嘗試通過java類訪問活動中的方法時，null對象引用上的android.content.Context.getPackageName（）'

Question

我正在實時運行一個基本的人臉檢測應用程序，並計算它在FaceDetectionProcessor.java類中檢測到面部的次數（用作參考 - > https://github.com/firebase/quickstart-android / tree / master / mlkit / app / src / main / java / com / google / firebase / samples / apps / mlkit / java ）並且在特定計數值下，它使用FaceDetectionProcessor.java對象調用Activity的對象一種啟動新活動的方法，然后崩潰。

嘗試了FaceDetectActivity.this，簡單地說就是getApplicationContext（），甚至是一個全局Context對象，它在FaceDetectActivity的onCreate（）中將其值設置為“this”

public class FaceDetectActivity extends AppCompatActivity implements ActivityCompat.OnRequestPermissionsResultCallback,CompoundButton.OnCheckedChangeListener{
    private static final String FACE_DETECTION = "Face Detection";
    private static final String TAG = "LivePreviewActivity";
    private static final int PERMISSION_REQUESTS = 1;
    private CameraSource cameraSource = null;
    private CameraSourcePreview preview;
    private GraphicOverlay graphicOverlay;
    private String selectedModel = FACE_DETECTION;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
.
.
.
.
}


private void createCameraSource(String model) {
        // If there's no existing cameraSource, create one.
        if (cameraSource == null) {
            cameraSource = new CameraSource(this, graphicOverlay);
        }
        try {
            switch (model) {
                case FACE_DETECTION:
                    Log.i(TAG, "Using Face Detector Processor");
                    cameraSource.setMachineLearningFrameProcessor(new FaceDetectionProcessor());
                    break;
                default:
                    Log.e(TAG, "Unknown model: " + model);
            }
        } catch (Exception e) {
            Log.e(TAG, "can not create camera source: " + model);
        }
    }

public void alertAndMove(){
        Intent i=new Intent(FaceDetectActivity.this,SelectingParty.class);
        startActivity(i);
    }

public class FaceDetectionProcessor extends VisionProcessorBase<List<FirebaseVisionFace>> {
    int count=0;

    private static final String TAG = "FaceDetectionProcessor";

    private final FirebaseVisionFaceDetector detector;

    public FaceDetectionProcessor() {
        FirebaseVisionFaceDetectorOptions options =
                new FirebaseVisionFaceDetectorOptions.Builder()
                        .setClassificationMode(FirebaseVisionFaceDetectorOptions.ALL_CLASSIFICATIONS)
                        .enableTracking()
                        .build();

        detector = FirebaseVision.getInstance().getVisionFaceDetector(options);
    }

    .
    .
    .
    . 
    . 
    .
    .

    @Override
    protected void onSuccess(
            @NonNull List<FirebaseVisionFace> faces,
            @NonNull FrameMetadata frameMetadata,
            @NonNull GraphicOverlay graphicOverlay) {
        graphicOverlay.clear();
        for (int i = 0; i < faces.size(); ++i) {
            FirebaseVisionFace face = faces.get(i);
            FaceGraphic faceGraphic = new FaceGraphic(graphicOverlay);
            graphicOverlay.add(faceGraphic);
            faceGraphic.updateFace(face, frameMetadata.getCameraFacing());
            Log.d(TAG, "onSuccess: Face detected and count= "+count);
            count++;
            if(count>=15)
                new FaceDetectActivity().alertAndMove();
        }
    }

    @Override
    protected void onFailure(@NonNull Exception e) {
        Log.e(TAG, "Face detection failed " + e);
    }
}

當調用新的FaceDetectActivity（）。alertAndMove（）時，應用程序在alertAndMove（）中的行Intent i = new Intent（FaceDetectActivity.this，......）崩潰

對於這種情況非常令人沮喪的是，相同的邏輯在我最初測試邏輯的朋友筆記本電腦上運行相同的代碼。

Answer 1

在FaceDetectActivity中，我將Intent和上下文發送到FaceDetectionProcessor.java：

case FACE_DETECTION:
                    Log.i(TAG, "Using Face Detector Processor");
                    Intent i=new Intent( this,SelectingParty.class);
                    cameraSource.setMachineLearningFrameProcessor(new FaceDetectionProcessor(this,i));
                    break;

在FaceDetectionProcessor.java的構造函數中，我初始化了實例Context obj和Intent intent，如下所示：

public FaceDetectionProcessor(Context obj, Intent intent) {
        this.obj=obj;
        this.intent=intent;
.
.
.

最后在FaceDetectionProcessor.java中的onSuccess方法中我做了：

if(count>=15){
                obj.startActivity(intent);
            }

而已 ！ 我仍然無法關閉以前的活動，任何建議我如何實現的評論都會有很大的幫助！ PS我試過CLEAR_TASK，CLEAR_TOP，NEW_TASK，沒用，但我可能會遺漏一些東西。