I tried your code out.
I had to add some import statements to get it to compile:

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

class PasswordBox extends JFrame


I invoked the class  from another java source as follows:
import javax.swing.*;

public class Test extends JFrame{

public Test(){

}

public static void main (String[] args){
Test test = new Test();
test.init();
}


public void init(){
PasswordBox box = new PasswordBox();
box.show();
}


}