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I am having trouble with running a jar file

I try to run a java program with the following script:
#!/usr/bin/env bash
java -cp dl.jar Elevator
Elevator is the main class. But I get an error every time.
Error: Could not find or load main class Elevator
Caused by: java.lang.ClassNotFoundException: Elevator
My classes are all in the .jar file under the directory src/hw4. I have tried adding src/hw4 to the classpath, but to no avail.

You should not use direct class name. You have to use the full package.
If in your IDE you wrote for exammple: package hw4; and you want to run the class Elevator, use this :
java -cp dl.jar hw4.Elevator
It seems to correspond to your jar file.
The src folder is not on the class path. It's just a directory in your IDE to order all files as project files and project content.

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 execute a large compiled Java project via command line, without using any software (Windows)?

I have a Java project with 5 packages and 30 classes. I want to test this project on a different computer, but I can't install any sotware on that computer so I can't use things like Maven, Eclipse etc. Is there a way I can execute the program on that computer?
What I tried to do, is to compile the project using Eclipse on my computer, then went to the other computer and tried to execute the project main class via the folder that the main class .class file is at.
I.E., say that the main class name is Hello in package Greetings and Hello.class is at folder named folder. So I opened the command line window at folder and typed the command:
java Greetings.Hello
That didn't work....
Edit: After doing this I got the message: Error: Could not find or load main class Greetings.Hello

If the package name is Greetings and you want to run Hello.class
Hello class must have main method.
Hello.class must in folder name Greetings (package name).
Execute java Greetings.Hello from the one level above of Greetings folder
It seems to me Hello.class is not inside of Greetings folder

If javac is installed on the system you can directly compile on the system. you can compile even large projects including many packages with choosing different options provided by javac.
The javac tool reads class and interface definitions, written in the Java programming language, and compiles them into bytecode class files. It can also process annotations in Java source files and classes.
There are two ways to pass source code file names to javac:
For a small number of source files, simply list the file names on the command line.
For a large number of source files, list the file names in a file, separated by blanks or line breaks. Then use the list file name on the javac command line, preceded by an # character.
Source code file names must have .java suffixes, class file names must have .class suffixes, and both source and class files must have root names that identify the class. For example, a class called MyClass would be written in a source file called MyClass.java and compiled into a bytecode class file called MyClass.class.
Inner class definitions produce additional class files. These class files have names combining the inner and outer class names, such as MyClass$MyInnerClass.class.
You should arrange source files in a directory tree that reflects their package tree. For example, if you keep all your source files in C:\workspace, the source code for com.mysoft.mypack.MyClass should be in C:\workspace\com\mysoft\mypack\MyClass.java.
By default, the compiler puts each class file in the same directory as its source file. You can specify a separate destination directory with -d source

Given that you have Eclipse and you ran the code in Eclipse: the quickest way is to use Eclipse and export it to executable JAR.
If you have Run Configuration (e.g. named Hello) that you use for running the code:
Menu -> Export -> Runnable JAR file
Launch Configuration: Hello
Select export destination: (e.g. C:\tmp\Hello.jar)
Set Extract required libraries into generated JAR
Click finish.
This will create Hello.jar file you can execute typing in:
java -jar Hello.jar

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

running a java program in a unix terminal

I have a java program with multiple class files and they are all stored in the same folder called lab7. I coded the project in NetBeans so used "package lab7" in all the files. My main application java file is called lab7.java. Now, when i try to run this on the terminal i get "Exception in main thread: NoClassDefFoundError". I do the following inside the folder lab7.
javac *.java
java lab7
I don't know why get this error. It should be some basic class path error. Thanks for the help.

Normally class names should start with a capital letter. So you should rename your main class to Main. If it's inside the lab7 package, run this:
java lab7.Main
This should be run in the directory that contains the lab7 directory. So if you're in the lab7 directory itself, go up one level first.

Use
java lab7.lab7
You do have a lab7.java file with a public static void main(String[]) method, right?