我正在开发一款android应用程序，其代码基础在稳步增长；该应用程序的目的是扫描和处理用于存储的手持设备（而不是智能手机）上的二维码和条形码。它有两个包含程序逻辑主要部分的活动；因此，我想存储代码的主要部分，其中包含处理外部服务（称为scanner service）中扫描器输入的功能，在那里实现方法并在其他活动中使用方法；
但是，我在使用context、getapplicationcontext（）和服务中活动的引用时遇到了一个主要问题，反之亦然；尽管我认为我已经引用并初始化了服务中活动的textviews，但应用程序仍会不断崩溃，并显示以下错误消息：

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)

我知道这是什么意思，但我不知道´我不知道我怎么才能看到风景

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

这样我就可以在服务和活动中使用它，应用程序就不会崩溃了。
因此，任何提示或帮助将不胜感激，提前谢谢你。
主要活动：

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) {

    }
}

scanservice类：

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) {
    }
}