从服务中的活动引用文本视图，反之亦然

Question

I am working on an Android app with a steadily growing Code basis; the aim of the app is to scan and process QR Codes and Barcodes on Handheld Devices for Storages (not Smartphones). It hast two Activities that contain major parts of the programmatical logic; hence, I want to store major parts of the Code that contains the Functionality for processing the Scanner input in an external Service, called Scanner Service, implement the methods there and use the methods in other Activites;

however, I have a major issue with the use of Context, getApplicationContext() and the reference of the Activity in the Service and vice versa; although I think I have referenced and initialised the Textviews from the Activity in the Service, the app keeps on crashing with the following error message:

AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.app.Activity.findViewById(int)' on a null object reference
        at com.xxxx.ScanService.<clinit>(ScanService.java:16)

I know what it means, but I don´t know how I could access the View

private static TextView content = (TextView) a.findViewById(R.id.content_detail);

in such a way that I can use it in the Service and in the Activity and the app stops crashing.

Therefore, any hints or help would be very much appreciated, thank you in advance.

The MainDetailActivity:

package com.example.xxx_app;

import android.app.Activity;
import android.app.Dialog;
import android.content.Context;
import android.content.Intent;
import android.graphics.drawable.ColorDrawable;
import android.os.Bundle;
import android.util.Log;
import android.view.KeyEvent;
import android.view.Menu;
import android.view.View;
import androidx.appcompat.app.AppCompatActivity;
import androidx.appcompat.widget.Toolbar;
import androidx.recyclerview.widget.RecyclerView;
import android.view.MenuItem;
import android.view.Window;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageButton;
import android.widget.ImageView;
import android.widget.ListView;
import android.widget.PopupMenu;
import android.widget.Spinner;
import android.widget.TextView;
import android.widget.Toast;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;

import static com.xxx.ScanService.*;
import static com.xxx.SimpleItemRecyclerViewAdapter.TAGG;

/**
 * An activity representing a single Main detail screen. This
 * activity is only used on narrow width devices. On tablet-size devices,
 * item details are presented side-by-side with a list of items
 * in a {@link MainListActivity}.
 */
public class MainDetailActivity extends AppCompatActivity implements AdapterView.OnItemSelectedListener,
        PopupMenu.OnMenuItemClickListener {
    Context context;
    private RecyclerView recyclerView;
    private RecyclerviewAdapter recyclerviewAdapter;
    private RecyclerTouchListener touchListener;
    private ListView listView;
    public TextView textView4;
    public String code;
    public static final String TAG = "Barcode ist:" ;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_detail);
        context = this;
        //TextView headerView = findViewById(R.id.txt1);
        Spinner spinner = (Spinner) findViewById(R.id.spinner);
        EditText editBarcode = (EditText) findViewById(R.id.editText);
        TextView content = (TextView) findViewById(R.id.content_detail);
        TextView editTextNumber = findViewById(R.id.editTextNumber);
        Button addBooking = findViewById(R.id.button);
        Button removeBooking = findViewById(R.id.button2);
        removeBooking.setEnabled(false);
        spinner.setOnItemSelectedListener(this);

        //String selectedItem = spinner.getSelectedItem().toString();
        // Create an ArrayAdapter using the string array and a default spinner layout
        ArrayAdapter<CharSequence> adapter = ArrayAdapter.createFromResource(this,
                R.array.mockdata, android.R.layout.simple_spinner_item);
        // Specify the layout to use when the list of choices appears
        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
        // Apply the adapter to the spinner
        spinner.setAdapter(adapter);
        String scannedCode = getIntent().getStringExtra("scannedCode");
        Log.d(TAG, "scannedCode" + scannedCode);
        if (scannedCode != null && (content.getText().toString().equals(""))) {
          content.setText(scannedCode);
        }


        Button btn = findViewById(R.id.imageView4);
        btn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                PopupMenu popup = new PopupMenu(MainDetailActivity.this, v);
                popup.setOnMenuItemClickListener(MainDetailActivity.this);
                popup.inflate(R.menu.popup_menu);
                popup.show();
            }
        });

        editBarcode.setOnKeyListener(new View.OnKeyListener() {

            @Override
            public boolean onKey(View v, int keyCode, KeyEvent event) {

                // TODO Auto-generated method stub
                String code = editBarcode.getText().toString();
                if (code.matches("")) //{ if(code.trim().isEmpty())
                //|| editBarcode.getText().toString() > 100 )
                {
                    Log.d(TAG, "Code ist leer");
                }


                if (keyCode == KeyEvent.KEYCODE_ENTER && code.length() > 0) {
                    editBarcode.setText("");
                    ScanService.checkEnteredCode(code);
                    return true;
                }
                return false;
            }
        });

        recyclerView = findViewById(R.id.recyclerview);
        recyclerviewAdapter = new RecyclerviewAdapter(this);
        Intent newIntent = getIntent();
        String receivedPalNo =  newIntent.getStringExtra("palNo");
        String receivedNo =  newIntent.getStringExtra("no");
        String receivedType =  newIntent.getStringExtra("type");
        String receivedRack =  newIntent.getStringExtra("rack");
        String receivedCountItems =  newIntent.getStringExtra("count_items");
        content.setText(receivedCountItems);
        RestClient.getPaletteItems(getApplicationContext(),recyclerviewAdapter,receivedPalNo);

        Log.d(TAGG,"Intent 1" + receivedPalNo);
        Log.d(TAGG, "Intent 2" + receivedNo);
        Log.d(TAGG, "Intent 3" + receivedType);
        Log.d(TAGG,"Intent 4" + receivedRack);
        Log.d(TAGG, "Intent 5" + receivedCountItems);

        final ArrayList<Items> itemList = new ArrayList<>();
        /*
        Items[] items = new Items(12345,123456, 200, 500);
        itemList.add(items);*/


        recyclerviewAdapter.setItemList((ArrayList<Items>) itemList);
        recyclerView.setAdapter(recyclerviewAdapter);

        touchListener = new RecyclerTouchListener(this,recyclerView);
        RecyclerviewAdapter finalRecyclerviewAdapter = recyclerviewAdapter;
        touchListener
                .setClickable(new RecyclerTouchListener.OnRowClickListener() {
                    @Override
                    public void onRowClicked(int position) {
                        //Toast.makeText(getApplicationContext(),itemList.get(position), Toast.LENGTH_SHORT).show();
                    }

                    @Override
                    public void onIndependentViewClicked(int independentViewID, int position) {

                    }
                })
                .setSwipeOptionViews(R.id.delete_task,R.id.edit_task)
                .setSwipeable(R.id.rowFG, R.id.rowBG, new RecyclerTouchListener.OnSwipeOptionsClickListener() {
                    @Override
                    public void onSwipeOptionClicked(int viewID, int position) {
                        switch (viewID){
                            case R.id.delete_task:
                                itemList.remove(position);
                                finalRecyclerviewAdapter.setItemList(itemList);
                                break;
                            case R.id.edit_task:
                                Toast.makeText(getApplicationContext(),"Edit Not Available",Toast.LENGTH_SHORT).show();
                                break;

                        }
                    }
                });
        recyclerView.addOnItemTouchListener(touchListener);



        class StableArrayAdapter extends ArrayAdapter<String> {

            HashMap<String, Integer> mIdMap = new HashMap<String, Integer>();

            public StableArrayAdapter(Context context, int textViewResourceId,
                                      List<String> objects) {
                super(context, textViewResourceId, objects);
                for (int i = 0; i < objects.size(); ++i) {
                    mIdMap.put(objects.get(i), i);
                }
            }

            @Override
            public long getItemId(int position) {
                String item = getItem(position);
                return mIdMap.get(item);
            }

            @Override
            public boolean hasStableIds() {
                return true;
            }

        }

    // savedInstanceState is non-null when there is fragment state
        // saved from previous configurations of this activity
        // (e.g. when rotating the screen from portrait to landscape).
        // In this case, the fragment will automatically be re-added
        // to its container so we don"t need to manually add it.
        // For more information, see the Fragments API guide at:
        //
        // http://developer.android.com/guide/components/fragments.html
        //
        String text = getIntent().getStringExtra("palNo");
        //headerView.setText(text);

        if (receivedType != null && receivedType.equals("FE")) {
            ImageView mImgView = findViewById(R.id.id_col_code);
            mImgView.setBackgroundResource(R.drawable.backgroun_blue);
        }
        if (receivedType != null && receivedType.equals("UFE")) {
            ImageView mImgView = findViewById(R.id.id_col_code);
            mImgView.setBackgroundResource(R.drawable.backgroun_yellow);
        }

    }

    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }


    public boolean onMenuItemClick(MenuItem item) {
        Toast.makeText(this, "Selected Item: " +item.getTitle(), Toast.LENGTH_SHORT).show();
        switch (item.getItemId()) {
            case R.id.search_item:
                // do your code
                return true;
            case R.id.upload_item:
                // do your code
                return true;
            case R.id.copy_item:
                // do your code
                return true;
           /* case R.id.print_item:
                // do your code
                return true;*/
            case R.id.share_item:
                // do your code
                return true;
            /*case R.id.bookmark_item:
                // do your code
                return true;*/
            default:
                return false;
        }
    }


        public void newDialog(Activity activity) {
            final Dialog dialog = new Dialog(activity);
            dialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
            dialog.setCancelable(true);
            dialog.setContentView(R.layout.sortiment_layout);
            dialog.getWindow().setBackgroundDrawable(new ColorDrawable(android.graphics.Color.TRANSPARENT));

            Button okButton = dialog.findViewById(R.id.ok);
            okButton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    //Toast.makeText(getApplicationContext(),"Ok" ,Toast.LENGTH_SHORT).show();
                    dialog.dismiss();
                }
            });
            Button cancelButton = dialog.findViewById(R.id.cancel);
            cancelButton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    //Toast.makeText(getApplicationContext(),"Abbrechen" ,Toast.LENGTH_SHORT).show();
                    dialog.cancel();
                }
            });
            dialog.show();
        }

        public void showDialog(Activity activity) {
            final Dialog dialog = new Dialog(activity);
            dialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
            dialog.setCancelable(true);
            dialog.setContentView(R.layout.newcustom_layout);
            dialog.getWindow().setBackgroundDrawable(new ColorDrawable(android.graphics.Color.TRANSPARENT));


            Button okButton = dialog.findViewById(R.id.ok);
            okButton.setOnClickListener(new View.OnClickListener() {
                @Override
                public void onClick(View v) {
                    //Toast.makeText(getApplicationContext(),"Ok" ,Toast.LENGTH_SHORT).show();
                    dialog.dismiss();
                }
            });
            dialog.show();
        }


    public void onItemSelected(AdapterView<?> parent, View view,
                               int pos, long id) {
        // An item was selected. You can retrieve the selected item using
        // parent.getItemAtPosition(pos)
    }

    public void onNothingSelected(AdapterView<?> parent) {
        // Another interface callback
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        int id = item.getItemId();
        if (id == android.R.id.home) {
            // This ID represents the Home or Up button. In the case of this
            // activity, the Up button is shown. For
            // more details, see the Navigation pattern on Android Design:
            //
            // http://developer.android.com/design/patterns/navigation.html#up-vs-back
            //
            navigateUpTo(new Intent(this, MainListActivity.class));
            return true;
        }
        return super.onOptionsItemSelected(item);
    }

    @Override
    public void onResume() {
        super.onResume();
        recyclerView.addOnItemTouchListener(touchListener);
    }

    @Override
    public void onPointerCaptureChanged(boolean hasCapture) {

    }
}

The ScanService Class:

package com.example.xxx;

import android.app.Activity;
import android.content.Context;
import android.util.Log;
import android.widget.TextView;


public class ScanService {


    private static final String TAG = "Scan Service Tag";
    private static Context mContext;
    private static Activity a = (MainDetailActivity)mContext;
    private static TextView content = (TextView) 
    a.findViewById(R.id.content_detail);
    private static TextView editTextNumber = (TextView) 
    a.findViewById(R.id.editTextNumber);

    public ScanService (Context mContext) {
        this.mContext = mContext;
    }

    public static void checkEnteredCode(String code, Activity a) {
        content.setText("");

        //PSP-H1-EA-F3
        if
        (code.matches("PSP-\\p{Upper}\\d\\p{Punct}\\p{Upper}\\" +
                "p{Upper}\\p{Punct}\\p{Upper}\\p{Digit}")) {
            content.setText("");
            content.setText(code);
            Log.d(TAG, "xxx");

        }
        if (code.matches("LF-[0-9]*")) {
            ///LF-(\d+)/gi
            content.setText("");
            content.setText(code);
            Log.d(TAG, "xxx");
        }
        if (code.matches("PAL-[0-9][0-9][0-9]")) {

            content.setText("");
            content.setText(code);
            Log.d(TAG, "xxx");
        }
        if (code.matches("P-[0-9][0-9][0-9]")) {

            content.setText("");
            content.setText(code);
            Log.d(TAG, "Palette");
        }
        if (code.matches("[0-9][0-9][0-9][0-9].[0-9][0-9].+DB")) {
           
            if(editTextNumber == null) {
                Log.d(TAG, "xxx");
            }
            else {
                editTextNumber.setText(code);
                Log.d(TAG, "xxx");
            }
            Log.d(TAG, "xxx");
        }
        if (code.matches("[0-9A-Z]*[0-9]*")) {
            //editBarcode.setText("");
            //editBarcode.setText(keyCode);
            Log.d(TAG, "xxx");
        }
        if (code.matches("\\d{13}")) {
            //newDialog(MainDetailActivity.this);
            //editBarcode.setText("");
            //editBarcode.setText(keyCode);
            if(editTextNumber == null) {
                Log.d(TAG, "xxx");
            }
            else {
                editTextNumber.setText(code);
                Log.d(TAG, "xxx");
            }
            Log.d(TAG, "xxx");
        }
        else {
            Log.d(TAG, "xxx");

        };
        //editBarcode.setText("");
        //editBarcode.setText(code);
            /* String code = editBarcode.getText().toString();
                if (code.matches("")) //{ if(code.trim().isEmpty())
                //|| editBarcode.getText().toString() > 100 )
                {
                    Log.d(TAG, "xxx");
                }
                //}
                checkEnteredCode(code);
                //editBarcode.setText("");
             return Boolean.parseBoolean(code);*/
        Log.d(TAG, code);

    }

    public static void checkEnteredCode(String code) {
    }
}

Answer 1

You cannot access any View s from a Service ! View s belong to the Activity . This is the wrong application architecture. A Service performs background processing (file I/O, network I/O, computation, etc.). The Activity is responsible for interacting with the user (inputs, display, etc.). If your Service wants to put data on the screen, you've broken the division of responsibilities. Your Service should simply notify your Activity (or any other component that is interested) when data has changed, and the Activity can then update the View itself. You can share data between the Service and your other components in a number of ways, including: event bus, publish/subscribe, shared preferences, broadcast Intent s, SQLite database, etc.