java6 and reinstalled java8 and now forms and jar both are working!.

In the short term,
the answer is yes. Since both JDK files are downloaded as jar fils it will ok to download both jar files. The reason to not opening after two java versions is as #Elliott said: "in the system is Java 6 can't run Java 8 compiled code, you should be getting an error." That's exactly true but the problem is how to use multiple versions of JDK in a single machine.
Then we have to move on to long term,
The tricky thing is to manage these multiple JDKs and IDEs. It’s a piece of cake if I just use Eclipse for compiling my code because the IDE allows me to configure multiple versions of Java runtime. Unfortunately (or fortunately), I have to use the command line/shell to build my code. So, it is important that I have the right version of JDK present in the PATH and other related environment variables (such as JAVA_HOME).
Manually modifying the environment variables every time I want to switch between JDKs, isn’t a happy task. But, thanks to Windows Powershell, I’m able to write a script that can do the heavy lifting for me.
Basically, what you want to achieve is to set the PATH variable to add the Java bin folder and set the JAVA_HOME environment variable and then launch the correct Eclipse IDE. And, I want to do this with a single command. Let’s do it.
Open a Windows Powershell.
I prefer writing custom Windows scripts in my profile file so that it is available to run whenever I open the shell. To edit the profile, run this command: notepad.exe $profile - the $profile is a special variable that points to your profile file.
Write the below script in the profile file and save it.
function myIDE{ $env:Path = “C:vraajavajdk7bin;” $env:JAVA_HOME = “C:vraajavajdk7” C:vraaideeclipseeclipse set-location C:vraaworkspacemyproject play }
function officeIDE{
$env:Path = "C:vraajavajdk6bin;"
$env:JAVA_HOME = "C:vraajavajdk6"
C:officeeclipseeclipse
}
Close and restart the Powershell.
Now you can issue the command myIDE which will set the proper PATH and environment variables and then launch the Eclipse IDE.
As you can see, there are two functions with different configurations. Just call the function name that you want to launch from the Powershell command line (myIDE).
If any issue please put a comment below!

See all the Java versions installed on Mac

How can I get all the Java versions installed on my Mac?
Is there a terminal command for this?
When I run java -version, I get the current JRE version that is in use in my system, but I want to list all the JREs installed on my Mac.

/usr/libexec/java_home -V
lists one line per Java environment installed (and known to the /usr/bin/java command).
You can still have other Java distributions which are not registered. They are typically downloaded as ZIP files without an installer, or using another package manager like homebrew.

The accepted solution didn't work for me.
The results it returned didn't include all the Java versions installed.
For example on my machine there are currently 5 installations, but the accepted solution only returns 3.
What eventually did work for me is:
mdfind -name 'java' | grep '/bin/java$'
It finds all java installations on the system regardless of how they were installed. This way I found the specific java installation which I was looking for.
Explanation of how it works:
mdfind is a native tool in MacOS to search for different files by given query, it's very fast (usually way faster than find)
The given command is looking for everything called java and then filter only the results that end with /bin/java which is the typical suffix of java installations

What does javac -version show?

Even the official documentation of javac
says nothing about the -version option.

Javac -version tells you about the JDK version. That is, it gives the information of Compiler.

Official page does mention about it:
http://docs.oracle.com/javase/6/docs/technotes/tools/windows/javac.html
Why you cannot find on the page is, -version is not available for javac until Java 6 I believe (I am sure it is not available in 1.4 and before). That's why it is not showing on your page, which is javac ref page for Java 1.5. However, go to java command reference page of Java 1.5 and you should be able to find the -version switch.

This command outputs your java compiler's version. In mine case it shows 1.6.0

Just to expand, both display the same; the version of Java that the tools are part of.
You can ASSUME that what one reports, the other will automatically also report and so it is overkill to have the option in both. Until the moment you encounter a machine where there are multiple versions of Java installed and you can compile something yet you can't run it. Displaying the versions of javac and java can then quickly tell you that the setup of the machine is messed up enough that different versions of Java are being used to compile and run. And then you thank the designers for adding it in.

You are looking at the official documentation for an OBSOLETE version of javac Java 5. The -version option is documented in the javac manual entry for Java 6 and Java 7.
And in response to your question about what -version actually shows (for javac and java) the best answer is "unspecified" or "it depends on what version / platform you are using". In practice, your best bet is to just try it for yourself.
If your reason for asking is that you want to extract some useful information about the Java platform from the version strings ... I recently tried searching for a web page that listed the version strings for the java command, and came up empty. A better approach would probably be to write a tiny Java application to print out the relevant properties from the System properties object. The javadoc for System lists a number of properties that are standardised.

Multiple Java versions running concurrently under Windows

We have a couple of applications running on Java 5 and would like now to bring in an application based on Java 6. Can both java versions live together under Windows?
Is there any control panel to set the appropriate Java version for different applications, or any other way to set up, what version of Java will be used to run that particular application?

Of course you can use multiple versions of Java under Windows. And different applications can use different Java versions. How is your application started? Usually you will have a batch file where there is something like
java ...
This will search the Java executable using the PATH variable. So if Java 5 is first on the PATH, you will have problems running a Java 6 application. You should then modify the batch file to use a certain Java version e.g. by defining a environment variable JAVA6HOME with the value C:\java\java6 (if Java 6 is installed in this directory) and change the batch file calling
%JAVA6HOME%\bin\java ...

I was appalled at the clumsiness of the CLASSPATH, JAVA_HOME, and PATH ideas, in Windows, to keep track of Java files. I got here, because of multiple JREs, and how to content with it. Without regurgitating information, from a guy much more clever than me, I would rather point to to his article on this issue, which for me, resolves it perfectly.
Article by: Ted Neward: Multiple Java Homes: Giving Java Apps Their Own JRE
With the exponential growth of Java as a server-side development language has come an equivablent
exponential growth in Java development tools, environments, frameworks, and extensions.
Unfortunately, not all of these tools play nicely together under the same Java VM installation. Some
require a Servlet 2.1-compliant environment, some require 2.2. Some only run under JDK 1.2 or above,
some under JDK 1.1 (and no higher). Some require the "com.sun.swing" packages from pre-Swing 1.0
days, others require the "javax.swing" package names.
Worse yet, this problem can be found even within the corporate enterprise, as systems developed using
Java from just six months ago may suddenly "not work" due to the installation of some Java Extension
required by a new (seemingly unrelated) application release. This can complicate deployment of Java
applications across the corporation, and lead customers to wonder precisely why, five years after the
start of the infamous "Installing-this-app-breaks-my-system" woes began with Microsoft's DLL schemes,
we still haven't progressed much beyond that. (In fact, the new .NET initiative actually seeks to solve the
infamous "DLL-Hell" problem just described.)
This paper describes how to configure a Java installation such that a given application receives its own,
private, JRE, allowing multiple Java environments to coexist without driving customers (or system
administrators) insane...

It is absolutely possible to install side-by-side several JRE/JDK versions. Moreover, you don't have to do anything special for that to happen, as Sun is creating a different folder for each (under Program Files).
There is no control panel to check which JRE works for each application. Basically, the JRE that will work would be the first in your PATH environment variable. You can change that, or the JAVA_HOME variable, or create specific cmd/bat files to launch the applications you desire, each with a different JRE in path.

We can install multiple versions of Java Development kits on the same machine using SDKMan.
Some points about SDKMan are as following:
SDKMan is free to use and it is developed by the open source community.
SDKMan is written in bash and it only requires curl and zip/unzip programs to be present on your system.
SDKMan can install around 29 Software Development Kits for the JVM such as Java, Groovy, Scala, Kotlin and Ceylon. Ant, Gradle, Grails, Maven, SBT, Spark, Spring Boot, Vert.x.
We do not need to worry about setting the _HOME and PATH environment variables because SDKMan handles it automatically.
SDKMan can run on any UNIX based platforms such as Mac OSX, Linux, Cygwin, Solaris and FreeBSD and we can install it using following commands:
$ curl -s "https://get.sdkman.io" | bash
$ source "$HOME/.sdkman/bin/sdkman-init.sh"
Because SDKMan is written in bash and only requires curl and zip/unzip to be present on your system. You can install SDKMan on windows as well either by first installing Cygwin or Git Bash for Windows environment and then running above commands.
Command sdk list java will give us a list of java versions which we can install using SDKMan.
Installing Java 8
$ sdk install java 8.0.201-oracle
Installing Java 9
$ sdk install java 9.0.4-open
Installing Java 11
$ sdk install java 11.0.2-open
Uninstalling a Java version
In case you want to uninstall any JDK version e.g., 11.0.2-open you can do that as follows:
$ sdk uninstall java 11.0.2-open
Switching current Java version
If you want to activate one version of JDK for all terminals and applications, you can use the command
sdk default java <your-java_version>
Above commands will also update the PATH and JAVA_HOME variables automatically. You can read more on my article How to Install Multiple Versions of Java on the Same Machine.

It should be possible changing setting the JAVA_HOME environment variable differently for specific applications.
When starting from the command line or from a batch script you can use set JAVA_HOME=C:\...\j2dskXXX to change the JAVA_HOME environment.
It is possible that you also need to change the PATH environment variable to use the correct java binary. To do this you can use set PATH=%JAVA_HOME%\bin;%PATH%.

I use a simple script when starting JMeter with my own java version
setlocal
set JAVA_HOME="c:\java8"
set PATH=%JAVA_HOME%\bin;%PATH%;
java -version
To have a java "portable"
you can use this method here:
https://www.whitebyte.info/programming/java/how-to-install-a-portable-jdk-in-windows-without-admin-rights

Or use links. While it is rather unpleasant to update the PATH in a running environment, it's easy to recreate a link to a new version of JRE/JDK. So:
install different versions of JDK you want to use
create a link to that folder either by junction or by built-in mklink command
set the PATH to the link
If other version of java is to be used, delete the link, create a new one, PATH/JAVA_HOME/hardcoded scripts remain untouched

Invoking Java with "java -version:1.5", etc. should run with the correct version of Java. (Obviously replace 1.5 with the version you want.)
If Java is properly installed on Windows there are paths to the vm for each version stored in the registry which it uses so you don't need to mess about with environment versions on Windows.

If you use Java Web Start (you can start applications from any URL, even the local file system) it will take care of finding the right version for your application.

Using Java Web Start, you can install multiple JRE, then call what you need.
On win, you can make a .bat file:
1- online version:
<your_JRE_version\bin\javaws.exe> -localfile -J-Djnlp.application.href=<the url of .jnlp file.jnlp> -localfile -J "<path_temp_jnlp_file_.jnlp>"
2- launch from cache:
<your_JRE_version\bin\javaws.exe> -localfile -J "<path_of_your_local_jnlp_file.jnlp>"