Develop Reference

GCJ throws error: "Undefined reference to main" when compiling

I´d wanted to compile a simple Java "Hello World" program like it was repesented on the GeeksforGeeks Hello World Tutorial, by using gcj in Linux Ubuntu. This is the source code:
class HelloWorld
{
public static void main(String args[])
{
System.out.println("Hello, World");
}
}
But gcj threw two errors:
(.text+0x18): undefined reference to main
collect2: error: ld returned 1 exit status
Original output from the terminal:
gcj -o helloworld HelloWorld.java
/usr/lib/gcc/i686-linux-gnu/4.6/../../../i386-linux-gnu/crt1.o: In function `_start':
(.text+0x18): undefined reference to `main'
collect2: error: ld returned 1 exit status
I´d take attention on the requirement, that the .java file should be named after the class which holds main:
Important Points :
The name of the class defined by the program is HelloWorld, which is same as name of file(HelloWorld.java). This is not a coincidence. In Java, all codes must reside inside a class and there is at most one public class which contain main() method.
By convention, the name of the main class(class which contain main method) should match the name of the file that holds the program.
What am I doing wrong?

You are missing the --main= option, from the documentation, this option is used when linking to specify the name of the class whose main method should be invoked when the resulting executable is run.
gcj -o helloworld --main=HelloWorld HelloWorld.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

I can't run a Java program

I was starting to learn Java. I followed the tutorial on how to install it.
I also checked by typing "javac"(without the quotation marks) in cmd if it works. And yes it gives a whole list of text which means its supposed to work, right?
This is my java code:
class apples{
public static void main(String args[]) {
System.out.printIn("hello youtube");
}
}
I saved it in a folder called 'test' in my c drive.
This is what I typed in cmd:
cd \
dir
now it lists everything in my c drive and one of them is test
cd test
dir
Now it lists everything in test and one of them is 'youtube.java'(the file I named), so I type
javac youtube.java
This doesn't work
This is what it gives me:
youtube.java:3: error: cannot find symbol
System.out.printIn("hello youtube");
symbol: method printIn(string)
location: variable out of type PrintStream
1 error
Can someone help me out with this?

You have a typo in your call. Change
System.out.printIn("hello youtube"); // capital 'I'
to
System.out.println("hello youtube"); // lowercase 'l'
And as has been mentioned already, in Java, the public class in a file must match the filename.

The name of the class must match the filename.
You should use
public class Youtube{
in your code(as class names are capitalized) and call the file Youtube.java.
Also you used In instead of ln.
Use:
System.out.println(
which means "print line to System.out".

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.

Different results when compiling with command prompt and BlueJ

I'm just starting Java ... again.
I just made a simple program
class first
{
public static void main()
{
System.out.println("Hello!");
}
}
This runs perfectly fine in BlueJ but it gives an error during run-time when running from command prompt.
This is the error
Exception in thread "main" java.lang.NoSuchMethodError: main
It's because I didn't give String args[] in the main parameter list
Till now, I used to give it subconsciously. I know that the string array contains all the parameter values when running but then why is it running in BlueJ?
(BlueJ is a student-friendly Java editor and compiler)

Your program is valid and will compile to the same thing whether you compile from BlueJ or from the command line.
However, blueJ will let you run any static method in a class (so you can test your functions) where as the command line java command will (only) look for a special main method to run. This main method tages a String array with all the command line parameters and your program should look like this even though you don't use these command line parameters:
class first
{
public static void main(String[] args)
{
System.out.println("Hello!");
}
}