DrJava Error

[pratzy]

Hi!

I am new to J2SE & I have recently installed DrJava IDE as I came to know that it consumes very less memory. I have kept all my programs in D:\JAVAPROG.
Whenever I run a piece of code like the following, I get an error.

"Welcome to DrJava. Working directory is D:\JAVAPROG
> java HelloComponent
Static Error: No static method in HelloComponent with name 'main' accepts arguments (String[])
> "

The code is as follows:

 import java.awt.*;
 import javax.swing.*;

    class HelloComponent extends JComponent {
      public void paintComponent( Graphics g ) {
        g.drawString( "Hello, Java!", 125, 95 );
      }
    }


    public class HelloJava {
      public static void main( String[] args ) {
        JFrame frame = new JFrame( "New Frame" );
        HelloComponent hello = new HelloComponent(  );
        frame.add( hello );
        frame.setSize( 300, 300 );
        frame.setVisible( true );
      }
    }

The above program compiles without any error but gives error when I run it. However, when I run a code having a single class, it gets executed perfectly. Therefore, I think the problem only occurs in a program with multiple classes where all the classes cannot contain main method.

Also, if I try to compile and execute the above program in DOS mode, it runs without any trouble. Looks like a DrJava config issue to me.

How do I overcome this trouble?? HELP!

[Eranga]

Did you write both class in the same file?

[pratzy]

Yes. Both the classes are written in the same file.

[Eranga]

You cannot create two classes like that, rule 1: you cannot have two public class in the same *.java file. rule 2: main method should be in the *.java class which you define, if you've multiple classes in the same *.java file.

[Eranga]

I think this is what you want to do.

import java.awt.*;
import javax.swing.*;

class HelloComponent extends JComponent {

    @Override
    public void paintComponent(Graphics g) {
        g.drawString("Hello, Java!", 125, 95);
    }

    public static void main(String[] args) {
        JFrame frame = new JFrame("New Frame");
        HelloComponent hello = new HelloComponent();
        frame.add(hello);
        frame.setSize(300, 300);
        frame.setVisible(true);
    }
}

[pratzy]

Thanks! This works!

U said that two classes cant be used in the same .java file. But my program executes pretty well in the DOS mode. How does that work?

[Eranga]

U said that two classes cant be used in the same .java file. But my program executes pretty well in the DOS mode. How does that work?

I think you get it wrong way. Say you've a java file named Test.java, then your content could be like this.

public class Test {
    public static void main(String[] args) {
         MyNewClass objClass = new MyNewClass();
         objClass.printMsg();
    }
}

class MyNewClass {
   public void printMsg() {
       System.out.println("message");
   }
}

but you cannot do this,

class Test {
    public static void main(String[] args) {
         MyNewClass objClass = new MyNewClass();
         objClass.printMsg();
    }
}

public class MyNewClass {
   public void printMsg() {
       System.out.println("message");
   }
}

or this,

public class Test {
    public static void main(String[] args) {
         MyNewClass objClass = new MyNewClass();
         objClass.printMsg();
    }
}

public class MyNewClass {
   public void printMsg() {
       System.out.println("message");
   }
}

Think about that why it is?

[htowninsomniac]

There are a number of problems here:
1. A class you want to run (using java MyClass) has to be public.
2. You can only have one public class per file, and the file has to have the same name as the public class.

These two problems are Java restrictions.

Hope this helps.