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"Could find or load main class" while running executable jar file in Java code

After converting my java program to executable jar file using commands in command prompt in windows 10,while executing jar file I am getting error:
Could find or load main class Combine.class" caused
by:java.lang.ClassNotFoundException:Combine.class
My jdk-11.0.1 has javamail api and excelapi.While executing I have set my classpath as:
classpath=%classpath%;C:\Program Files\Java\jdk-11.0.1\javamail_api\javax.mail-1.6.2.jar;C:\Program Files\Java\jdk-11.0.1\javamail_api\activation.jar;C:\Program Files\Java\jdk-11.0.1\jexcelapi\jxl.jar;.;
It was compiling and executing properly but after converting to executable jar file it is not running and giving above error.
Any help would be appreciated.
Thank you

The clue is in the exception message. It is trying to load a class with the name Combine.class. But the classes real name is Combine.
You have created the JAR file incorrectly.
echo Main-Class: Combine.class > manifest.txt
jar cmf manifest.txt FinalExecutable.jar Combine.class
If Combine is in the default package (i.e. it doesn't have a package statement) then the above should be:
echo Main-Class: Combine > manifest.txt
jar cmf manifest.txt FinalExecutable.jar Combine.class
If Combine is declared in package foo.bar, then the above should be.
echo Main-Class: foo.bar.Combine > manifest.txt
jar cmf manifest.txt FinalExecutable.jar foo/bar/Combine.class
and you need to be in the directory above the foo directory.
NB: the "Main-Class" attribute in the manifest must be a Java fully qualified class name, NOT a filename or file pathname.
It also should be noted that the CLASSPATH environment variable and the -cp argument will be ignored when you run a JAR using java -jar .... If your executable JAR depends on other JAR files, you should either combine them (to create a shaded JAR) or you should add a "Class-Path" attribute to the manifest; see https://docs.oracle.com/javase/tutorial/deployment/jar/downman.html
Finally, my advice would be to use a build tool (e.g. Maven) to compile your code, create the executable JAR file, etc rather than doing it by hand.

Java command terminal

Alright so i am trying to run my java file but it's not doing what I want it to from terminal.
I have a main directory called packageTester.
packageTester contains src and bin
src has packageA packageB
pacakgeA has HelloA.java
packageB has HelloB.java
bin has my class files so
bin has packageA packageB
pacakgeA has HelloA.class
packageB has HelloB.class
To compile the files I used the following command when I was in the pacakageTester directory:
javac -d bin -sourcepath source src/package*/* , which works !
Now how do I run HelloB.class which contains the main method and has an object of HelloA.
I thought when at the packageTester directory, I can do:
java bin/packageB/HelloB
but that does not work because it cannot seem to find the .class file.
HELP will be greatly appreciated to figure out how to execute the file correctly

The root of bin should be in your classpath in order for packageB.HelloB to be found as packageB/HelloB.class while parsing the classpath.
The easiest way to do so is to change directory to bin and execute java packageB.HelloB from there.
Alternatively, you can execute java -cp bin packageB.HelloB from your packageTester directory, or from somewhere else if you replace bin by an absolute path.

javac -classpath not doing the trick

I have a source file SerialTalk.java, in directory C:\javasrc\BattProj
This file imports classes from RXTXcomm.jar, eg.
import gnu.io.CommPortIdentifier;
import gnu.io.SerialPort;
...
RXTXcomm.jar is in the same directory as SerialTalk.java. I compile specifying a classpath pointing to the current directory:
javac -verbose -classpath . SerialTalk.java
Invariably, I get the following error. (Actually, many instances & variants of this error):
SerialTalk.java:3: error: package gnu.io does not exist
import gnu.io.CommPortIdentifier;
When I open the RXTXcomm.jar (eg. with 7-Zip) I can see the gnu.io structure, and the specific .class files that I'm trying to import.
So what am I doing wrong? The same .java (source) file has been compiled and run on another workstation within the Netbeans IDE. The difference here is I'm trying to compile it using javac from the command line. (Environment is Win7, 32 bit, jdk1.7.0_03)

So what am I doing wrong?
You're not putting the jar file on the class path. Putting the directory on the class path doesn't do it. That only tells javac where to find .class files in the directory structure, not jar files containing class files. You want:
javac -verbose -classpath .;RXTXcomm.jar SerialTalk.java

Cannot find class even when jar file is in working directory

I am struggling to get my Java program to run on AIX. I used Eclipse on Windows to create a runnable Jar file, jRams.jar below. I kept on getting a class not found error, until finally I put all the external libraries in the same directory.
$ ls
JAXB2_20081030.jar
JAXB2_20110601.jar
activation.jar
asjava.jar
commons-beanutils-1.8.3.jar
commons-beanutils-bean-collections-1.8.3.jar
commons-beanutils-core-1.8.3.jar
commons-codec-1.5.jar
commons-collections-3.2.1.jar
commons-configuration-1.6.jar
commons-digester-2.1.jar
commons-jxpath-1.3.jar
commons-lang-2.6.jar
commons-logging-1.1.1.jar
commons-logging-adapters-1.1.1.jar
commons-logging-api-1.1.1.jar
jRams.jar
jaxb-api.jar
jaxb-impl.jar
jaxb-xjc.jar
jaxb1-impl.jar
jremote.jar
jsr173_1.0_api.jar
log4j-1.2.16.jar
netty-3.2.4.Final.jar
$
Still, I get the class not found error.
$ java -jar jRams.jar
The java class is not found: com.jbase.jremote.JRemoteException
jremote.jar definitely contains JRemoteException. Why isn't this working?
UPDATE
Thank you for your straight-to-the-point answers. I now understand the nature of a java application and a manifest file far better.
Turns out my ftp client was transferring in ASCII mode and not Binary, so the jar files were corrupt. I have learned a great deal, nonetheless.

When using the -jar option, you need to specify which jar-files should be on your class path in the manifest file. Just having the required jar files in the same directory won't do it.
Add a line in your manifest that says:
Class-Path: JAXB2_20081030.jar:JAXB2_20110601.jar:....:netty-3.2.4.Final.jar
or skip the -jar option and launch using
java -cp JAXB2_20081030.jar:....:netty-3.2.4.Final.jar:jRams.jar pkg.JRamsMain
and it should work fine.
(Note that on *nix systems, as opposed to Windows machines, the jar files in the class paths should be separated using : instead of ;.)
Further reading:
The Java Tutorials: Adding Classes to the JAR File's Classpath

You need to add all those JARs to the runtime CLASSPATH by adding the -classpath parameter. AIX requires you to separate the JARs using :

You will have to specify the full path(if libraries not in the same directory as jRams) or just the names of the jar file in a manifest file (If all dependency jars are in the same folder). Alternative specify the path to all the dependent jars using -cp argument.
Example (This assume every dependency is in the same directory you are executing java command from):
java -cp commons-collections-3.2.1.jar; jaxb-impl.jar; etc.. ;jRams.jar package_to_class.MyMainClass.java
Where package_to_class is example: com.myproj.example.
EDITED.

Follow these steps to add "Class-Path" to existing jar file -
Create "newmanifest" file with following entry
Class-Path: additional/jars
Update existing jar file e.g. "classes.jar"
jar --update --manifest=newmanifest --file classes.jar
Inflate jar file
jar -xvf classes.jar
created: META-INF/
inflated: META-INF/MANIFEST.MF
Verify "Class-Path" is added to MANIFEST.MF
cat META-INF/MANIFEST.MF
Manifest-Version: 1.0
main-class: CLASSNAME
Created-By: 15.0.2 (Oracle Corporation)
Class-Path: additional/jars

Package not found; javac

This is annoying.
I have a directory structure like this
-lib
--some jar files
-packageName
--Main.java
--SomeOtherPackage
--SomeOtherJavaClass.java
Main.java imports SomeOtherPackage. And both java files uses jars in the lib.
What I do is add the jar files independently in the CLASSPATH. And then run as:
javac packageName/Main.java
but it gives the error that Package not found SomeOtherPackage . Shouldn't it automatically realize the dependency and build SomeOtherPackage as well? What would be the javac command and the classpath for the above case?
Thanks

The normal practice is to add the package root to the classpath.
When you're already in the package root, use -cp .. E.g.
cd /path/to/all/packages
javac -cp . packageName/Main.java
If you want to include JAR files as well, use the ; (or in *nix, the :) as classpath path separator:
javac -cp .;lib/file.jar packageName/Main.java
To save the time in repeating all the typing of shell commands, use a .bat (or in *nix a .sh) file. Or just an IDE if you're already familiar with java/javac and so on.

You need to add packageName to the CLASSPATH so it can find SomeOtherPackage