[FIXED] What am i missing here or i should be doing differently?

Issue

I have just started learning about classes in Java so I am not sure what I am missing here or what I should be doing differently. This is what I am doing:

class Student {

    private int roll;
    private String name;
    public void hW(){
        System.out.println("Hello world");
    }
}

public class classes {

    /**
     * @param args the command line arguments
     */
    public static void main(String args[]) {
      System.out.println("hello");  
      Student s = new Student();
      s.hW();
      // TODO code application logic here

    }
}

It's giving this error when I run this file:

error: can't find main(String[]) method in class: Student

What changes should I make??

I am using Netbeans IDE 11.3 on macOSX

Solution

Though you haven't mentioned in your question how you were trying to run the file, I was able to reproduce the issue with some hit and trial. Here is what you trying to do and why you faced the issue:

1. You have created classes.java in your IDE and in this file, you have typed the code which you have posted.
2. Now, you are going to Mac OS Terminal window and trying to run java classes.java probably thinking of Java 11 feature which allows you to run a single-file program without compiling.

Why you faced the problem?

You faced the problem because class Student is at the top but it does not have public static void main(String[] args) while this feature of Java requires the top-level class to be executable (i.e. having this main method)

Check the following paragraph from https://openjdk.java.net/jeps/330

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.

What is the solution?

Any of the following will work:

1. Move the class, classes up so that it becomes the first class in your file. Since the class, classes already has public static void main(String[] args), your command, java classes.java will execute it without any problem.
2. Put the following piece of code inside class Student and execute java classes.java from the Terminal window.

public static void main(String args[]) {
    System.out.println("hello");  
    Student1 s = new Student1();
    s.hW();
}

3. Execute the following command from the Mac OS Terminal window to run your class in the pre-Java11 way:

javac classes.java
java classes

4. Execute the file from your IDE.

On a side note, you should follow Java naming conventions e.g. the class, classes should be Classes as per the naming conventions.

Answered By - Arvind Kumar Avinash
Answer Checked By - Cary Denson (JavaFixing Admin)