Question

这个程序编译得很好。这是Head First Java的教程之一，我很确定我正确地输入了它，但我得到的索引超出了界限错误。我知道这个错误意味着什么，但是对于我的生活，我无法看到我出错的地方。

import java.awt.*;
import javax.swing.*;
import javax.sound.midi.*;
import java.util.*;
import java.awt.event.*;

public class BeatBox{

    JPanel mainPanel;
    ArrayList<JCheckBox> checkBoxList;
    Sequencer sequencer;
    Sequence sequence; 
    Track track;
    JFrame theFrame;

    String[] instrumentNames = {"Bass Drum", "Closed High-Hat",
    "Open Hi-Hat", "Acoustic Snare", "Crash Cymbal", "Hand Clap",
    "High Tom", "High Bongo", "Maracas", "Whistle", "Low Conga",
    "Open High Conga"};
    int[] instruments = {35,42,46,38,49,39,50,60,70,72,64,56,58,47,67,63};

    public static void main(String[] args){
        BeatBox box = new BeatBox();
        box.buildGUI();
    }

    public void buildGUI(){
        theFrame = new JFrame("Cyber BeatBox");
        theFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        BorderLayout layout = new BorderLayout();
        JPanel background = new JPanel(layout);
        background.setBorder(BorderFactory.createEmptyBorder(10,10,10,10));

        checkBoxList = new ArrayList<JCheckBox>();
        Box buttonBox = new Box(BoxLayout.Y_AXIS);

        JButton start = new JButton("Start");
        start.addActionListener(new MyStartListener());
        buttonBox.add(start);

        JButton stop = new JButton("Stop");
        stop.addActionListener(new MyStopListener());
        buttonBox.add(stop);

        JButton upTempo = new JButton("Tempo Up");
        upTempo.addActionListener(new MyUpTempoListener());
        buttonBox.add(upTempo);

        JButton downTempo = new JButton("Tempo Down");
        downTempo.addActionListener(new MyDownTempoListener());
        buttonBox.add(downTempo);

        Box nameBox = new Box(BoxLayout.Y_AXIS);
        for(int i = 0; i < 16; i++){
            nameBox.add(new Label(instrumentNames[i]));
        }

        background.add(BorderLayout.EAST, buttonBox);
        background.add(BorderLayout.WEST, nameBox);

        theFrame.getContentPane().add(background);

        GridLayout grid = new GridLayout(20,20);
        grid.setVgap(1);
        grid.setHgap(2);
        mainPanel = new JPanel(grid);
        background.add(BorderLayout.CENTER, mainPanel);

        for(int i = 0; i < 256; i++){
            JCheckBox c = new JCheckBox();
            c.setSelected(false);
            checkBoxList.add(c);
            mainPanel.add(c);
        }

        setUpMidi();

        theFrame.setBounds(50,50,300,300);
        theFrame.pack();
        theFrame.setVisible(true);
    }

    public void setUpMidi(){
        try{
            sequencer = MidiSystem.getSequencer();
            sequencer.open();
            sequence = new Sequence(Sequence.PPQ, 4);
            track = sequence.createTrack();
            sequencer.setTempoInBPM(120);
        }catch(Exception e){e.printStackTrace();}
    }

    public void buildTrackAndStart(){
        int[] trackList = null;

        sequence.deleteTrack(track);
        track = sequence.createTrack();

        for(int i = 0; i < 16; i++){
            trackList = new int[16];

            int key = instruments[i];

            for(int j = 0; j < 16; j++){

                JCheckBox jc = (JCheckBox)checkBoxList.get(j + (16 * i));
                if(jc.isSelected()){
                    trackList[j] = key;
                }else{
                    trackList[j] = 0;
                }
            }

            makeTracks(trackList);
            track.add(makeEvent(176,1,127,0,16));
        }

        track.add(makeEvent(192,9,1,0,15));
        try{
            sequencer.setSequence(sequence);
            sequencer.setLoopCount(sequencer.LOOP_CONTINUOUSLY);
            sequencer.start();
            sequencer.setTempoInBPM(120);
        }catch(Exception e){e.printStackTrace();}
    }

    public class MyStartListener implements ActionListener{
        public void actionPerformed(ActionEvent a){
            buildTrackAndStart();
        }
    }

    public class MyStopListener implements ActionListener{
        public void actionPerformed(ActionEvent a){
            sequencer.stop();
        }
    }

    public class MyUpTempoListener implements ActionListener{
        public void actionPerformed(ActionEvent a){
            float tempoFactor = sequencer.getTempoFactor();
            sequencer.setTempoFactor((float)(tempoFactor * 1.03));
        }
    }

    public class MyDownTempoListener implements ActionListener{
        public void actionPerformed(ActionEvent a){
            float tempoFactor = sequencer.getTempoFactor();
            sequencer.setTempoFactor((float)(tempoFactor * .97));
        }
    }

    public void makeTracks(int[] list){

        for(int i = 0; i < 16; i++){
            int key = list[i];

            if(key != 0){
                track.add(makeEvent(144,9,key,100,i));
                track.add(makeEvent(128,9,key,100,i+1));
            }
        }
    }

    public MidiEvent makeEvent(int comd, int chan, int one, int two, int tick){
        MidiEvent event = null;
        try{
            ShortMessage a = new ShortMessage();
            a.setMessage(comd, chan, one, two);
            event = new MidiEvent(a, tick);
        }catch(Exception e) {e.printStackTrace();}
        return event;
    }
}

Answer 1

此：

for(int i = 0; i < 16; i++) {
    nameBox.add(new Label(instrumentNames[i]));
}

instrumentNames中只有12个元素，因此您只能从0到11循环。当您尝试访问instrumentNames[12]时，您会获得ArrayIndexOutOfBoundsException。

改为使用数组长度：

for(int i = 0; i < instrumentNames.length; i++) {
    nameBox.add(new Label(instrumentNames[i]));
}

或for-each循环：

for (String instrumentName : instrumentNames) {
    nameBox.add(new Label(instrumentName));
}

获取数组索引超出绑定异常并且无法查找源

1 个答案: