Why is my Java Programme getting Problems ?? (Exception in thread "main" )

[haiderali]

HI

i have been working on this project for a pair of days and i dont remember if i had this problem at the start as well or not ...

because the programme sometime works perfectly and sometime Gives me this error :

Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:1090)
at java.awt.Container.add(Container.java:410)
at d.dproject.DPuzzle.<init>(DPuzzle.java:61)
at d.dproject.DDProject.main(DDProject.java:46)

anyway .. can you please help me to figure out the problem .. i am a beginner in java .. and need much progress

there are 3 classes ..

Main one is a GUI which calls other classes ...
one is a type of DnD of text ..
and last one which i was creating when i got the error is a puzzle in developing ..

package d.dproject;
import javax.swing.*;
import java.awt.*;;
import java.io.*;
import java.awt.event.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
 
 
 
 
public class DDProject implements ActionListener{
    JFrame DDFrame;
    JRadioButton puzzle,textDrag;
    DPuzzle a;
    TxTDrag b;
    //Costruttore
    DDProject (){
        a = new DPuzzle();
        b = new TxTDrag();
        DDFrame = new JFrame("Drag and Drop");
        DDFrame.setDefaultCloseOperation(DDFrame.EXIT_ON_CLOSE);
        DDFrame.setBounds(100,100,200,150);
        DDFrame.setLayout(new GridLayout(2,1));
 
 
        ButtonGroup radButtons = new ButtonGroup();
        puzzle = new JRadioButton("Puzzle");
        puzzle.addActionListener(this);
        textDrag = new JRadioButton("DnD Contenuto Files");
        textDrag.addActionListener(this);
        radButtons.add(puzzle);
        radButtons.add(textDrag);
 
 
        DDFrame.add(textDrag);
        DDFrame.add(puzzle);
        DDFrame.setVisible(true);
    }
 
 
    public static void main(String[] args) {
        new DDProject();
        new TxTDrag();
        new DPuzzle();
    }
 
    @Override
    public void actionPerformed(ActionEvent ae) {
        if (ae.getSource()==puzzle){
            a.DPFrame.setVisible(true);
        }
        if (ae.getSource()==textDrag){
            b.txtDragf.setVisible(true);
        }
    }
}

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package d.dproject;
 
import javax.swing.*;
import java.awt.*;;
import java.io.*;
import java.awt.event.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
 
 
 
 
/**
 *
 * @author Haider Ali
 */
public class TxTDrag implements ActionListener{
    JTextArea txtArea1,txtArea2;            //file 1 e 2
    JTextField nameFile1,nameFile2;         //Nomi dei File     
    JFrame txtDragf;                               //Frame Principale
    JButton show1,show2,writeF;                           //Bottoni file1 e file2
    JCheckBox ckBx;                                 //CheckBox
 
    public TxTDrag(){
        txtDragf = new JFrame("DnD Testo");
        //f.setLayout(new BorderLayout());
        txtDragf.setBounds(100,100,700,500);
 
 
        JPanel centerPane = new JPanel();
        centerPane.setLayout(new GridLayout(2,2,10,10));
        JPanel northPane = new JPanel();
        northPane.setLayout(new GridLayout(3,2,10,10));
 
 
        nameFile1 = new JTextField();
        nameFile2 = new JTextField();
 
        txtArea1 = new JTextArea();
        txtArea2 = new JTextArea();
        JScrollPane scr1 = new JScrollPane(txtArea1);
        JScrollPane scr2 = new JScrollPane(txtArea2);
 
        show1 = new JButton("Visualizza");
        show1.addActionListener(this);
        show2 = new JButton("Visualizza");
        show2.addActionListener(this);
        writeF = new JButton("Save");
        writeF.addActionListener(this);
 
        //CheckBox per Drag and Drop
        ckBx = new JCheckBox ("Attiva Drag & Drop del Testo");
        ckBx.addActionListener(this);
 
        northPane.add(new JLabel("Nome File1 :"));
        northPane.add(new JLabel("Nome File2 :"));
        northPane.add(nameFile1);
        northPane.add(nameFile2);
        northPane.add(new JLabel("TextArea per File1 :"));
        northPane.add(new JLabel("TextArea per File2 :"));
        centerPane.add(scr1);
        centerPane.add(scr2);
        centerPane.add(show1);
        centerPane.add(show2);
 
        JPanel southPane = new JPanel ();
        southPane.add(ckBx);
        southPane.add(writeF);
 
 
        txtDragf.add(northPane,BorderLayout.NORTH);
        txtDragf.add(centerPane,BorderLayout.CENTER);
        txtDragf.add(southPane,BorderLayout.SOUTH);
        txtDragf.setVisible(false);
    }
 
    @Override
    public void actionPerformed(ActionEvent ae) {
        //Bottone per File 1
 
        if (ae.getSource()==show1){
            String fileNome = nameFile1.getText();
            String par="";
            FileReader fr = null;
            try {
                fr = new FileReader(fileNome);
                BufferedReader br = new BufferedReader(fr);
                try {
                    par = br.readLine();
                    txtArea1.setText("");
 
                    while(par!=null){
                        txtArea1.append(par+"\n");
                        par = br.readLine();
                    }
                    br.close();
                } catch (IOException ex) {
                    System.out.println("Error");
                }
            } catch (FileNotFoundException ex) {
                JOptionPane.showMessageDialog(null, "Il Nome per File 1 inserito non Esiste", "Error Esistenza",JOptionPane.ERROR_MESSAGE);
            }
        }
 
        //Bottone per File 2
 
        if (ae.getSource()==show2){
            String fileNome = nameFile2.getText();
            String par="";
            FileReader fr = null;
            try {
                fr = new FileReader(fileNome);
                BufferedReader br = new BufferedReader(fr);
                try {
                    par = br.readLine();
                    txtArea2.setText("");
 
                    while(par!=null){
                        txtArea2.append(par+"\n");
                        par = br.readLine();
                    }
                    br.close();
                } catch (IOException ex) {
                    System.out.println("Error");
                }
 
 
            } catch (FileNotFoundException ex) {
                JOptionPane.showMessageDialog(null, "Il Nome per File 2 inserito non Esiste", "Error Esistenza",JOptionPane.ERROR_MESSAGE);
            }
        }
        //fine Source button
 
 
        //Inizio CheckBox
        if (ae.getSource()==ckBx){
            if (ckBx.isSelected()){             //se checkbox è selezionato
                txtArea1.setDragEnabled(true);  //il contenuto di txtArea sara dragAble
                txtArea2.setDragEnabled(true);  
            }else{
                txtArea1.setDragEnabled(false);
                txtArea2.setDragEnabled(false);
            }
        }
        //Fine Source checkBox
 
 
 
 
        //Inizio Scrittura
        if (ae.getSource()==writeF){
 
            String fileNome1 = nameFile1.getText();
            String par1="";
            String fileNome2 = nameFile2.getText();
            String par2="";
 
            PrintWriter outputStream1 = null;
            try {
                outputStream1 = new PrintWriter(new FileOutputStream(fileNome1));
                par1 = txtArea1.getText();
                outputStream1.println(par1);
                outputStream1.close();
            } catch (FileNotFoundException ex) {
                System.out.println("Error "+fileNome1);
                System.exit(0);
            }
 
            PrintWriter outputStream2 = null;
            try {
                outputStream2 = new PrintWriter(new FileOutputStream(fileNome2));
                par2 = txtArea2.getText();
                outputStream2.println(par2);
 
                outputStream2.close();
            } catch (FileNotFoundException ex) {
                System.out.println("Error "+fileNome2);
                System.exit(0);
            }
 
 
        } 
        }
    }

/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
package d.dproject;
import javax.swing.*;
import java.awt.*;;
import java.io.*;
import java.awt.event.*;
import java.awt.event.ActionListener;
import java.awt.event.ActionEvent;
import java.util.*;
 
public class DPuzzle implements ActionListener{
    JFrame DPFrame;
    ImageIcon []img;
 
    DPuzzle(){
        DPFrame = new JFrame("PUZZLE");
        //DPFrame.setDefaultCloseOperation(DPFrame.EXIT_ON_CLOSE);
        DPFrame.setBounds(200,100,700,500);
        DPFrame.setLayout(new BorderLayout());
 
 
        JPanel westPane = new JPanel();
        westPane.setLayout(new GridLayout(3,3,7,7));
        JPanel eastPane = new JPanel();
        eastPane.setLayout(new GridLayout(3,3,7,7));
 
        img = new ImageIcon[9];
        img[0] = new ImageIcon("1.jpg");
        img[1] = new ImageIcon("2.jpg");
        img[2] = new ImageIcon("3.jpg");
        img[3] = new ImageIcon("4.jpg");
        img[4] = new ImageIcon("5.jpg");
        img[5] = new ImageIcon("6.jpg");
        img[6] = new ImageIcon("7.jpg");
        img[7] = new ImageIcon("8.jpg");
        img[8] = new ImageIcon("9.jpg"); 
 
        JLabel []lbl = new JLabel[9];
 
        int []vet = new int[9];
        int n;    
        boolean trovato= false;
 
        for (int i=0;i<9;i++){
            do{
                trovato=false;
                n=(int)(Math.random()*9);
                for (int j=0;j<5;j++){
                    if (n==vet[j])
                        trovato=true;
                }
            }while(trovato);
            vet[i]=n;
            lbl[n]= new JLabel(img[i]);
        }
 
        for (int i=0;i<9;i++) {
            westPane.add(lbl[i]);
        }
        //Fine Pannello Ovest
 
 
 
 
 
 
 
 
        DPFrame.add(westPane,BorderLayout.WEST);
        DPFrame.add(eastPane,BorderLayout.EAST);
        DPFrame.setVisible(false);
    }
 
    @Override
    public void actionPerformed(ActionEvent ae) {
 
    }
 
}
[ATTACH]2064[/ATTACH]

[Norm]

Exception in thread "main" java.lang.NullPointerException
at java.awt.Container.addImpl(Container.java:1090)
at java.awt.Container.add(Container.java:410)
at d.dproject.DPuzzle.<init>(DPuzzle.java:61)

There is a variable with a null value on line 61. Look at line 61 in the your source and see what variable is null. Then backtrack in the code to see why that variable does not have a valid value.
If you can not tell which variable it is, add a println just before line 61 and print out the values of all the variables on that line.

The code in the main() method looks unusual. It creates three class objects and doesn't use the reference returned by the new statements anywhere.

Also variable names don't follow coding standards of starting with a lowercase letter. That makes the code confusing to read because the naming convention says that class names start with uppercase letters.