Develop Reference

java11 allows the compilation and execution in a single line

in a file named filename.java
class filename{
public static void main(String[] a){
System.out.println("From filename main method");
}
}
public class ClassName{
public static void main(String[] a){
System.out.println("From First main method");
}
}
Observe below commands:
Command 1:
C:\javaDJ>java filename.java
From filename main method
Command 2:
C:\javaDJ>javac filename.java
filename.java:7: error: class ClassName is public, should be declared in a file named ClassName.java
public class ClassName{
^
1 error
Observation:
command 1 compiles (i assume internally ) and executes successfully.
command 2 throws compilation error.
Problem Statement :
How is java cmd able to compile the file called filename.java, when the file(filename.java) contains a public class (ClassName)which is not named 'filename.java' (the name of the file-name.) ?

To highlight a specific section from the JEP#Launch Single-File Source-Code Programs with regards to the behavior
In source-file mode, execution proceeds as follows:
The class to be executed is the first top-level class found in the
source file. It must contain a declaration of the standard public
static void main(String[]) method.

The feature which enabled you to execute Command 1 successfully was introduced in Java 11. The feature allows you to execute a Java source code file directly using the java interpreter. The source code is compiled in memory and then executed by the interpreter, without producing a .class file on disk. Check this for more information.
The error you got in Command 2 has been there since the beginning of Java.

Trying to understand how to create a jar file that opens terminal and takes in a argument

I finished creating a program but I was told that my program
must be a Java application that takes as a command line argument the name of the file."
I understand I can use the jar command in terminal but I don't undestand how you open the terminal and take a file name as a argument. I was wondering if someone could explain what code is required to do this.
Thanks alot.
I tried creating a basic jar file in terminal with the line "jar cvf findOptimalTransport.jar ." but the jar file does not open, I think its because the current implementation takes the users input with a scannar in the code and prints via the terminal. However, this wont work because a terminal window is not opened with this command.

It doesn't have to be a jar file. Command line arguments can be entered from the command line, when you run your application.
Let me give you an example, about how this works. Let's say you have the below simple Java application:
public class MyApplication{
public static void main(String[] arguments){
System.out.println("Hello World!");
}
}
That public static void main() is a method; and more specifically the main method of your application which is what is executed when compiled and ran.
To compile and then run it, you type in the command line/terminal:
javac MyApplication.java //this will compile it
java MyApplication //this will run the main method of MyApplication
But what is that parameter in the main method? What is String[] arguments ?
When you run your program, whatever you type after the application name is an argument, of type String and it is stored in the String array String[] arguments (or most commonly String[] args).
What this means, is that, if you execute your application like this:
java MyApplication some_file.txt // Run application with one arg.
You can access that argument like so:
public class MyApplication{
public static void main(String[] arguments){
System.out.println("Hello World!");
System.out.println("You entered: " + arguments[0]);
}
}
Output:
Hello World!
You entered: some_file.txt
Note: To run a jar file, you need to navigate to the folder that the jar file is in and from the command line you can run it by typing:
java -jar <jarname>.jar

Unable to run java with command line arguments

I have been trying to run Java with command line arguments, but for some reason the class can not be found. I am very certain the directory is correct. Is there any way to fix this?
CLDemo.java file
public class Demo {
public static void main(String[] args) {
System.out.print("It works!!!");
}
}

You need to do cd out\production before java CLDemo.
The default compile output of IntelliJ IDEA is under out\production folder, and Java needs to run at the corresponding package (folder) of your compile output.

Java error Could not find or load main class package

When I run this code from eclips it will give the error print:
package Chapter08_JavaIOFundamentals;
import java.io.Console;
public class Echo {
public static void main(String []args) {
Console console = System.console();
if(console == null) {
System.err.println("Cannot retrive console object - are you running your application from an IDE? Exiting the application ... ");
ystem.exit(-1); }
console.printf(console.readLine());
}
}
But when i try to run it from a command line like this.
javac Echo.java //(this will gives no errors)
java Echo
it will give a error:
Error: Could not find or load main class Echo.
Other programma's like Hello World give no problem.
What is the problem here?

You have to call java -cp . Chapter08_JavaIOFundamentals.Echo from the parent directionry of Chapter08_JavaIOFundamentals

You need to check that you added the location of your .class file to your classpath. If it is in the current folder then you can a dot . in your classpath

Opening class in package from console in Java

So when i try to open a java class that's not in a package from the command prompt it all works fine, but when I try to open a class that's in a package it gives me NoClassDefFoundError.
And when I list the package when I try to open the class (java somepackage/someclass) it says that it can't load or find the main class.
Any help?

What I can infer is, you have two classes:
Test.java:
// no package defined here
class Test{
public static void main(String[] args){
System.out.println("Test");
}
}
so you can compile and run it using:
javac Test.java
java Test
Another class:
Test.java:
package test; // package defined here
class Test{
public static void main(String[] args){
System.out.println("Test");
}
}
And thus doing same thing gives you error.
For this you need to be in the parent directory of 'test' folder in your terminal or cmd and use:
java test.Test
No problem with compiler. You can compile as usual using javac Test.java from inside 'test' folder.

NoClassDefFoundError means that your JVM can't find your class at runtime. This could be because it's not visible (set to private or protected, or just no access modifier).
It could also be missing from your build path

package pkg1;
public class Dataguise{
public static void main(String [] args){
System.out.println("My name is Maninder Singh");
}
}
Suppose this is my code. My package name is pkg1.
1. First you need to create pkg1 dirrectory if not existed.
2. Run "javac Dataguise.java" command
3. It will generate the "Dataguise.class" file and move this file to "pkg1" folder
4. Now run "pkg1.Dataguise" command it will work.
I was having the same issue so sharing my experience.

What I assume is, you are creating a package and a class inside it, for example.
package com.vishwa.hello.commandLineArgs;
public class ComandLineArguments {
public static void main(String[] args) {
// TODO Auto-generated method stub
for (int i = 0; i < args.length; i++) {
System.out.println("Command line arg is "+args[i]);
}
}
}
When I try to compile and run the above code in command prompt (from the package folder) as
javac ComandLineArguments.java.
java ComandLineArguments 123 456.
You get the below error:
Error: Could not find or load main class ComandLineArguments
Caused by: java.lang.NoClassDefFoundError: com/vishwa/hello/commandLineArgs/ComandLineArguments (wrong name: ComandLineArguments)
There are 2 ways to solve this:
Go to the root path of your project and then run.
java com/vishwa/hello/commandLineArgs/ComandLineArguments 123 456
789.
Command line arg is 123.
Command line arg is 456.
Command line arg is 789.
If you want to run the program from the package directory, you need to specify the class path along with the complete package
reference.
java -cp
/Users/16399/Documents/workspace-spring-tool-suite-4-4.12.1.RELEASE/CoreJava/src/
com/vishwa/ hello/commandLineArgs/ComandLineArguments 123 456 789.
Command line arg is 123. Command line arg is 456. Command line arg is 789.