Android：制作相机活动

Question

我正在关注this指南，让我的相机活动。以下是用户打开相机并单击图片的活动代码：

public class MainActivity extends Activity {

private static final String TAG = "MainActivity";

private Camera mCamera;
private CameraPreview mPreview;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    if(checkCameraHardware(this)) {
        mCamera = getCameraInstance();
    } else {
        Log.d(TAG, "Check camera hardware is false");
    }

    if(mCamera == null) {
        Log.d(TAG, "Camera is null");
    } else {
        Log.d(TAG, "Camera isn't null");
    }


    mPreview = new CameraPreview(this, mCamera);
    FrameLayout preview = (FrameLayout) findViewById(R.id.camera_preview);
    preview.addView(mPreview);
}

    private boolean checkCameraHardware(Context context) {
        if (context.getPackageManager().hasSystemFeature(PackageManager.FEATURE_CAMERA)){
            return true;
        } else {
            return false;
        }
    }

    public static Camera getCameraInstance(){
        Camera c = null;
        try {
            c = Camera.open();
        }
        catch (Exception e){
            Log.e(TAG, e.toString());
        }
        return c;
    }

这是CameraPreview：

public class CameraPreview extends SurfaceView implements SurfaceHolder.Callback {

private SurfaceHolder mHolder;
private Camera mCamera;
private static final String TAG = "CameraPreview";

public CameraPreview(Context context, Camera camera) {
    super(context);
    mCamera = camera;

    mHolder = getHolder();
    mHolder.addCallback(this);

    mHolder.setType(SurfaceHolder.SURFACE_TYPE_PUSH_BUFFERS);
}

@Override
public void surfaceCreated(SurfaceHolder holder) {

    try {
        mCamera.setPreviewDisplay(holder);
        mCamera.startPreview();
    } catch (IOException e) {
        Log.d(TAG, "Error in setting the camera preview: " + e.getMessage());
    }
}

@Override
public void surfaceChanged(SurfaceHolder holder, int format, int width, int height) {
    if(mHolder.getSurface() == null) {
        return;
    }
    try {
        mCamera.stopPreview();
    } catch (Exception e) {

    }

    try {
        mCamera.setPreviewDisplay(mHolder);
        mCamera.startPreview();
    } catch (Exception e) {
        Log.d(TAG, "Error in starting camera preview: " + e.getMessage());
    }

}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {

}
}

在运行应用程序时，会显示以下错误：

FATAL EXCEPTION: main
Process: com.example.prakhar.camerainterfacesample, PID: 13275
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.hardware.Camera.setPreviewDisplay(android.view.SurfaceHolder)' on a null object reference
at com.example.prakhar.camerainterfacesample.CameraPreview.surfaceCreated(CameraPreview.java:34)
at android.view.SurfaceView.updateWindow(SurfaceView.java:580)
at android.view.SurfaceView$3.onPreDraw(SurfaceView.java:176)
at android.view.ViewTreeObserver.dispatchOnPreDraw(ViewTreeObserver.java:944)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1970)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1061)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:5891)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:767)
at android.view.Choreographer.doCallbacks(Choreographer.java:580)
at android.view.Choreographer.doFrame(Choreographer.java:550)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:753)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5300)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)

第34行是：

mCamera.setPreviewDisplay(holder);

清单文件是：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.prakhar.camerainterface">

    <uses-permission android:name="android.permission.CAMERA" />
    <uses-feature android:name="android.hardware.camera" />
    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

    <application
        android:allowBackup="true"
        android:label="@string/app_name"
        android:supportsRtl="true">
        <activity android:name=".MainActivity"></activity>
    </application>

</manifest>

我哪里错了？