我看不到为什么我得到了NullPointerException

Question

首先，我是整个Android派对的初学者。 我试图编写代码以单击按钮来启动和停止相机预览。 我最终希望从预览中提取单个帧，但是在正确启动预览时遇到了问题。

下面是我的代码。 我的相机预览是通过单击launchFrames按钮开始/停止的。 即真/假每次切换按钮A布尔值告诉onClickListener是否调用camera_start()或camera_stop()方法。 目前，我的应用程序在创建按钮之后（在实现onClickListener之前）崩溃。

我通过添加Log消息来确定这是崩溃的地方，如所附的LogCat 。 请阐明导致此错误的原因以及您在我的代码中可能注意到的任何其他问题（常规结构或特定内容）。 让我知道我是否未能提供有关我的问题的足够信息。

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

这是我的活动的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

给定您的日志消息， launchFrames必须为null 。 您可以注意到堆栈跟踪告诉您崩溃的确切行号，从而更直接地告诉您。

尝试清理您的项目（例如，Eclipse主菜单中的Project> Clean），看看是否有帮助。