Develop Reference

Very first hello world java program works in Eclipse but not in command prompt

I know this question has been answered before but as a total newbie trying to start learning Java i'd be grateful please for a specific answer?
In Eclipse when i press run with the following code it works fine
public class MySweetProgram {
public static void main(String[] args) {
System.out.println("Hello there!");
}
}
However when i go to C:\Program Files (x86)\Java\jre1.8.0_241\bin in a command prompt and type java MySweetProgram i get an error saying cannot load or find main class MySweetProgram
I've searched through other threads that advise to set the classpath. Mine is set to C:\Program Files (x86)\Java\jre1.8.0_241\lib.
I changed the folder in Classpath from lib to bin but got the same error, so i changed it back again
I do a java -version and get the following :
java version "1.8.0_241"
Java(TM) SE Runtime Environment (build 1.8.0_241-b07)
Java HotSpot(TM) Client VM (build 25.241-b07, mixed mode)
Would someone be able to advise me please on what they think may be the issue?
I apologise if this is an ignorant question
Thanks
Jimmy

welcome! Eclipse will be compiling the code for you and then running it. It does this all for you when you click run, to make your life as a developer slightly easier. If you would like to compile and run the command from the command line, you will need to ensure you have access to the jdk (which must be installed somewhere by Eclipse, but may not be immediately obvious where... a folder called /something/jdk/, presumably). With that on the path (in the %PATH% variable), the following should work for you:
cd /to/where/your/code/is
javac MySweetProgram.java
java -cp . MySweetProgram
Have a look into compilation and what it means, so you gain a better understanding of what Eclipse is doing. And good luck to you!

When you compile a .java file, in your case the MySweetProgram.java, you can decide where you want the executable file to go. That file will be called MySweetProgram.class. Eclipse may keep those two files in separate directories.
Normally (before Java11) you could only run a .class file. So you'd have to add classpath (-cp) argument for java to find your file:
java -cp path MySweetProgram (don't put the .class)
Now (after Java11) you can run the java file directly because when you give it a .java file, it will compile it for you just before executing it:
java MySweetProgram.java (if you're in the directory)
java path\MySweetProgram.java
If you have a MySweetProgram.class in the directory, this will give an error.
It gets a little more complicated when you use packages, but Oracle has some very nice Offical Java Tutorials

Ant is unable to find a javac compiler

I am trying to build some binaries of an application. When i am trying to make the build, I am getting this:
BUILD FAILED
/home/.../ant-build.xml:84: Unable to find a javac compiler;
com.sun.tools.javac.Main is not on the classpath.
Perhaps JAVA_HOME does not point to the JDK.
It is currently set to "/usr/lib/jvm/java-8-openjdk-amd64/jre"
The application requires java 7, and i have it installed but i had it along java 8. The build requires for tools.jar which is located in java 7 only, therefore after several attempts i uninstalled java 8. However, when I am trying make again, i get the same error, even though i don't have java8 installed at all!
I set the JAVA_HOME properly:
$ echo $JAVA_HOME
/usr/lib/jvm/java-7-openjdk-amd64/
I doubt i need the path but i set that one up just in case as well :
$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/usr/lib/jvm/java-7-openjdk-amd64/bin
I'm running out of ideas here :P any help would be appreciated.
Also something maybe important, I had to add the repo manually to get java-7 otherwise it wasnt able to locate the package. Thats when i got double jdk-s.

I guess you are calling the JAVAC command from any other source/tool like Jenkins/Bamboo/Teamcity. Make sure you have set the environment variable of that tool. usually it lies in the manage tool option.
I was facing same issue while using Jenkins.If you don't set up an environment variable in tool from which you are making a call it take your
"/usr/lib/jvm/java-8-openjdk-amd64/jre" so go and set the the env variable JAVA_HOME of you tools. attaching screen shot of Jenkins issue. Go to Manage Jenkins -->configure system --> environment variables and set you java_home path.
Cheers

I just encountered the same situation.
That is because JAVA_HOME is not correctly set in sudo environment.
I added export JAVA_HOME=/path/to in /root/.bashrc, and eventually it was built right.

It seems that you are using javac of java8 with JAVA_HOME or CLASS_PATH pointing to java7.
You can use which java to check that.
export JAVA_HOME=/lib/jvm/java-7-openjdk-amd64
export export ${PATH}=${JAVA_HOME}:/bin:${PATH}`

Correctly install Java for Matlab

I am trying to extract a jar file from Matlab code using javac from Library Compiler (java package).
I have set JAVA_HOME to:
C:\Program Files\Java\jdk1.7.0_71
and added to PATH:
C:\Program Files\Java\jdk1.7.0_71\bin.
When I enter java -version in my console, I get java version jdk1.7.0_71 and running javac -version shows jdk1.7.0_71.
However, it seems that matlab could not find javac, thus, I am not able to compile my .m code into a .jar file. When I tried to compile .m code I got the following:
Error: An error occurred while shelling out to javac (error code = -1).
Unable to build executable.
Executing command: ""C:\Program Files\Java\Java\jdk1.7.0_11\bin\javac" -verbose - classpath "...\MATLAB\R2014a\toolbox\javabuilder\jar\javabuilder.jar" -d "...\features_extraction\for_testing\classes" "...\features_extraction\for_testing\features_extraction\Class1.java" "...\features_extraction\for_testing\features_extraction\Features_extractionMCRFactory.java" "...\for_testing\features_extraction\Class1Remote.java" "...\for_testing\features_extraction\package-info.java""
This is the directory of my java bin folder and javac.exe exists : ...\Java\jdk1.7.0_71\bin\bin\javac? I found this link for compatibility between Java and Matlab2014a(which is my version) here. What should I check in order to correctly link Matlab to Java? A few days ago Matlab was working correctly with Java.
In my PATH I noticed the existence of the following path: C:\ProgramData\Oracle\Java\javapath in which there are three wrong shortcuts to java.exe, javac.exe, and javax.exe. When I type in matlab console getenv JAVA_HOME I am getting C:\Program Files\Java\jdk1.7.0_11 while when I wrote version -java I got: Java 1.7.0_11-b21 with Oracle Corporation Java HotSpot(TM) 64-Bit Server VM mixed mode. When I write javac in matlab console I am getting: Undefined function or variable 'javac'.
When I try to built C++ library I got a similar issue: Error: An error occurred while shelling out to mbuild (error code = -1).
Unable to build executable.
EDIT: Does the absence of the proper MCR counts in my case?

You said
getenv JAVA_HOME I am getting C:\Program Files\Java\jdk1.7.0_11 while when I wrote version -java I got: Java 1.7.0_11-b21
try to use setenv in MATLAB, doc here http://www.mathworks.com/help/matlab/ref/setenv.html
setenv('JAVA_HOME','C:\Program Files\Java\jdk1.7.0_71');
setenv('PATH','C:\Program Files\Java\jdk1.7.0_71\bin');

Following from the answers in this link
http://www.mathworks.com/matlabcentral/answers/131639-matlab-deploytool-not-working-fails-while-building
seems like you have to install JDK 1.7.0_11-b21, which is the one your MATLAB version is compatible with for compiling into jar. Also, after installing that specific version, you should modify the JAVA_HOME and PATH environment variables. Try that to see if it helps.

I was having a very similar problem with my installation.
I set the environment variable (press > type env > Enter) JAVA_HOME (under "User variables") to my JDK home directory (not the bin folder) without any trailing \ or ;.
The problem occurs because MATLAB cannot execute javac properly, however, in order to work out exactly what MATLAB is trying to execute (which is not working) we need to use the command window and not the Library Compiler.
It appears you have already done this but for completeness I will add the process here for others.
To find what MATLAB executes, click "Open log file" and copy the entire first line.
Paste the first line in the MATLAB command window and add -v to the end.
Press enter to run the command and note the "Executing command:" that is printed to the screen.
Once you have the "executing command" that MATLAB returns for you, check the first path in that command and make sure it matches your javac file.
In your original post you have done this and posted
C:\Program Files\Java\Java\jdk1.7.0_11\bin\javac
a path which contains \Java\Java, if this isn't a typo then it may be a problem as it is not the default Java installation path. You also say after that your installation is at
...\Java\jdk1.7.0_71\bin\bin\javac
which contains \bin\bin and is also not the default java bin install directory, perhaps another typo?
If the path to javac matches exactly you could try copying and pasting the "executing command" into your OS's command window (+R > cmd > Enter) and looking for errors. By doing this we can remove ourselves from the MATLAB installation and if errors still exist it likely has nothing to do with MATLAB.
To see if the same problem could be caused by other environment variables I removed every other path to a java installation (keeping JAVA_HOME) in my environment variables. MATLAB uses the absolute path to the java installation and so it was still able to compile with javac using only the JAVA_HOME environment variable.

javac not working in CMD

I'm having a issue with compiling java from my Command Prompt. At first it was saying javac isn't a recognised internal or external command and by reading up I see you need to change the path and thats what I did exactly to...
variable name: path
variable value:%SystemRoot%\system32;%SystemRoot%;%SystemRoot%\System32\Wbem;%path%;C:\Program Files\Java\jre7\bin;
I closed commpand prompt and attempted it again however the same issue came up.
I then tried echo %path% and the entire path line was echoed(entire line in terms of that variable value)
I also have tried "for %i in (javac.exe) do #echo %~$PATH:i" and that returns ECHO ON.
I'm quite stumped and confused now. My javafile is in and named main.java however I am trying to compile it whilst in the javawork folder in CMD
C:\Users\MYNAME\Documents\Javawork
Thanks,

You want the path to the JDK, not the JRE, in your system path. The JRE doesn't come with a compiler (javac).

To compile Java code you need to install the Java Development Kit (JDK) as javac.exe isn't included in the Java Runtime Environment (JRE).
The path should point to the bindirectory of the JDK.

Why javac can compile in this case

My environment is Windows 7 and JDK 1.7.
I have not set the CLASSPATH environment variable; echo %CLASSPATH% outputs nothing.
Java compiler: C:\Program Files\Java\jdk1.7.0_10\bin
Java source: is in D:\tmpmulu\Tj.java
I run the command like below:
C:\>"C:\Program Files\Java\jdk1.7.0_10\bin\javac.exe" -cp d:\tmpmulu\ d:\tmpmulu\Tj.java
It works. The command set the classpath and compiled the file.
But when I change the command to use . instead of d:\tmpmulu\ as my classpath:
C:\>"C:\Program Files\Java\jdk1.7.0_10\bin\javac.exe" -cp . d:\tmpmulu\Tj.java
It's also OK.
That confused me. The . means the current path, it should be c:\. How did it run successfully?
Another question is command like below:
C:\>"C:\Program Files\Java\jdk1.7.0_10\bin\javac.exe" -cp d:\tmpmulu\ Tj.java
As my thinking, the classpath is set to d:\tmpmulu\, it should find the Tj.java file. But the result is 'file not found Tj.java'.
Can anyone tell me details?

Well, presumably your code doesn't rely on having anything in the classpath, basically. If it only uses classes from the JDK, that's absolutely fine.
Note that the classpath is only used to find class files - not source code. That explains both the lack of failure when your source path isn't on the classpath, and then failure when you try to use the classpath to locate Tj.java.

First, if you're compiling with javac -cp . myClass.java, you can omit it the -cp . completely, since it's the default classpath.
Second, -cp should be used when you need to specify references to additional JARs file not included in the standard JDK library, such as a jdbc connector. If your class doesn't require any additional library, then it doesn't really matter what directory you tell javac to look into.