通过我的这个小项目，我创建了一个底部有较大插槽的网格。我想添加鼠标的当前坐标，所以在 GridPanel 构造函数中我做了：
addMouseMotionListener(new MouseMotionAdapter(){
... mouseMoved and mouseDragged methods ...
});
renderGrid()但是，这样做会导致在我的方法中引发空指针异常。dbg = dbi.getGraphics();我之前遇到错误的错误被抛出，dbi = createImage(1000,1000);返回null。在稍微改变了一些东西之后，然后将它恢复到原来的样子，它就开始工作了。我不知道是什么导致了这种变化。这是我的完整代码：
主要的：
package tileWorld;
import javax.swing.JFrame;
public class Main extends JFrame
{
private GridPanel grid;
public Main()
{
setVisible(true);
setResizable(false);
setSize(1000, 1000);
setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
grid = new GridPanel();
add(grid);
}
public static void main(String[] args)
{
Main m = new Main();
}
}
网格面板：
package tileWorld;
import java.awt.Color;
import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Image;
import java.awt.Toolkit;
import java.awt.event.MouseEvent;
import java.awt.event.MouseMotionAdapter;
import javax.swing.JPanel;
public class GridPanel extends JPanel implements Runnable{
public static final int GRID_WIDTH = 1000, GRID_HEIGHT = 900, PANEL_WIDTH = 1000, PANEL_HEIGHT = 1000;
Dimension panelDim = new Dimension(PANEL_WIDTH, PANEL_HEIGHT);
private Image dbi;
private Graphics dbg;
private boolean running = false;
private Thread gridThread;
private Grid grid;
public GridPanel()
{
grid = new Grid();
setPreferredSize(panelDim);
setVisible(true);
startGrid();
// ========= USER INPUTS ===============
addMouseMotionListener(new MouseMotionAdapter(){
public void mouseMoved(MouseEvent e)
{
System.out.println("Moving!!");
}
public void mouseDragged(MouseEvent e)
{
System.out.println("Dragging!!");
}
});
}
private void paintScreen()
{
Graphics g;
try
{
g = this.getGraphics();
if(dbg != null && g != null)
{
g.drawImage(dbi, 0, 0, null);
}
Toolkit.getDefaultToolkit().sync();
g.dispose();
}catch(Exception e)
{
e.printStackTrace();
}
}
private void draw(Graphics g)
{
requestFocus();
grid.draw(g);
}
private void updateGrid()
{
}
private void renderGrid()
{
if(dbi == null)
{
dbi = createImage(1000,1000);
dbg = dbi.getGraphics();
dbg.setColor(Color.GRAY);
dbg.fillRect(0,0,1000,1000);
}
draw(dbg);
}
@Override
public void run()
{
while(running)
{
updateGrid();
renderGrid();
paintScreen();
}
}
private void startGrid()
{
if(gridThread == null)
{
gridThread = new Thread(this);
running = true;
gridThread.start();
}
}
}
网格：
package tileWorld;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Rectangle;
public class Grid
{
private final int GRID_NUM = (GridPanel.GRID_WIDTH / 10)*(GridPanel.GRID_HEIGHT / 10); // the number of "blocks" in the grid.
// Grid is 100 x 90 blocks (1000x900 px) // ((height/10) x (width/10)) for 10 x 10 px blocks
private Rectangle gridRects[];
private final int BLOCK_WIDTH = 10, BLOCK_HEIGHT = 10;
private int rectX, rectY, actionX1 = 50, actionX2 = 50;
// private final int MOUSE_COORDS_X = 5, MOUSE_COORDS_Y = 950;
public Grid()
{
gridRects = new Rectangle[GRID_NUM];
createGrid();
}
private void createGrid()
{
for (int i =0; i<GRID_NUM; i++)
{
if(rectX >= 1000)
{
rectX = 0;
rectY += 10;
}
gridRects[i] = new Rectangle(rectX, rectY, BLOCK_WIDTH, BLOCK_HEIGHT);
rectX += 10;
}
}
public void draw(Graphics g)
{
// ======= Draw the lines for the gride
g.setColor(Color.BLACK);
for (int i =0; i<GRID_NUM; i++)
{
g.drawRect(gridRects[i].x, gridRects[i].y, 10, 10);
}
// ====== Draw the action bar slots
for(int i=0; i < 9; i++) // 9 lines creates 10 slots.. buffer 50 px on each side. 900 px wide, 100 px tall. 90px x 100 px.
{
g.drawLine(actionX1, 900, actionX2, 1000);
actionX1 += 90;
actionX2 = actionX1;
}
// ======= Draw coordinates of mouse current location
}
}
错误信息：
Exception in thread "Thread-2" java.lang.NullPointerException
at tileWorld.GridPanel.renderGrid(GridPanel.java:86)
at tileWorld.GridPanel.run(GridPanel.java:99)
at java.lang.Thread.run(Unknown Source)
感谢您的时间和建议！我是否应该将其全部复制并粘贴到一个新项目中？有时当我使用 C++ Visual Studios 时就是这种情况。（目前正在使用 Eclipse）。再次感谢你！我会努力解决这个问题，如果我发现任何问题，我会回复。