我创建了一个简单的相机应用程序，就像他们在这个例子中所做的那样http://developer.android.com/guide/topics/media/camera.html#custom-camera。但问题是我无法让它在Android模拟器上工作。系统相机应用程序在模拟器上完美运行，但是当我尝试运行它时，我的应用程序崩溃了。模拟器是一个没有皮肤的nexus 5，模拟相机。我尝试了不同的更改设备，但仍然是同样的问题。 这是崩溃后的logcat：
`10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: FATAL EXCEPTION: main
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: Process: bajsa.myapplication, PID: 3146
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.hardware.Camera.setPreviewDisplay(android.view.SurfaceHolder)' on a null object reference
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at bajsa.myapplication.CameraPreview.surfaceCreated(CameraPreview.java:30)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.view.SurfaceView.updateWindow(SurfaceView.java:582)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.view.SurfaceView$3.onPreDraw(SurfaceView.java:177)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.view.ViewTreeObserver.dispatchOnPreDraw(ViewTreeObserver.java:944)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2055)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1107)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6013)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.view.Choreographer$CallbackRecord.run(Choreographer.java:858)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.view.Choreographer.doCallbacks(Choreographer.java:670)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.view.Choreographer.doFrame(Choreographer.java:606)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:844)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.os.Handler.handleCallback(Handler.java:739)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:95)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.os.Looper.loop(Looper.java:148)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:5417)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
10-03 15:53:16.750 3146-3146/? E/AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)`
`10-03 15:53:36.270 1796-1808/? E/StrictMode: A resource was acquired at attached stack trace but never released. See java.io.Closeable for information on avoiding resource leaks.`
`10-03 15:57:50.330 1027-1027/? I/perfprofd: profile collection skipped (running in emulator)
10-03 16:08:07.460 1349-1365/? E/BluetoothAdapter: Bluetooth binder is null
10-03 16:08:07.460 1349-1365/? E/KernelCpuSpeedReader: Failed to read cpu-freq
10-03 16:08:07.460 1349-1365/? E/KernelCpuSpeedReader: java.io.FileNotFoundException: /sys/devices/system/cpu/cpu0/cpufreq/stats/time_in_state: open failed: ENOENT (No such file or directory)`
为什么不起作用？