Help creating an applet (prime numbers and numbers divisible by 4)

Question

Hi all,
I am very new to java, and I have exhausted all of my resources before posting here. I have checked the archives but what I am looking for is not really there. I have an exam in 2 days, and there is a question like this one I cannot get to work.
I am supposed to ask for two numbers, the startNumber and endNumber. Then I am supposed to print in ascending or ascending order (this is already done)only the numbers divisible by 4 and the prime numbers. I have seen in the archives how you can use a boolean to check for prime numbers, but given the complexity of this question, I cannot put it into practice. Anybody can help me please?
import java.awt.*;
import java.applet.*;
import java.awt.event.*;
public class Div4OrPrime extends Applet implements ActionListener
    //DECLARE THE GUIS
    public Button showB;
    public Label startL, endL;
    public TextField startN, endN;
    private TextArea screen;
    //DECLARE THE PANELS
    public Panel topPanel, middlePanel, bottomPanel;
    //DECLARING THE VARIABLES
    public int startNumber, endNumber;
    public int result;
    public String output="";
    public int counter;
    //INITIALIZATION OF THE APPLET
    public void init()
        //CREATION OF THE BUTTONS
          showB = new Button ("Show numbers");
       //CREATION OF LABELS
           startL = new Label ("Starting Number");
           endL = new Label ("Ending Number");
       //CREATION OF THE TEXTAREAS
          startN = new TextField(8);
          endN = new TextField(8);
          screen=new TextArea();   
       //CREATION OF THE PANELS
         topPanel=new Panel (new GridLayout (1,1));
         middlePanel = new Panel (new GridLayout (1,4));
         bottomPanel = new Panel (new GridLayout (1,1));
         setLayout (new GridLayout (3,3));
       //ADDING THE WIDGETS TO THE PANELS
          topPanel.add(showB);
          middlePanel.add(startL); middlePanel.add(startN);
          middlePanel.add(endL); middlePanel.add(endN);
          bottomPanel.add(screen);
          add(topPanel);
          add(middlePanel);
          add(bottomPanel);
       //ADD THE ACTION SOURCE LISTENER
          showB.addActionListener(this); 
          startN.addActionListener(this);
          endN.addActionListener(this);
      //SET THE ACTION
       public void actionPerformed(ActionEvent e)
           output="";
           if (e.getActionCommand().equals("Show numbers"))
       startNumber=Integer.parseInt(startN.getText());
                  endNumber=Integer.parseInt(endN.getText());
               if  (startNumber<endNumber)
                  for (a = startNumber  ; a<=endNumber; a++)
                           output= output +a+"\n";
                           screen.setText(output);         
                      else if (startNumber>endNumber)
                          for (a=startNumber; a>=endNumber; a--)
                           output= output +a+"\n";
                           screen.setText(output);         
                     repaint();
}

Answer 1

Hi all,
Iam very new to java, and I have exhausted all of my
resources before posting here. I have checked the
archives but what I am looking for is not really
there. I have an exam in 2 days, and there is a
question like this one I cannot get to work.
I am supposed to ask for two numbers, the startNumber
and endNumber. Then I am supposed to print in
ascending or ascending order (this is already
done)only the numbers divisible by 4 and the prime
numbers. I have seen in the archives how you can use a
boolean to check for prime numbers, but given the
complexity of this question, I cannot put it into
practice. Anybody can help me please?
Code SNIP
I'm sure there are better ways than this but it's the only one I can think of offhand.
Create a boolean array from 2 to the largest number, set them all to true, then go through and on the first true number set all the multiples to false. Then do the same with the next true number(Sieve of Erasthoenes). Then you can go through and set all the mutiples of 4 to true with another loop. Then print the numbers in the range.