当我在构造函数中调用我创建的方法draw（）时，我不断收到NullPointerException。这特别令人沮丧，因为我找到了解决方法，但这不是我想要的。这是有效的代码。 公共类TutorialGrid扩展了JFrame {
private JPanel contentPane;
private Graphics g;
private int currentlength;
private int currentwidth;
private Integer[][] maze = new Integer[20][30];
private static TutorialGrid frame;
public static JTable table;
private JTextField title;
/**
* Launch the application.
*/
public static void main(String[] args) {
EventQueue.invokeLater(new Runnable() {
public void run() {
try {
frame = new TutorialGrid();
frame.setVisible(true);
} catch (Exception e) {
e.printStackTrace();
}
}
});
}
/**
* Create the frame.
*/
public TutorialGrid(){
setResizable(false);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(0, 0, 1366, 768);
contentPane = new JPanel();
contentPane.setBorder(new BevelBorder(BevelBorder.LOWERED, null, null, null, null));
setContentPane(contentPane);
contentPane.setLayout(null);
ImageIcon icon = new ImageIcon("C:\\Users\\Brendan\\Desktop\\GAME\\Images\\BG7.jpg");
input();
table = new JTable();
table.addMouseListener(new MouseAdapter() {
@Override
public void mouseClicked(MouseEvent e) {
draw(maze);
}
});
table.setAutoResizeMode(JTable.AUTO_RESIZE_OFF);
table.setAutoCreateRowSorter(true);
table.setCellSelectionEnabled(true);
table.setColumnSelectionAllowed(true);
table.setDoubleBuffered(true);
table.setDragEnabled(true);
table.setFillsViewportHeight(true);
table.setFocusCycleRoot(true);
table.setFocusTraversalPolicyProvider(true);
table.setIgnoreRepaint(true);
table.setInheritsPopupMenu(true);
table.setSurrendersFocusOnKeystroke(true);
table.setBackground(new Color(0, 0, 0));
table.setForeground(new Color(255, 255, 255));
table.setBorder(new LineBorder(new Color(139, 0, 0)));
table.setBounds(180, 40, 1000, 600);
contentPane.add(table);
title = new JTextField();
title.addMouseListener(new MouseAdapter() {
@Override
public void mouseClicked(MouseEvent e) {
draw(maze);
}
});
title.setHorizontalAlignment(SwingConstants.CENTER);
title.setText("Click to Start");
title.setBorder(new LineBorder(new Color(0, 128, 0)));
title.setEditable(false);
title.setForeground(Color.WHITE);
title.setBackground(new Color(0, 0, 0));
title.setBounds(606, 11, 151, 20);
contentPane.add(title);
title.setColumns(10);
JLabel lblBgpanel = new JLabel("", icon,JLabel.CENTER);
lblBgpanel.setBounds(0, 0, 1360, 740);
contentPane.add(lblBgpanel);
}
正如您所看到的，我在表和标题上都有mouseListeners，它们成功地调用了draw方法，没有任何问题。它在桌面上绘制我想要的网格，但是为了绘制它，我必须单击其中一个容器。我希望它在JFrame初始化时绘制网格。但如果我简单地画画（迷宫）;在构造函数中，它给了我一个空指针异常。这是绘制和输入方法的代码，用于绘制网格。
public void draw(Integer[][] maze){
int x= 125;
int y =50;
int width1 =25;
int length1 =25;
g=table.getGraphics();
for(int i=0; i<20; i++)
{
for(int j=0; j<30; j++)
{
if(maze[i][j] == maze[currentlength][currentwidth])
{
g.setColor(Color.YELLOW);
g.fillRect(x,y,width1,length1);
g.setColor(Color.RED);
g.drawRect(x,y,width1,length1);
x = x+25;
}
else if(maze[i][j] == 1)
{
g.setColor(Color.BLACK);
g.fillRect(x,y,width1,length1);
g.setColor(Color.RED);
g.drawRect(x,y,width1,length1);
x = x+25;
}
else if(maze[i][j] == 0)
{
g.setColor(Color.BLUE);
g.fillRect(x,y,width1,length1);
g.setColor(Color.RED);
g.drawRect(x,y,width1,length1);
x = x+25;
}
else if(maze[i][j] == -2)
{
g.setColor(Color.GREEN);
g.fillRect(x,y,width1,length1);
g.setColor(Color.RED);
g.drawRect(x,y,width1,length1);
x = x+25;
}
else if(maze[i][j] == -10)
{
g.setColor(Color.WHITE);
g.fillRect(x,y,width1,length1);
g.setColor(Color.RED);
g.drawRect(x,y,width1,length1);
x = x+25;
}
}
y=y+25;
x=125;
}
}
public void input(){
//Imports and reads grid file
Scanner scan = null;
try
{
FileReader grid = new FileReader("C:\\Users\\Brendan\\Desktop\\tutorialgrid.txt");
scan = new Scanner(grid);
}
catch(FileNotFoundException e)
{
System.out.println(e.getMessage() + "Could not find that file");
System.exit(0);
}
for(int i=0; i<20; i++)
{
for(int j=0; j<30; j++)
{
maze[i][j]=scan.nextInt();
if(maze[i][j] == -1)
{
currentlength = i;
currentwidth = j;
}
if(maze[i][j] == -10)
{
}
}
}
}
}
这一切都在同一个班级里面。这就是我想要做的但却给我一个错误。我在构造函数的底部添加了绘图（迷宫），当我尝试运行它时，它会在我身上爆炸。
public TutorialGrid(){
setResizable(false);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
setBounds(0, 0, 1366, 768);
contentPane = new JPanel();
contentPane.setBorder(new BevelBorder(BevelBorder.LOWERED, null, null, null, null));
setContentPane(contentPane);
contentPane.setLayout(null);
ImageIcon icon = new ImageIcon("C:\\Users\\Brendan\\Desktop\\GAME\\Images\\BG7.jpg");
input();
table = new JTable();
table.addMouseListener(new MouseAdapter() {
@Override
public void mouseClicked(MouseEvent e) {
draw(maze);
}
});
table.setAutoResizeMode(JTable.AUTO_RESIZE_OFF);
table.setAutoCreateRowSorter(true);
table.setCellSelectionEnabled(true);
table.setColumnSelectionAllowed(true);
table.setDoubleBuffered(true);
table.setDragEnabled(true);
table.setFillsViewportHeight(true);
table.setFocusCycleRoot(true);
table.setFocusTraversalPolicyProvider(true);
table.setIgnoreRepaint(true);
table.setInheritsPopupMenu(true);
table.setSurrendersFocusOnKeystroke(true);
table.setBackground(new Color(0, 0, 0));
table.setForeground(new Color(255, 255, 255));
table.setBorder(new LineBorder(new Color(139, 0, 0)));
table.setBounds(180, 40, 1000, 600);
contentPane.add(table);
title = new JTextField();
title.addMouseListener(new MouseAdapter() {
@Override
public void mouseClicked(MouseEvent e) {
draw(maze);
}
});
title.setHorizontalAlignment(SwingConstants.CENTER);
title.setText("Click to Start");
title.setBorder(new LineBorder(new Color(0, 128, 0)));
title.setEditable(false);
title.setForeground(Color.WHITE);
title.setBackground(new Color(0, 0, 0));
title.setBounds(606, 11, 151, 20);
contentPane.add(title);
title.setColumns(10);
JLabel lblBgpanel = new JLabel("", icon,JLabel.CENTER);
lblBgpanel.setBounds(0, 0, 1360, 740);
contentPane.add(lblBgpanel);
draw(maze);
}
这是错误。
java.lang.NullPointerException
at game.TutorialGrid.draw(TutorialGrid.java:136)
at game.TutorialGrid.<init>(TutorialGrid.java:111)
at game.TutorialGrid$1.run(TutorialGrid.java:41)
at java.awt.event.InvocationEvent.dispatch(Unknown Source)
at java.awt.EventQueue.dispatchEvent(Unknown Source)
at java.awt.EventDispatchThread.pumpOneEventForFilters(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForFilter(Unknown Source)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.pumpEvents(Unknown Source)
at java.awt.EventDispatchThread.run(Unknown Source)
game.TutorialGrid.draw第136行是draw方法开始的行。 game.TutorialGrid。第111行是构造函数中的最后一行，我放置了绘图（迷宫）。 game.TutorialGrid $ 1.run第41行是line frame = new TutorialGrid（）;
非常感谢帮助。
答案 0 :(得分：0)
getGraphics（）返回null，直到看得见。