朋友..我正在尝试绘制一些图表。我的应用程序在第一次尝试时运行正常，但是当我尝试重复绘制图形2,3次尝试时，它显示空指针异常......解决方案是什么？这是我的代码：
public class ECG extends Activity {
/** Called when the activity is first created. */
Context cnt;
Callback c;
Camera mCamera;
boolean mFinished;
DrawMyGraph g;
ImageView clock,graph_Text,heart_Index,graph_View;
static long startTime = 0;
static int beats = 0;
volatile boolean show=false;
SurfaceHolder sh;
SurfaceView sv;
TextView show_BPM;
Button on,off;
int no,count=0;
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
show_BPM=(TextView)findViewById(R.id.showBPM);
sv=(SurfaceView)findViewById(R.id.surfaceView1);
clock=(ImageView)findViewById(R.id.clock);
graph_Text=(ImageView)findViewById(R.id.graphText);
heart_Index=(ImageView)findViewById(R.id.heartIndex);
graph_View=(ImageView)findViewById(R.id.graphview);
graph_Text.setBackgroundResource(R.drawable.graphtext2);
on=(Button)findViewById(R.id.on);
off=(Button)findViewById(R.id.off);
Random r=new Random();
no=r.nextInt(84-60)+60;
Context context = getApplicationContext();
CharSequence text = "cover the camera with your index finger......!";
int duration = 100000;
Toast toast = Toast.makeText(context, text, duration);
toast.show();
///////////////////////////////
////////////////////////////
g = new DrawMyGraph(this, 26);
addContentView(g, new LayoutParams(LayoutParams.MATCH_PARENT,
LayoutParams.MATCH_PARENT));
startCamPreview();
}
public void startCamPreview() {
SurfaceView sv = (SurfaceView) findViewById(R.id.surfaceView1);
SurfaceHolder sh = sv.getHolder();
mCamera = Camera.open();
try {
mCamera.setPreviewDisplay(sh);
} catch (IOException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
sh.addCallback(new Callback() {
@Override
public void surfaceDestroyed(SurfaceHolder holder) {
// TODO Auto-generated method stub
System.out.println("Deleted");
mCamera.setPreviewCallback(null);
mCamera.stopPreview();
mCamera.release();
mCamera = null;
}
@Override
public void surfaceCreated(SurfaceHolder holder) {
System.out.println("Created");
mCamera.setPreviewCallback(new PreviewCallback() {
public void onPreviewFrame(byte[] data, Camera camera) {
Camera.Parameters params = camera.getParameters();
int width = params.getPreviewSize().width;
int height = params.getPreviewSize().height;
byte[] mYUVData = new byte[data.length];
int[] mRGBData = new int[width * height];
System.arraycopy(data, 0, mYUVData, 0, data.length);
decodeYUV420SP(mRGBData, mYUVData, width, height);
int[] mRedHistogram = new int[256];
calculateIntensityHistogram(mRGBData, mRedHistogram,
width, height, 0);
double imageRedMean = 0;
double redHistogramSum = 0;
for (int bin = 0; bin < 256; bin++) {
imageRedMean += mRedHistogram[bin] * bin;
redHistogramSum += mRedHistogram[bin];
} // bin
imageRedMean /= redHistogramSum;
double[] mBinSquared;
mBinSquared = new double[256];
for (int bin = 0; bin < 256; bin++) {
mBinSquared[bin] = ((double) bin) * bin;
}
double imageRed2ndMoment = 0;
for (int bin = 0; bin < 256; bin++) {
imageRed2ndMoment += mRedHistogram[bin]
* mBinSquared[bin];
} // bin
imageRed2ndMoment /= redHistogramSum;
double imageRedStdDev = Math.sqrt((imageRed2ndMoment
- imageRedMean * imageRedMean));
int val = (new Double(imageRedStdDev * 100000)
.intValue());
show_BPM.setText(String.valueOf(no));
int v = new Double(imageRedStdDev).intValue();
for (int l = 0; l < 1; l++) {
}
int yVal=250;
if (imageRedStdDev < 15 && val != 0)
{
show=true;
g.addPoint(yVal);
g.addPoint(yVal);
float flt=(float)Math.random();
g.addPoint(yVal+flt*25);
g.addPoint(yVal);
g.addPoint(yVal);
flt=(float)Math.random();
g.addPoint(yVal-flt*20);
g.addPoint(yVal);
g.addPoint(yVal);
flt=(float)Math.random();
g.addPoint(yVal-flt*30);
g.addPoint(yVal);
g.addPoint(yVal);
g.addPoint(yVal);
g.addPoint(yVal);
flt=(float)Math.random();
g.addPoint(yVal+flt*35);
try {
Thread.sleep(500);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
} else {
show=false;
if(val==0){
count++;
g.addPoint(yVal);
g.addPoint(yVal);
g.addPoint(yVal);
g.addPoint(yVal);
show_BPM.setText(String.valueOf(0));
}else{
count++;
g.addPoint(yVal);
g.addPoint(yVal);
g.addPoint(yVal);
g.addPoint(yVal);
show_BPM.setText(String.valueOf(0));
}
}
}
});
}
@Override
public void surfaceChanged(SurfaceHolder holder, int format,
int width, int height) {
System.out.println("Changed");
Camera.Parameters parameters = mCamera.getParameters();
parameters.setPreviewSize(320, 240);
parameters.setPreviewFrameRate(15);
parameters.setSceneMode(Camera.Parameters.SCENE_MODE_NIGHT);
parameters.setFocusMode(Camera.Parameters.FOCUS_MODE_AUTO);
mCamera.setParameters(parameters);
mCamera.startPreview();
g.invalidate();
}
});
// fl.addView(b);
}
static public void decodeYUV420SP(int[] rgb, byte[] yuv420sp, int width,
int height) {
final int frameSize = width * height;
for (int j = 0, yp = 0; j < height; j++) {
int uvp = frameSize + (j >> 1) * width, u = 0, v = 0;
for (int i = 0; i < width; i++, yp++) {
int y = (0xff & ((int) yuv420sp[yp])) - 16;
if (y < 0)
y = 0;
if ((i & 1) == 0) {
v = (0xff & yuv420sp[uvp++]) - 128;
u = (0xff & yuv420sp[uvp++]) - 128;
}
int y1192 = 1192 * y;
int r = (y1192 + 1634 * v);
int g = (y1192 - 833 * v - 400 * u);
int b = (y1192 + 2066 * u);
if (r < 0)
r = 0;
else if (r > 262143)
r = 262143;
if (g < 0)
g = 0;
else if (g > 262143)
g = 262143;
if (b < 0)
b = 0;
else if (b > 262143)
b = 262143;
rgb[yp] = 0xff000000 | ((r << 6) & 0xff0000)
| ((g >> 2) & 0xff00) | ((b >> 10) & 0xff);
}
}
}
static public void calculateIntensityHistogram(int[] rgb, int[] histogram,
int width, int height, int component) {
for (int bin = 0; bin < 256; bin++) {
histogram[bin] = 0;
} // bin
if (component == 0) // red
{
for (int pix = 0; pix < width * height; pix += 3) {
int pixVal = (rgb[pix] >> 16) & 0xff;
histogram[pixVal]++;
} // pix
} else if (component == 1) // green
{
for (int pix = 0; pix < width * height; pix += 3) {
int pixVal = (rgb[pix] >> 8) & 0xff;
histogram[pixVal]++;
} // pix
} else // blue
{
for (int pix = 0; pix < width * height; pix += 3) {
int pixVal = rgb[pix] & 0xff;
histogram[pixVal]++;
} // pix
}
}
public void onWindowFocusChanged(boolean hasFocus) {
super.onWindowFocusChanged(hasFocus);
clock.setBackgroundResource(R.anim.clockanimation);
AnimationDrawable clock_Anim=new AnimationDrawable();
clock_Anim=(AnimationDrawable)clock.getBackground();
clock_Anim.start();
timer.start();
}
CountDownTimer timer=new CountDownTimer(30000,1000){
@Override
public void onFinish()throws NullPointerException {
// TODO Auto-generated method stub
mCamera.stopPreview();**//This is the line for which shows NullPointer exception**
g.invalidate();
Canvas.freeGlCaches();
graph_Text.setBackgroundResource(R.drawable.graphtext3);
if(count>0)
{
System.out.println("Hiiiiiii");
show_BPM.setText(String.valueOf(0));
}
else
{
System.out.println("Helllooo");
show_BPM.setText(String.valueOf(no));
}
}
@Override
public void onTick(long arg0) {
// TODO Auto-generated method stub
}
};
class DrawMyGraph extends View {
public LinkedList<Float> yValues = null;
private int size = 0;
private LinkedList<Integer> xValues = null;
private LinkedList<Long> beatList = new LinkedList<Long>();
public DrawMyGraph(Context context, int size) {
super(context);
yValues = new LinkedList<Float>();
int inc = 5;
this.size = size * inc;
xValues = new LinkedList<Integer>();
int cnt = 468;
for (int i = 0; i < this.size * inc; i++) {
xValues.add(cnt);
cnt = cnt - (inc - 2);
}
}
@Override
protected void onDraw(Canvas canvas) {
Bitmap bitmap=Bitmap.createBitmap(2 , 2, Bitmap.Config.RGB_565);
graph_View.setImageBitmap(bitmap);
Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
paint.setStyle(Paint.Style.STROKE);
paint.setStrokeWidth(2);
paint.setColor(Color.RED);
invalidate();
for (int i = 0, j = 1; i < yValues.size() && j < yValues.size(); i++, j++) {
canvas.drawLine(xValues.get(i) + 30, (yValues.get(i) - 10)*2,
xValues.get(j) + 30, (yValues.get(j) - 10)*2, paint);
}
super.onDraw(canvas);
}
public int getSize() {
return size;
}
public void addPoint(float p) {
if (yValues.size() > size) {
yValues.removeLast();
}
yValues.addFirst(p);
}
public void addBeat(long beat) {
beatList.add(beat);
}
}
public void onBackPressed() {
finish();
return;
}
}
这就是我的logcat所说的
02-10 17：34：25.570：W / dalvikvm（3681）：threadid = 1：线程退出 未捕获的异常（组= 0x40015578）02-10 17：34：25.574： E / AndroidRuntime（3681）：致命例外：主02-10 17：34：25.574： E / AndroidRuntime（3681）：java.lang.NullPointerException 02-10 17：34：25.574：E / AndroidRuntime（3681）：at com.android.ecg.ECG $ 1.onFinish（ECG.java:312)02-10 17：34：25.574： E / AndroidRuntime（3681）：at android.os.CountDownTimer $ 1.handleMessage（CountDownTimer.java:118） 02-10 17：34：25.574：E / AndroidRuntime（3681）：at android.os.Handler.dispatchMessage（Handler.java:99）02-10 17：34：25.574：E / AndroidRuntime（3681）：at android.os.Looper.loop（Looper.java:123）02-10 17：34：25.574： E / AndroidRuntime（3681）：at android.app.ActivityThread.main（ActivityThread.java:3687)02-10 17：34：25.574：E / AndroidRuntime（3681）：at java.lang.reflect.Method.invokeNative（Native Method）02-10 17：34：25.574：E / AndroidRuntime（3681）：at java.lang.reflect.Method.invoke（Method.java:507)02-10 17：34：25.574： E / AndroidRuntime（3681）：at com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:842） 02-10 17：34：25.574：E / AndroidRuntime（3681）：at com.android.internal.os.ZygoteInit.main（ZygoteInit.java:600)02-10 17：34：25.574：E / AndroidRuntime（3681）：at dalvik.system.NativeStart.main（原生方法）
答案 0 :(得分：0)
欢迎来到SO。建议不要发布完整的活动，只要它是一个“简单”的NPE。代码量使得很难看到问题。
我猜你的计时器在相机准备好之前访问mCamera对象。在调用对象上的方法之前，应检查mCamera是否为null。另外，请确保在相机准备好之前不会启动计时器。