所以我正在为我的班级制作一个 GUI 项目，但我有点卡住了。我的问题与我的 GUI 方面有关，所以我想您可以忽略与排序有关的其他方法。到目前为止，我主要关心的是让 GUI 正常工作。但是，我一直遇到错误，确切地说是空指针异常：
Java.lang.NullPointerException
at SortDriver$SortCanvas.paint(SortDriver.java:253)
at SortDriver.init(SortDriver.java:197)
at sun.applet.AppletPanel.run(AppletPanel.java:436)
at java.lang.Thread.run(Thread.java:679)
在阅读了我的代码之后，我认为我将其范围缩小到了我的 SortCanvas 类。我以前从未使用过它，但它是我的教授给我们的存根的一部分。也就是说它可以正常工作并正确显示图像，但看起来我的实现不正确。有人可以帮我弄清楚如何正确实现我的 SortCanvas“图片”吗？我阅读了 Canvas 的 Java 文档，但我仍然不明白我做错了什么。这是我的代码：
import java.awt.*;
import java.applet.Applet;
import javax.swing.*;
import java.awt.event.*;
public class SortDriver extends Applet {
private int array[]; // array to be sorted
private int limit = 1000; // size of array to be sorted - you may have to make
// this bigger for faster sorts
private int largestNum; // need to know for color scaling purposes in paint()
// flag to tell paint() whether to paint a single location or the whole array
private enum PaintType {ALL, SINGLE};
private PaintType doPaint = PaintType.ALL;
private int index = -1; // index of single array location to be painted
//this listener object responds to button events
private ButtonActionListener buttonListener;
//button to start the sort
private JButton sortButton;
//basic window frame
private JFrame mainFrame;
//layouts
private BorderLayout initialLayout;
private FlowLayout northLayout;
private BorderLayout centerLayout;
private BorderLayout southLayout;
//basic panel for window frame
private JPanel initialPanel;
//panels for window layout
private JPanel northPanel;
private JPanel centerPanel;
private JPanel southPanel;
//panels for radio buttons
private JPanel radioOrderPanel;
private JPanel radioSortPanel;
private JPanel radioColorPanel;
//north panel header labels
private JLabel topTitleLabel;
private JLabel bottomTitleLabel;
private JLabel arraySizeLabel;
//radio buttons for list order (radioOrderButton)
//random set, ordered set, reverse set
private JRadioButton rOB1, rOB2, rOB3;
//radio buttons for sort type (radioSortButton)
//bubblesort, insertionsort, mergesort, quicksort
private JRadioButton rSB1, rSB2, rSB3, rSB4;
//radio buttons for color choice (radioColorButton)
//green, red, white, blue
private JRadioButton rCB1, rCB2, rCB3, rCB4;
//radio button groups for each radio panel
private ButtonGroup orderGroup, sortGroup, colorGroup;
//text field for size of the array
private JTextField arraySizeTextField;
// the picture of the sort will appear on this canvas
private SortCanvas picture;
private final int pictureWidth = 500; // size of the sort bar 1001
private final int pictureHeight = 50;
public void init() {
buttonListener = new ButtonActionListener();
array = new int[limit];
// load the array
largestNum = array[0] = (int) (Math.random()*1000000.0);
for (int i=1; i<limit; i++) {
array[i] = (int) (Math.random()*1000000.0);
// also keep track of the largest so that we can scale by it in paint()
if (array[i] > largestNum) largestNum = array[i];
}
//set up the main frame
mainFrame = new JFrame();
initialPanel = (JPanel) mainFrame.getContentPane();
initialLayout = new BorderLayout();
mainFrame.setResizable(false);
mainFrame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
mainFrame.setSize(650, 750);
initialPanel.setLayout(initialLayout);
//set up north panel
northPanel = new JPanel();
northLayout = new FlowLayout();
topTitleLabel = new JLabel("SortIt!");
bottomTitleLabel = new JLabel("A program by Mike Sevilla");
northPanel.setLayout(northLayout);
northPanel.add(topTitleLabel, BorderLayout.NORTH);
northPanel.add(bottomTitleLabel, BorderLayout.NORTH);
initialPanel.add(northPanel, BorderLayout.NORTH);
//set up center panel
centerPanel = new JPanel();
centerLayout = new BorderLayout();
centerPanel.setLayout(centerLayout);
//place array size label
arraySizeLabel = new JLabel("Size:");
//place array size text field w/ space for 5 chars
arraySizeTextField = new JTextField("", 5);
//place sort button
sortButton = new JButton("Sort it!");
// the listener is triggered when the button is clicked
sortButton.addActionListener(buttonListener);
centerPanel.setLayout(centerLayout);
//place sort bar on top of center layout
picture = new SortCanvas();
centerPanel.add(picture, BorderLayout.CENTER);
centerPanel.add(arraySizeLabel, BorderLayout.CENTER);
centerPanel.add(arraySizeTextField, BorderLayout.CENTER);
centerPanel.add(sortButton, BorderLayout.CENTER);
initialPanel.add(centerPanel, BorderLayout.CENTER);
//set up south panel
southPanel = new JPanel();
southLayout = new BorderLayout();
southPanel.setLayout(southLayout);
//set radio buttons and format layouts
radioOrderPanel = new JPanel();
radioOrderPanel.setLayout(new BoxLayout(radioOrderPanel, BoxLayout.Y_AXIS));
radioSortPanel = new JPanel();
radioSortPanel.setLayout(new BoxLayout(radioSortPanel, BoxLayout.Y_AXIS));
radioColorPanel = new JPanel();
radioColorPanel.setLayout(new BoxLayout(radioColorPanel, BoxLayout.Y_AXIS));
//define radio buttons
rOB1 = new JRadioButton("Random Order", true);
rOB1.addActionListener(buttonListener);
radioOrderPanel.add(rOB1);
rOB2 = new JRadioButton("In Order", false);
rOB2.addActionListener(buttonListener);
radioOrderPanel.add(rOB2);
rOB3 = new JRadioButton("In Reverse", false);
rOB3.addActionListener(buttonListener);
radioOrderPanel.add(rOB3);
rSB1 = new JRadioButton("Bubble Sort", true);
rSB1.addActionListener(buttonListener);
radioSortPanel.add(rSB1);
rSB2 = new JRadioButton("Insertion Sort", false);
rSB2.addActionListener(buttonListener);
radioSortPanel.add(rSB2);
rSB3 = new JRadioButton("Merge Sort", false);
rSB3.addActionListener(buttonListener);
radioSortPanel.add(rSB3);
rSB4 = new JRadioButton("Quick Sort", false);
rSB4.addActionListener(buttonListener);
radioSortPanel.add(rSB4);
rCB1 = new JRadioButton("Green", true);
rCB1.addActionListener(buttonListener);
radioColorPanel.add(rCB1);
rCB2 = new JRadioButton("Red", false);
rCB2.addActionListener(buttonListener);
radioColorPanel.add(rCB2);
rCB3 = new JRadioButton("White", false);
rCB3.addActionListener(buttonListener);
radioColorPanel.add(rCB3);
rCB4 = new JRadioButton("Blue", false);
rCB4.addActionListener(buttonListener);
radioColorPanel.add(rCB4);
//add radio buttons to a button group
orderGroup = new ButtonGroup();
orderGroup.add(rOB1);
orderGroup.add(rOB2);
orderGroup.add(rOB3);
sortGroup = new ButtonGroup();
sortGroup.add(rSB1);
sortGroup.add(rSB2);
sortGroup.add(rSB3);
sortGroup.add(rSB4);
colorGroup = new ButtonGroup();
colorGroup.add(rCB1);
colorGroup.add(rCB2);
colorGroup.add(rCB3);
colorGroup.add(rCB4);
initialPanel.add(southPanel, BorderLayout.NORTH);
picture.paint(picture.getGraphics());
mainFrame.setVisible(true);
}
// this object is triggered whenever a button is clicked
private class ButtonActionListener implements ActionListener {
public void actionPerformed(ActionEvent event) {
// find out which button was clicked
Object source = event.getSource();
// start sort button was clicked
if (source == sortButton) {
// call the sort
doBubblesort();
}
// called when user hits return in text field
if (source == arraySizeTextField) {
int size = Integer.parseInt(arraySizeTextField.getText());
}
}
}
private void doBubblesort() {
int temp;
// this is just bubblesort
for (int i=0; i<limit-1; i++) {
for (int j=0; j<limit-1-i; j++) {
if (array[j]>array[j+1]) {
temp = array[j]; array[j] = array[j+1]; array[j+1] = temp;
// redraw only locations j and j+1
doPaint = PaintType.SINGLE; // try changing this to ALL and see what happens
index = j;
picture.paint(picture.getGraphics());
index = j+1;
picture.paint(picture.getGraphics());
}
}
}
}
class SortCanvas extends Canvas {
// this class paints the sort bar
SortCanvas() {
setSize(pictureWidth, pictureHeight);
setBackground(Color.white);
}
public void paint(Graphics g) {
if (doPaint == PaintType.ALL) {
// paint whole array - this takes time so it shouldn't be done too frequently
setBackground(Color.white);
g.setColor(Color.white);
g.fillRect(0, 0, pictureWidth, pictureHeight);
for (int i=0; i<limit; i++) {
// the larger the number, the brighter green it is
// green is between 0.0 and 1.0
// divide by the largest number to get a value between 0 and 1
float green = (float)(array[i]/(float)largestNum);
// clamp if necessary - it shouldn't be
if (green<0f) green = 0f;
if (green>1f) green = 1f;
g.setColor(new Color(0.0f, green, 0.0f));
// array location 0 is painted at left;
// array location limit-1 is painted to right
//this is a single vertical line in the bar
g.drawLine((int)(i*pictureWidth/limit), 0,
(int)(i*pictureWidth/limit), pictureHeight);
}
}
else {
// just paint one location on the bar
float green = (float)(array[index]/(float)largestNum);
if (green<0f) green = 0f;
if (green>1f) green = 1f;
g.setColor(new Color(0.0f, green, 0.0f));
g.drawLine((int)(index*pictureWidth/limit), 0,
(int)(index*pictureWidth/limit), pictureHeight);
}
}
}
}