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

public class ForDiFrame
{
  public static final int nFinestre = 10;
  public static JFrame [] tFrame = new JFrame[nFinestre];
  
  public ForDiFrame()
  {
    Toolkit tk = Toolkit.getDefaultToolkit();    
    Dimension d = tk.getScreenSize();
    int h = d.height/nFinestre;
    int w = d.width/ nFinestre;
    
    for(int i = 0; i < tFrame.length; i++)
    {  
      tFrame [i].setSize((w *(i+1)), (h * (i+1)));
      tFrame [i].setLocation(( d.width - w * (i+1)/2),
      ( d.height - h * (i+1)));
      
      tFrame [i].setVisible(true);
    }
  }
  
  public static void main(String args[])
  {
    ForDiFrame start = new ForDiFrame();
  }
}
>Exit code: 0
>java ForDiFrame

Exception in thread "main" java.lang.NullPointerException

at ForDiFrame.<init>(ForDiFrame.java:19)
at ForDiFrame.main(ForDiFrame.java:29)

Perchè al momento dell'eseguzione mi lancia l'eccazione?
Grazie anticipatamente