NullPointerException,why?

[Jack Zheng]

I am learning the "Head First JaVa",and I tried to write my own "BeatBox" codes. The BeatBox.java was compiled sucessfully and the swing Gui was successfully loaded,as the following picture:

But when I chose some checkboxs and pressed "startbutton", the program throwed a "NonPointerException"at BeatBox.makeTrack(BeatBox.java:71),I don't know why? Can somebody help me to explain this?

import javax.sound.midi.*;
import javax.swing.*;
import java.awt.event.*;
import java.awt.*;
import java.util.*;
public class BeatBox{
  String[] instrumentNames={"Bass Drum","Closed Hi-Hat","Open Hi-Hat",
"Acoustic Snare","Crash Cymbal",
"Hand Clap","High Tom","Hi Bongo","Maracas","Whistle","Low Conga",
"CowBell","Vibraslap","Low-mid Tom","High Agogo","Open Hi Conga"};
  int[] instrumentNumbers={36,42,46,38,57,39,50,60,70,72,64,56,58,47,67,63};
  ArrayList<JCheckBox> checkBoxList=new ArrayList<JCheckBox>();
  Sequencer sequencer;
  Track track;
  public static void main(String[] args){
      BeatBox beatbox=new BeatBox();
      beatbox.setupGui();
  } 
  public void setupGui(){
    JFrame frame=new JFrame("Cyber BeatBox");
    BorderLayout layout=new BorderLayout();
    JPanel background=new JPanel(layout);
    GridLayout grid=new GridLayout(16,16);
    JPanel mainPanel=new JPanel(grid);
    frame.getContentPane().add(background);
    background.add(BorderLayout.CENTER,mainPanel);
    background.setBorder(BorderFactory.createEmptyBorder(10,10,10,10));
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    //basic layerout

    Box buttonBox=new Box(BoxLayout.Y_AXIS);
    JButton startButton=new JButton("start");
    buttonBox.add(startButton);
    startButton.addActionListener(new startButtonListener()); 
    JButton stopButton=new JButton("stop");
    buttonBox.add(stopButton);
    stopButton.addActionListener(new stopButtonListener()); 
    JButton tempoUpButton=new JButton("tempo up");
    buttonBox.add(tempoUpButton); 
    tempoUpButton.addActionListener(new upButtonListener());  
    JButton tempoDownButton=new JButton("tempo down");
    buttonBox.add(tempoDownButton);
    tempoDownButton.addActionListener(new downButtonListener()); 
    background.add(BorderLayout.EAST,buttonBox);
    //buttonbox
  
    Box labelBox=new Box(BoxLayout.Y_AXIS); 
    for(int i=0;i<16;i++){
      labelBox.add(new Label(instrumentNames[i]));
    }
    background.add(BorderLayout.WEST,labelBox);
    //labelbox
  
    grid.setHgap(2);
    grid.setVgap(1);
    for(int i=0;i<256;i++){
      JCheckBox c=new JCheckBox();
      checkBoxList.add(c);
      c.setSelected(false);
      mainPanel.add(c);
    }
    // checkbox
    frame.pack();
    frame.setSize(400,400);
    frame.setVisible(true);
  }
  public Track makeTrack(ArrayList<JCheckBox> checkBoxList){
    for(int i=0;i<16;i+=1){   
        for(int j=0;j<16;j+=1){
          if(checkBoxList.get(j+(16*i)).isSelected()){
            track.add(makeEvent(144,9,instrumentNumbers[j],100,j));
            track.add(makeEvent(128,9,instrumentNumbers[j],0,j+1));
          }else{
            track.add(makeEvent(128,1,100,0,j));
          }
        }
    }
    return track;  
  }
  public static MidiEvent makeEvent(int status,int chn,
  int first,int second,int tick){
    MidiEvent event=null;
    try{
      ShortMessage message=new ShortMessage(); 
      message.setMessage(status,chn,first,second);
      event=new MidiEvent(message,tick);
    }
    catch(Exception ex){
      ex.printStackTrace();
    }
    return event;
  }
  public class startButtonListener implements ActionListener{
    public void actionPerformed(ActionEvent event){
      try{
        Sequencer sequencer=MidiSystem.getSequencer();
        sequencer.open();
        Sequence sequence=new Sequence(Sequence.PPQ,4);
        Track track=sequence.createTrack();
        BeatBox beatbox=new BeatBox();
        beatbox.makeTrack(checkBoxList);
        sequencer.setSequence(sequence);
        sequencer.setTempoInBPM(220);
        sequencer.start();  
      }
      catch(Exception ex){ex.printStackTrace();
      }
    }
  }
  public class stopButtonListener implements ActionListener{
    public void actionPerformed(ActionEvent event){
      sequencer.stop();
    }
  }
  public class upButtonListener implements ActionListener{
    public void actionPerformed(ActionEvent event){
      sequencer.setTempoInBPM(440);
    }
  }
  public class downButtonListener implements ActionListener{
    public void actionPerformed(ActionEvent event){
      sequencer.setTempoInBPM(110);
    }
  }
}

[Jayesh A Lalwani]

NullPointerException occurs when you use a reference that s assigned to null. Look at line 71. Which variables might be null?

[Rajdeep Biswas]

Welcome number 3 to you today.

Jack Zheng wrote:..the program throwed a "NonPointerException"at BeatBox.makeTrack(BeatBox.java:71)..

If you are using an IDE, you can add debug points and run the program in debug mode. And then you can easily find out which variables having null are trying to access instance methods or fields.
If not IDE, then you can add print statements that print the state of each of the variables on console.

As user aboves has suggested to find out, do so.

[Jack Zheng]

I used the IDE and found the null variables. Thank you！

[Campbell Ritchie]

And welcome to the Ranch