Question

首先，我是整个Android派对的初学者。我试图编写代码来点击按钮开始和停止相机预览。我最终想要从预览中提取单个帧，但我遇到了使预览正确启动的问题。

以下是我的代码。我的相机预览是在点击launchFrames按钮时开始/停止。每次按钮告诉onClickListener是否调用camera_start()或camera_stop()方法时，切换为true / false的布尔值。目前，我的应用程序在创建按钮后（以及onClickListener实现之前）崩溃。

我通过添加日志消息确定了崩溃的位置，如附加的LogCat所示。请详细说明导致此错误的原因以及您可能会注意到我的代码（一般结构或特定事物）的任何其他问题。如果我未能提供有关我的问题的足够信息，请告诉我。

public class MainActivity extends ActionBarActivity implements SurfaceHolder.Callback{

public static final String EXTRA_MESSAGE = "com.example.myfirstapp.MESSAGE";
public static final int REQUEST_VIDEO_CAPTURE = 1;
public static final String TAG = "MAIN";
private static boolean toggleFrameLauncher = false;
Button launchFrames;
Camera mcamera;
SurfaceView surfaceView;
SurfaceHolder surfaceHolder;
boolean previewing = false;
LayoutInflater controlInflater = null;

@Override
protected void onCreate(Bundle savedInstanceState) {
    Log.i(TAG, "Started onCreate method"); ////////// DEBUG ///////////
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Log.i(TAG, "setContentView executed successfully"); /////// DEBUG //////

    Button launchFrames = (Button)findViewById(R.id.button_frames);
    Log.i(TAG, "Created button reference successfully");//// DEBUG ////////

    launchFrames.setOnClickListener(new Button.OnClickListener() {
        @Override
        public void onClick(View arg0) {
            if (!toggleFrameLauncher) {
                toggleFrameLauncher = !toggleFrameLauncher;
                start_camera();
            } else {
                toggleFrameLauncher = !toggleFrameLauncher;
                stop_camera();
            }
        }
    });

    Log.i(TAG, "Button configured correctly"); /DEBUG-Code does not reach here/
    surfaceView = (SurfaceView)findViewById(R.id.surfaceView);
    surfaceHolder = surfaceView.getHolder();
    surfaceHolder.addCallback(this);

    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
        .add(R.id.container, new PlaceholderFragment())
        .commit();
    }
}

public void start_camera() {

    try {
        mcamera = Camera.open();
    } catch(RuntimeException e) {
        Log.e(TAG, "init_camera: " + e);
        return;
    }
    Camera.Parameters param;
    param = mcamera.getParameters();
    //modify parameter
    //param.setPreviewFrameRate(20);
    param.setPreviewSize(480,320);
    param.setPreviewFormat(ImageFormat.RGB_565);
    mcamera.setParameters(param);
    try {
        mcamera.setPreviewDisplay(surfaceHolder);
        mcamera.startPreview();
    } catch (Exception e) {
        Log.e(TAG, "init_camera: " + e);
        return;
    }
}

private void stop_camera() {
    mcamera.stopPreview();
    mcamera.release();
}

@Override
public void surfaceChanged(SurfaceHolder arg0, int arg1, int arg2, int arg3) {
    if(previewing) {
        mcamera.stopPreview();
        previewing = false;
    }

    if (mcamera != null) {
        try {
            mcamera.setPreviewDisplay(surfaceHolder);
            mcamera.startPreview();
            previewing = true;
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

@Override
public void surfaceCreated(SurfaceHolder holder) {
    mcamera = Camera.open();
}

@Override
public void surfaceDestroyed(SurfaceHolder holder) {
    mcamera.stopPreview();
    mcamera.release();
    mcamera = null;
    previewing = false;
}

}

这是我的LogCat：

05-24 18:58:33.463: I/MAIN(19366): Started onCreate method
05-24 18:58:33.503: I/MAIN(19366): setContentView executed successfully
05-24 18:58:33.503: I/MAIN(19366): Created button reference successfully
05-24 18:58:33.503: W/dalvikvm(19366): threadid=1: thread exiting with uncaught exception (group=0x4153aba8)
05-24 18:58:33.503: E/AndroidRuntime(19366): FATAL EXCEPTION: main
05-24 18:58:33.503: E/AndroidRuntime(19366): Process: com.example.myfirstapp, PID: 19366
05-24 18:58:33.503: E/AndroidRuntime(19366): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.myfirstapp/com.example.myfirstapp.MainActivity}: java.lang.NullPointerException
05-24 18:58:33.503: E/AndroidRuntime(19366):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at android.app.ActivityThread.access$800(ActivityThread.java:135)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at android.os.Handler.dispatchMessage(Handler.java:102)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at android.os.Looper.loop(Looper.java:136)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at android.app.ActivityThread.main(ActivityThread.java:5017)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at java.lang.reflect.Method.invokeNative(Native Method)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at java.lang.reflect.Method.invoke(Method.java:515)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at dalvik.system.NativeStart.main(Native Method)
05-24 18:58:33.503: E/AndroidRuntime(19366): Caused by: java.lang.NullPointerException
05-24 18:58:33.503: E/AndroidRuntime(19366):    at com.example.myfirstapp.MainActivity.onCreate(MainActivity.java:46)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at android.app.Activity.performCreate(Activity.java:5231)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
05-24 18:58:33.503: E/AndroidRuntime(19366):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
05-24 18:58:33.503: E/AndroidRuntime(19366):    ... 11 more

这是我活动的XML文件：

<?xml version="1.0" encoding="utf-8"?>
    <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
        xmlns:tools="http://schemas.android.com/tools"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation = "horizontal">

    <Button
        android:id="@+id/button_frames"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/text_box_1"
        android:layout_toRightOf = "@id/button_video"
        android:text="@string/button_frames" />

    <SurfaceView
        android:id="@+id/surfaceView"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content" />

</RelativeLayout>

Answer 1

根据您的日志消息，launchFrames必须为null。通过注意堆栈跟踪告诉您崩溃的确切行号，您可以更直接地告诉它。

尝试清理项目（例如，项目＆gt;在Eclipse主菜单中清理），看看是否有帮助。

我无法理解为什么我会收到NullPointerException

1 个答案: