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About compiling Java through terminal javac

I am a Java newbie, just started learning it in college, and my class is using NetBeans but I'd like to use VSCode.
The professor told us that every Java file should start with:
package nameofthepackage
So that Java knows to which package the class (file) I created belongs to.
So I always create this structure:
I create a folder with the name of the main class, and inside this folder I make a src folder that will store the Java files. Eg:
MyJavaProject/src/MyJavaProject.java
I always name the main .Java file the same as the project folder name.
And when I compile I use javac with the -cp parameter to specify that the src folder is the classpath folder, where it should look for the .Java files I create.
I also always tell javac to compile all the files inside the src folder, using the * wildcard.
The issue is that with this line on top of my .Java files, javac compiles all the files, but I can't execute the bytecode because it complains it can't find the classes I created, even the main one.
As Soon as I remove the package line from the top of the files, I can compile and run the code.
So far it's good, but for any more complex projects this is gonna be really annoying.
Any ideas how can I fix this?

You need to specify the full name (with package) of the class that contains the main function.
Assuming the class MyJavaProject is the one containing your main, that would be:
java nameofthepackage.MyJavaProject
This is assuming you built it with
javac -d . MyJavaProject.java
It would create your target class in a directory structure like:
nameofthepackage\MyJavaProject.class

how to run jar files depend on jar files

my program consist of
"javax.mail.jar"
and a simple myProgram.java (Contain Main class) ("package:com.test.myprogram")
and anotherProgram.java (contain Main class) ("package:com.test.myprogram")
I have converted it to "myprogram.jar" file
how do I run "myProgram.java" in shell? using "myprogram.jar"

If your application is in (say) "myprogram.jar" and it depends on "javax.mail.jar", then you would run it like this:
$ java -cp myprogram.jar:javax.mail.jar com.test.myprogram.Main
(On Windows, you need to use ; instead of : as the classpath separator.)
However, it doesn't make a lot of sense to have "myProgram.java" and "anotherProgram.java" BOTH declare a Main class ... by which I assume you mean a class called Main. The problem is that since both versions of the Main class are in the same package, compiling one source file will overwrite the Main class produced when you compile the other source file.

java -classpath yourjarfilename.jar yourpackagename.classname

How to compile and run project with multiple classes in package?

I just realized I'm about to graduate and I still don't know how to handle this situation:
Say I have a java package named mystuff.project1 and inside the package I have Project1.java (which has the main method) and ThingThatDoesStuff.java . Both are public classes, have the package declaration at the top of the file, etc. I can debug this project fine in Eclipse.
For maximum simplicity, I move the project to C:\ so the java files are located in c:\mystuff\project1
I navigate into c:\mystuff\project1 and type javac *.java to compile the class files
Now how the blazes do I run my application?
java Project1 doesn't work
java mystuff.project1.Project1 doesn't work
java -cp . Project1 doesn't work
java -cp . mystuff.project1.Project1 doesn't work
All of the above give me "Error: Could not find or load main class"
I've been searching SO and elsewhere to try to understand this problem but I'm completely baffled.
I've:
cleared my CLASSPATH variable
triple-checked PATH etc
successfully tried compiling and running standalone class files that aren't in a package
I understand this is a stupid newbie question but I just can't figure it out. I realized every other time I've run into this problem I similarly couldn't find a solution and put all of the classes into a single file. I'd rather learn how to stop doing that now.

I think you have to change to C:\ and then enter:
javac mystuff\project1\*.java
java mystuff\project1\Project1
and this should work.
It's either that or:
javac mystuff/project1/*.java
java mystuff/project1/Project1

Mac Terminal: Could not find or load main class CLASSNAME

I am trying to run a java program through the Terminal on Mac, yet getting:
Error: Could not find or load main class (MY CLASSNAME)
I compiled this application with Eclipse, and when I run this with Eclipse, it works fine.
Furthermore, I am in the right directory, as when I type "ls" in the Terminal, it lists all the files, includes the class file I am trying to run.
This is what I type:
java mainClass
I would very much appreciate help to solve this!
Thank you,
Dean
EDIT: Solution - instead of java mainClass, it must have package too: java startPackage.mainClass

Start by making sure you are at the directory above the top level package
If the class belongs to the package com.foo.bar, you want to be in the directory above com.
In your case, you want to be in the directory above startPack.
Then you need to use the fully qualified name to run the class...
java statPack.mainClass
For example...

Make sure you have the current directory inside your CLASSPATH.
java -cp . mainClass
To set this globally, you can use export CLASSPATH=$CLASSPATH:. inside .bash_profile.
Separately, if your class lives inside a package such as com.foo.bar, then you will need to go to the parent directory of com and run your application with the full path.
java com.foo.bar.mainClass

I too faced this on Mac machine and then what I had to do to make it work was:
Problem Statement:
I had one package xyz under the root of project i.e src/main/java and then inside xyz package I had one class Student.java
my current directory is /Users/username/projectname/src/main/java/xyz:
I can see Student.java exists here
and I compiled it using javac Student.java
Now I see class file has been created at this location. But when I try to run the class file using java Student
I get the error: Error: Could not find or load main class Student
Solution:
Now the solution is to go one step back in the directory and go to root path:/Users/username/projectname/src/main/java and run the command
java xyz.Student
and it will work.
Link to follow: https://javarevisited.blogspot.com/2015/04/error-could-not-find-or-load-main-class-helloworld-java.html

For people dumb like me, make sure you are typing java HelloWorld - and NOT java HelloWorld.class - to run the compiled file with the name HelloWorld.class. This is especially so if you are used to hitting the tab key to complete the file name, as the terminal will give you java HelloWorld.class if you hit the tab key for autocomplete after typing something like java He...
This answer is here because it took 3 sites, including this answer, and 25 mintues before I figured out what I was doing wrong.
Logic is easy, typing is hard.

Using the absolute path can also resolve this problem:
java -classpath /Users/xingliu/IdeaProjects/springproject/src/main/java/ startPackage.mainClass

Package name is different than the folder structure but still Java code compiles

I am using Notepad++ to write my Java code and Command Prompt to compile and run it.
Following is my sample Java code,
package abraKadabra;
public class SuperClass{
protected int anInstance;
public static void main(String [] abc){
System.out.println("Hello");
}
}
However, this file is in the following folder structure :
"usingprotected\superPkg" (usingProtected is a folder somewhere in the hierarchy in C:)
So, my package name here should be something like usingProtected.superPkg instead of abraKadabra as I wrote it.
But, when I compile this Java code from command prompt, it compiles fine with no error or warnings. Why is it so? Shouldn't the package name adhere to the folder structure?
And if it should, how would it adhere?
For e.g. if my package name is usingProtected.superPkg, will the compiler check in the reverse order. The present working directory should be superPkg, then the parent directory should be usingProtected and its done. Is it how it checks the folder structure with package name?

The Java language specification doesn't force files to be in a certain directory. It optionally allows the compiler to require that public classes are in files with the same name of the class, but I don't think there's anything similar for packages. Section 7.2.1 talks about possible storage options in a file system, but it doesn't say anything about enforcing source code structure, as far as I can see.
However, it's best practice - and a pretty much universally accepted convention - to reflect the package structure in the source directory structure... and javac will use this to try to find source files which aren't explicitly specified to be compiled.
Note that if you're compiling from the command line, by default each class will appear in the same location as the corresponding source file, but if you use the "-d" option (e.g. "-d bin") the compiler will build an appropriate output directory structure for you, rooted in the specified directory.

After experimenting a bit, I got the way how to use package name and run Java class files from command prompt.
Suppose following is my Java source file:-
package mySample;
public abstract class Sample{
public static void main(String... a){
System.out.println("Hello ambiguity");
}
}
This file is in directory "D:\Code N Code\CommandLine".
Now, when compile the source code (by going to the above directory from cmd) using following command:-
javac -d . Sample.java
This automatically creates "mySample" folder in my current directory. So, my class file Sample.class is present in directory "D:\Code N Code\CommandLine\mySample". Compiler created this new folder "mySample" from the package name that I gave in my source code.
So if I had given my package name to be "package com.mySample", compiler would create two directories and place my class file in "D:\Code N Code\CommandLine\com\mySample".
Now, I am still in the present working directory i.e. in "D:\Code N Code\CommandLine". And to run my class file, I give the following command:
java mySample.Sample
So, I give the complete hierarchy of package and then the class name. The Java Interpreter will search the current directory for "mySample" directory and in that for "Sample.class". It gets it right and runs it successfully. :)
Now, when I asked that why it compiles my wrong package source code, it would compile the code successfully though, but it gives NoClassDefFoundError when I run my class file. So above method can be used to use package names from command line.

If you're compiling a single class, javac doesn't need to look elsewhere for it. It'll just compile the file as is and put the resulting .class into the same folder. However, you generally won't be able to use the class til you put it into an "abraKadabra" directory in one of the directories in the class path.
If your class uses another class in the package, though, you might have problems compiling it where it is, for the same reason (javac wants to find the class and make sure it has the methods and such that your class uses).

Java compiler does not check the directory structure when it compiles source files. As you mentioned, suppose you have a source file that starts with the directive
package abraKadabra;
You can compile the file even if it is not contained in a subdirectory .../abraKadabra . The source file will compile without errors if it doesn’t depend on other packages. However, the resulting program will not run (unless also including package name in execution). The virtual machine won’t find the resulting classes when you try to run the program.