Null pointer exception thrown in paintComponent

[Michael Nana]

Hey guys,
I am working on a l-system assignment to draw strings on a panel and when you click the draw button it expands the string and redraws the diagram larger and larger.
Well I have finished coding up but I have a null pointer exception in my paintcomponent which i don't really see the reason why. I initialized all the variables I am using in the paintcomponent so I don't see why I get flagged with it. I'll greatly appreciate it if you would take a look.

 import java.awt.*;
import java.awt.event.*;
import java.io.File;
import java.io.FileNotFoundException;
import java.util.Scanner;
import java.util.*;

import javax.swing.*;

public class  L_GuiApp{
    public static void main(String args[]) {
    	
        new  L_Gui();
    }
}

 
class L_Gui extends JFrame {
	//Graphics g =getGraphics();

	
	
	
	
    /** default constructor  */
    public L_Gui() {
        initComponents();
        
		
        
    }
    
    // all the initializations in one method
    private void initComponents() {
    	
      
        jLabel1      = new JLabel("Iterations");
        jLabel2      = new JLabel("Branch Angle");
        jLabel3      = new JLabel("Scale");
        jLabel4      = new JLabel();

        jTextField1  = new JTextField();
        jTextField2  = new JTextField();
        jTextField3  = new JTextField();
        jTextField4  = new JTextField();
        
        jButton1     = new JButton("DRAW");
        loadButton   = new JButton("LOAD" );
        jButton2     = new JButton ("expand");

        // make a drawing surface
          p = new DrawPanel();
        
        // make the panel of buttons
          controlPanel = new JPanel();
          controlPanel.setPreferredSize( new Dimension(100,400));
          controlPanel.setLayout(new FlowLayout());
          controlPanel.setBackground(Color.yellow);
        
        // set the Layout to FlowLayout
          getContentPane().setLayout(new FlowLayout());

        // add a WindowListener to terminate the program when window is closed
          addWindowListener(new WindowAdapter() {
            public void windowClosing(WindowEvent evt) {
                System.exit(1);
            }
          });


        // set up jButton1 (draw)
          final DrawPanel z= new DrawPanel ();
          jButton2.setSize(80,20);
          jButton2.addActionListener(new ActionListener() {
              public void actionPerformed(ActionEvent evt) {
             	 
             	
             }
           });
          jButton1.setSize(80,20);
          jButton1.addActionListener(new ActionListener() {
            public void actionPerformed(ActionEvent evt) {
            	 handler(evt);
            	
            }
          });
          jTextField1.setText("jTextField1");
          jTextField2.setText("jTextField2");
          jTextField3.setText("jTextField3");
          jTextField3.addActionListener(new ActionListener() {
              public void actionPerformed(ActionEvent evt) {
                  System.out.println(p.Angle);
              }
          });
          
          

          // Fix up loadButton
          loadButton.setSize(80,20);
          loadButton.addActionListener(new ActionListener() {
             public void actionPerformed(ActionEvent evt) {
            	
            	// jTextField3ActionPerformed();
             }
          });
        
        // Add the components to the control panel

          controlPanel.add(jTextField1);        
          controlPanel.add(jLabel1);        
          controlPanel.add(jTextField2);        
          controlPanel.add(jLabel2); 
          controlPanel.add(jTextField3);
          controlPanel.add(jLabel3);        
          controlPanel.add(loadButton);
          controlPanel.add(jButton1);
          controlPanel.add(jButton2);
        
        // add the panels to the L_GUI
          getContentPane().add(p);
          getContentPane().add(controlPanel);

          pack();
          setVisible(true);
    } // _________  end of InitComponents

    
    // method that will be called to handle reading in of file
    private void jTextField3ActionPerformed() {
    	//p.expandCode();
    	Scanner input = null;
		File f = new File("l-system1.txt");
		
		try{
		input = new Scanner(f);
		
		//first line (axiom)
		p.axiom = input.next();
		System.out.println("axiom: "+p.axiom);
		input.nextLine();
		
		//second line (instructions)
		p.instructions = input.nextLine();
		System.out.println("instructions: "+p.instructions);
		
		//third line (angle)
		p.Angle = input.nextInt();
		input.nextLine();
		System.out.println("initial angle: "+p.Angle);
		
		//fourth line (number of units to move each time)
		p.distance = input.nextInt();
		input.nextLine();
		System.out.println("distance: "+p.distance);
		
		//fifth line (initial x coordinate)
		p.initialX = input.nextInt();
		input.nextLine();
		System.out.println("initial x: "+p.initialX);
		
		//sixth line (initial y coordinate)
		p.initialY = input.nextInt();
		input.nextLine();
		System.out.println("initial y: "+p.initialY);
		
		//seventh line (initial direction)
		p.initialDirection = input.next();
		setDirection(p.initialDirection);
		
		System.out.println("initial direction: "+p.initialDirection);
		
		}  
		catch (FileNotFoundException e) {
		e.printStackTrace();
		}
		
		jTextField1.setText(p.instructions);
		jTextField2.setText(Double.toString(p.Angle));
		jTextField3.setText(Integer.toString(p.distance));
		
    }
    
    public void setDirection (String s){
    	if (s.equals("East"))
    	p.AngleInUse=0;
    	
    	if (s.equals("West"))
    			p.AngleInUse= 180;
    	if (s.equals("North"))
			p.AngleInUse= 270;
    	if (s.equals("South"))
			p.AngleInUse= 90;
    	System.out.println (p.Angle);
    	
		
		}
    
    
    // method that is called when the draw button is pressed
    private void handler(ActionEvent evt) {
      if(evt.getSource()==jButton1){
    	  System.out.println("draw button was pressed");
          //p.DrawBoy();
          p.expandCode();
          String newer = p.expandCode();
         // p.repaint();
      }
    }
    
   

    public class DrawPanel extends JPanel{
    	private int height,width;
		private double  step_angle;
		private int step_length;
	    double angle;
	    String axiom;
		String instructions;
		//int initialAngle;
		int distance;
		int initialX;
		int initialY;
		String initialDirection;
		double Angle;
		double AngleInUse;
		int newY;
		int newX;
		int chara;
	    //String instructions;
	    
		private Stack<Integer> Base_x=new Stack<Integer>();
		private Stack<Integer> Base_y=new Stack<Integer>();
		private Stack<Double> Base_angle= new Stack<Double>();
	
    
  	  
        public DrawPanel(){
        	/*super();
			setVisible(true);
			repaint();*/
           setBackground(Color.white);
           setPreferredSize( new Dimension( 800,800));
           
        }

        public void paintComponent(Graphics g){
        //	jTextField3ActionPerformed();
      	    
        	super.paintComponent(g);
        	
        	//System.out.println("Hey"+instructions);

        	double rad_angle;
        	double angle=0.0;
        	int newx=0;
        	int newy=0;
        	int x=width/2;
        	int y = height/2;
        	
        			
          
               //g.setColor(Color.black);
               g.drawLine(1, 1, 100, 100);
               //int length = instructions.length();
               for(int i=0;i<instructions.length();i++)
   			{
   				char c=instructions.charAt(i);
   			    switch(c)
   				{
   				  	case 'f': //rad_angle=Math.PI*angle/180;
   							  newx= initialX + (int) (distance*Math.cos(Math.toRadians(AngleInUse)));
   						     newy=initialY+(int) (distance*Math.sin(Math.toRadians(AngleInUse)));
   						   //  if((newx>controlPanel.getHeight() && newx>controlPanel.getHeight())||(newy>controlPanel.getWidth() && newy>controlPanel.getWidth())){
   						 	//		distance=distance/2;}
   						 	  g.drawLine(initialX, initialY, newx, newy);
   						 	  System.out.println(initialX);
   						 	  
   							 g.setColor(Color.red);
   							 g.drawOval(newx,newy,1,1);
   							 g.setColor(Color.blue);
     							 initialX=newx;
   							initialY=newy;
   							
   						    break;
   					case '+': AngleInUse=AngleInUse+Angle;
   					
   					      	break;
   					case '-': AngleInUse=AngleInUse-Angle;
   					
   						   break;
   					case '[': Base_x.push(new Integer(initialX));
   					          Base_y.push(new Integer(initialY));
   					          Base_angle.push(new Double(Angle));
   					          break;
   					case ']': initialX=((Integer)Base_x.pop()).intValue();
   					          initialY=((Integer)Base_y.pop()).intValue();
   					         Angle=((Double)Base_angle.pop()).doubleValue();
   					          break; 
   					
               
        }
   			}
        }
        public String expandCode(){
        	//jTextField3ActionPerformed();
        	String expander = new String();
        	for(int x =0; x<instructions.length();x++){
        	if(instructions.charAt(x)=='f'){
        	expander+=instructions;
        		System.out.println("hey");
        		}
        	else 
        		expander+=x;
        }
        	axiom=expander;
        	instructions=expander;
        	System.out.println(instructions);
        	return axiom;
        }
        
        public void DrawBoy (){
        	
            Graphics g = getGraphics();
            System.out.println(instructions);
         	 int length= instructions.length();
         	 int  oldX = initialX;
         	 int oldY= initialY;
         	 int newX=0;
         	int  newY=0;
         	 
         	 
         	 //AngleInUse= 0;
         	 
         	 for (int x=0; x<length; x++){
         		 /*if (newX>=360)
         			 newX= newX-360;
         		 
         		 if (newY>=360)
         			 newY=newY-360;*/
         		 if(AngleInUse>=360)
         			 AngleInUse=AngleInUse-360;
         		 if(AngleInUse<0){
         			 AngleInUse=AngleInUse+360;
         		 }
         		 if (instructions.charAt(x)=='f'){
         		 //newX=(int)(distance*Math.cos(Math.toRadians(AngleInUse))+initialX);
         		 //newY=(int)(distance*Math.sin(Math.toRadians(AngleInUse))+initialY);
         		if (AngleInUse>=0&&AngleInUse<=90)
         			newX=(int)(oldX+(distance*Math.cos(Math.toRadians(AngleInUse))));
         		if (AngleInUse>=270&&AngleInUse<=360)
         			newX=(int)(oldX+(distance*Math.cos(Math.toRadians(AngleInUse))));
         		if (AngleInUse>=90&&AngleInUse<=270)
         			newX=(int)(oldX-(distance*Math.cos(Math.toRadians(AngleInUse))));
         		if (AngleInUse>=0&&AngleInUse<=180)
         			newY=(int)(oldY-(distance*Math.sin(Math.toRadians(AngleInUse))));
         		if (AngleInUse>=180&&AngleInUse<=360)
         			newY=(int)(oldY-(distance*Math.sin(Math.toRadians(AngleInUse))));
         		 
         		 System.out.println(newX);
         		 g.drawLine(oldX, oldY, newX, newY);
         		 oldX=newX;
         		 oldY=newY;
         		// instructions=instructions.replace('f', instructions);
         	 }
         	 
         	 else if (instructions.charAt(x)=='+'){
         		 AngleInUse=AngleInUse-Angle;
         	 }
         	 else {
         		 AngleInUse= AngleInUse+Angle;
         	 }
           }
       }
       
    }
      
 // end---------------------------------------------TreePanel


 // Variable declarations - do not modify
    private JLabel      jLabel1;
    private JLabel      jLabel2;
    private JLabel      jLabel3;
    private JLabel      jLabel4;
    
    private JTextField  jTextField3;
    private JTextField  jTextField2;
    private JTextField  jTextField1;
    private JTextField  jTextField4;
    
    private JButton     jButton1;
    private JButton     loadButton; 
    private JButton     jButton2;
    

    private DrawPanel   p;
    private JPanel      controlPanel;
}

[Steve Luke]

Michael Nana wrote:Hey guys,
... I initialized all the variables I am using in the paintcomponent so I don't see why I get flagged with it.

That is incorrect - if you did than you would not get a NullPointerException. The key is to recognize which Object was not initialized before it is used. If you give us the error message and the line where the error occurs it will probably help solve the problem. A quick look indicates that the String instructions may not be initialized before use. It looks like it (or any of the other Strings) might not get assigned until the actionPerformed on jTextField3 gets executed... Which means that you would get this NPE anytime the DrawPanel is visible without the text getting edited before hand.

[Michael Nana]

Steve Luke wrote:

Michael Nana wrote:Hey guys,
... I initialized all the variables I am using in the paintcomponent so I don't see why I get flagged with it.

That is incorrect - if you did than you would not get a NullPointerException. The key is to recognize which Object was not initialized before it is used. If you give us the error message and the line where the error occurs it will probably help solve the problem. A quick look indicates that the String instructions may not be initialized before use. It looks like it (or any of the other Strings) might not get assigned until the actionPerformed on jTextField3 gets executed... Which means that you would get this NPE anytime the DrawPanel is visible without the text getting edited before hand.

Oh yeah sorry, the error is on line 265, instructions.length

Yeah I assumed that and that's why I have the jtextfield3 () method called in the paint component commented. When I remove the comment, I no longer get the NPE however the program doesn't behave like I wish. I have a method expandcode that I want it to expand the instructions when called but it doesn't do it.

[Steve Luke]

You will have to do something to initialize those values prior to displaying the panel. Calling the jtextfield's action performed method at the start of the paintComponent method is probably not the correct thing to do because it could cause side effects (like re-setting the object values or a long loop where the paint triggers an action which triggers a paint ... making the application unresponsive).

The simplest thing is probably to define some meaningful default values when the Object gets first created. Like the empty String ("") or something descriptive ("Enter Text") depending on what seems to make sense in your scenario. But you would need to make meaningful defaults (meaningful == will cause expected, nice looking display and not cause errors) for all your Strings and Objects used in the paintComponent method, and these values should be assigned when the class is created / initialized and before it gets displayed.