所以我有以下代码:
public class Minesweeper extends MIDlet implements CommandListener {
public static String error = "";
public void startApp() throws MIDletStateChangeException {
Display display = Display.getDisplay(this);
canvas = new MCanvas();
canvas.addCommand(exitCommand);
canvas.addCommand(okCommand);
canvas.addCommand(newCommand);
canvas.setCommandListener(this);
try{
display.setCurrent(canvas);
} catch (Exception e) {
error = e.toString();
}
}
}
当我将display.setCurrent(canvas);
留在try块之外时,该应用程序会失败并显示NullPointerException
。当我注释掉那一行时,该应用程序可以正常工作(虽然显然没有添加任何画布)。 因此错误是由该行或该行导致的错误引起的。
所以我用try / catch来说明这一行。尽管错误是由该行引起的,但当该行被try / catch包围时仍会发生错误。 如何捕获错误?(我也尝试使用Throwable
,但仍未捕获。
MCanvas:
import javax.microedition.lcdui.Canvas;
import javax.microedition.lcdui.Font;
import javax.microedition.lcdui.Graphics;
public class MCanvas extends Canvas {
protected void paint(Graphics g){
//Minesweeper.p("repaint");
Space[] data = Minesweeper.topaint;
for(int x=0; x<data.length; x++){
data[x].print();
int r = data[x].row * 10;
int c = data[x].col * 10;
int v = data[x].value;
String s = "";
//System.out.println("r:"+Integer.toString(r)+" c:"+Integer.toString(c)+" s:"+Integer.toString(v));
g.setColor(250, 0, 0);
//Minesweeper.p("if");
if(data[x].open){
switch(v){
case 0:
g.setColor(50, 50, 50);
break;
case 1:
g.setColor(100, 50, 50);
s = "1";
break;
case 2:
g.setColor(150, 50, 50);
s = "2";
break;
case 3:
g.setColor(200, 50, 50);
s = "3";
break;
case 4:
g.setColor(250, 50, 50);
s = "4";
break;
case 5:
g.setColor(250, 100, 100);
s = "5";
break;
case 6:
g.setColor(250, 125, 125);
s = "6";
break;
case 7:
g.setColor(250, 150, 150);
s = "7";
break;
case 8:
g.setColor(250, 175, 175);
s = "8";
break;
case 9:
g.setColor(250, 200, 200);
break;
default:
g.setColor(250, 100, 100);
}
} else {
g.setColor(0,0,0);
}
g.fillRect(c, r, 10, 10);
g.setColor(250, 250, 250);
Font font = Font.getFont(Font.FACE_SYSTEM, Font.STYLE_PLAIN, Font.SIZE_SMALL);
g.setFont(font);
g.drawString(s, c+5, r+8, Graphics.HCENTER | Graphics.BASELINE);
if(data[x].hover){
g.setColor(250, 250, 250);
g.drawLine(c, r, c, r+9);
g.drawLine(c, r, c+9, r);
g.drawLine(c+9, r, c+9, r+9);
g.drawLine(c, r+9, c+9, r+9);
}
//Minesweeper.p("here?");
}
//Minesweeper.p("here");
//Minesweeper.p(Minesweeper.error);
if(Minesweeper.error != null){
g.drawString(Minesweeper.error, 10, 10, Graphics.HCENTER | Graphics.BASELINE);
}
Minesweeper.p("msg:"+Minesweeper.message);
g.setColor(0, 0, 0);
Font font = Font.getFont(Font.FACE_SYSTEM, Font.STYLE_PLAIN, Font.SIZE_LARGE);
g.setFont(font);
g.drawString(Minesweeper.message, this.getWidth()/2, this.getHeight()-10, Graphics.HCENTER | Graphics.BASELINE);
Font fontsm = Font.getFont(Font.FACE_SYSTEM, Font.STYLE_PLAIN, Font.SIZE_SMALL);
g.setFont(fontsm);
}
protected void keyPressed(int keyCode) {
int gameaction = getGameAction(keyCode);
int c = Minesweeper.selected.col;
int r = Minesweeper.selected.row;
switch (gameaction) {
case UP:
Minesweeper.p("UP");
if(r>0){
Minesweeper.selected.leavehere();
Minesweeper.getSpace(Minesweeper.selected.row - 1, Minesweeper.selected.col).gohere();
}
break;
case DOWN:
Minesweeper.p("DOWN");
if(r<Minesweeper.height-1){
Minesweeper.selected.leavehere();
Minesweeper.getSpace(Minesweeper.selected.row + 1, Minesweeper.selected.col).gohere();
}
break;
case LEFT:
Minesweeper.p("LEFT");
if(c>0){
Minesweeper.selected.leavehere();
Minesweeper.getSpace(Minesweeper.selected.row, Minesweeper.selected.col - 1).gohere();
}
break;
case RIGHT:
Minesweeper.p("RIGHT");
if(c<Minesweeper.length-1){
Minesweeper.selected.leavehere();
Minesweeper.getSpace(Minesweeper.selected.row, Minesweeper.selected.col + 1).gohere();
}
break;
}
repaint();
}
}
答案 0 :(得分:1)
如Display.setCurrent API javadocs中所述,
...
setCurrent()
方法立即返回,无需等待更改发生......
由于上述原因,setCurrent
触发的调用中可能发生的异常可能(并且很可能 )会漏掉您的try-catch。
为了能够捕获并报告此类异常,我们应该研究setCurrent触发的调用(在您的情况下,这些是在Canvas, Event Delivery section的API javadoc中解释的),在适当的情况下通过try-catch块覆盖这些调用并设计适当的方式来报告异常情况。
在您的情况下,try-catch可能会围绕MCanvas.paint
中的代码(这可能是NPE发生的地方),例如可以通过显示带有错误消息的适当屏幕(例如Alert
)来报告异常通过catch块为该屏幕调用setCurrent
。
答案 1 :(得分:0)
如果我是你,我会继续在MCanvas类中插入try / catch块。
如果没有看到更多的代码,任何人都很难找出NullPointerException的位置。
从目前为止粘贴的代码中我唯一能提出的问题是:
你的Minesweeper类是否包含一个名为topaint的静态对象空间数组?你有没有声明它但可能忘了用数据填充它?
Space[] topaint = new Space[20]; // Declared, but nothing in it yet.
尝试访问Minesweeper.topaint [0]会产生NullPointerException,除非你也这样做
topaint[0] = new Space();