java - 尝试在 Android Studio 中对空对象引用调用虚拟方法 'android.content.Context.getApplicationInfo()'

Question

我在应用程序中遇到了尝试在空对象引用上调用虚拟方法“java.util.List com.example.demo.Database.Database.getName()”错误，我解决了使用 Database database = new Database(this); 代码时出现该错误。

现在有一个新错误

E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.demo, PID: 19678 java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.demo/com.example.demo.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3132) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415) at android.app.ActivityThread.-wrap17(ActivityThread.java) at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821) at android.os.Handler.dispatchMessage(Handler.java:102) at android.os.Looper.loop(Looper.java:148) at android.app.ActivityThread.main(ActivityThread.java:7325) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:162) at com.readystatesoftware.sqliteasset.SQLiteAssetHelper.(SQLiteAssetHelper.java:109) at com.readystatesoftware.sqliteasset.SQLiteAssetHelper.(SQLiteAssetHelper.java:129) at com.example.demo.Database.Database.(Database.java:20) at com.example.demo.MainActivity.(MainActivity.java:28) at java.lang.Class.newInstance(Native Method) at android.app.Instrumentation.newActivity(Instrumentation.java:1096) at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3122) at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3415)  at android.app.ActivityThread.-wrap17(ActivityThread.java)  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1821)  at android.os.Handler.dispatchMessage(Handler.java:102)  at android.os.Looper.loop(Looper.java:148)  at android.app.ActivityThread.main(ActivityThread.java:7325)  at java.lang.reflect.Method.invoke(Native Method)  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120) 

这是我的 MainActivity.java 代码

公共(public)类 MainActivity 扩展 AppCompatActivity {

RecyclerView recyclerView;
RecyclerView.LayoutManager layoutManager;
SearchAdapter adapter;
private Context context;
MaterialSearchBar materialSearchBar;
List<String> suggestList = new ArrayList<>();
Database database = new Database(this);
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    recyclerView = (RecyclerView)findViewById(R.id.recycler_search);
    layoutManager = new LinearLayoutManager(this);
    recyclerView.setLayoutManager(layoutManager);
    recyclerView.setHasFixedSize(true);

    materialSearchBar =(MaterialSearchBar)findViewById(R.id.search_bar);
    materialSearchBar.setHint("Search");
    materialSearchBar.setCardViewElevation(10);
    LoadSuggestList();
    materialSearchBar.addTextChangeListener(new TextWatcher() {

        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {

        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            List<String> suggest = new ArrayList<>();
            for(String search:suggestList)
            {
                if(search.toLowerCase().contains(materialSearchBar.getText().toLowerCase()))
                    suggest.add(search);
            }
            materialSearchBar.setLastSuggestions(suggest);
        }

        @Override
        public void afterTextChanged(Editable s) {


        }
    });
    materialSearchBar.setOnSearchActionListener(new MaterialSearchBar.OnSearchActionListener(){
        @Override
        public void onSearchStateChanged(boolean enabled) {
            if(!enabled)
            {
                adapter = new SearchAdapter(getBaseContext(),database.getPharmacy());
                recyclerView.setAdapter(adapter);
            }
                //recyclerView.setAdapter(adapter);
        }

        @Override
        public void onSearchConfirmed(CharSequence text) {
            startSearch(text.toString());
        }

        @Override
        public void onButtonClicked(int buttonCode) {

        }
    });

    adapter = new SearchAdapter(this,database.getPharmacy());
    recyclerView.setAdapter(adapter);
}

private void startSearch(String text){
    adapter = new SearchAdapter(this, database.getPharmacyByCity(text));
    recyclerView.setAdapter(adapter);
}
private void LoadSuggestList() {
    suggestList = database.getName();
    materialSearchBar.setLastSuggestions(suggestList);
}

这是我的 Database.java 代码

public class Database extends SQLiteAssetHelper {

private static final String DB_NAME = "pharmacyDemo.db";
private static final int DB_VER = 1;

public Database(Context context) {
    super(context, DB_NAME, null, DB_VER);
}

public List<Pharmacy> getPharmacy(){
    SQLiteDatabase db = getReadableDatabase();
    SQLiteQueryBuilder qd = new SQLiteQueryBuilder();

    String[] sqlSelect ={"ID","Name","City","Phone"};
    String tableName = "Pharmacy_List";

    qd.setTables(tableName);
    Cursor cursor = qd.query(db,sqlSelect,null, null ,null , null , null);
    List<Pharmacy> result = new ArrayList<>();
    if(cursor.moveToFirst())
    {
        do{
            Pharmacy pharmacy = new Pharmacy();
            pharmacy.setID(cursor.getInt(cursor.getColumnIndex("ID")));
            pharmacy.setName(cursor.getString(cursor.getColumnIndex("Name")));
            pharmacy.setCity(cursor.getString(cursor.getColumnIndex("City")));
            pharmacy.setPhone(cursor.getString(cursor.getColumnIndex("Phone")));
            result.add(pharmacy);
        }
        while(cursor.moveToNext());
    }
    return result;
}

public List<String> getName(){
    SQLiteDatabase db = getReadableDatabase();
    SQLiteQueryBuilder qd = new SQLiteQueryBuilder();

    String[] sqlSelect ={"Name"};
    String tableName = "Pharmacy_List";

    qd.setTables(tableName);
    Cursor cursor = qd.query(db,sqlSelect,null, null ,null , null , null);
    List<String> result = new ArrayList<>();
    if(cursor.moveToFirst())
    {
        do{
            result.add(cursor.getString(cursor.getColumnIndex("Name")));
        }
        while(cursor.moveToNext());
    }
    return result;
}

public List<Pharmacy> getPharmacyByCity(String city){
    SQLiteDatabase db = getReadableDatabase();
    SQLiteQueryBuilder qd = new SQLiteQueryBuilder();

    String[] sqlSelect ={"ID","Name","City","Phone"};
    String tableName = "Pharmacy_List";

    qd.setTables(tableName);
    Cursor cursor = qd.query(db,sqlSelect,"City = ?", new String[]{city},null , null , null);
    List<Pharmacy> result = new ArrayList<>();
    if(cursor.moveToFirst())
    {
        do{
            Pharmacy pharmacy = new Pharmacy();
            pharmacy.setID(cursor.getInt(cursor.getColumnIndex("ID")));
            pharmacy.setName(cursor.getString(cursor.getColumnIndex("Name")));
            pharmacy.setCity(cursor.getString(cursor.getColumnIndex("City")));
            pharmacy.setPhone(cursor.getString(cursor.getColumnIndex("Phone")));
            result.add(pharmacy);
        }
        while(cursor.moveToNext());
    }
    return result;
}

Answer 1

尝试使用 context = this; 在 Database.java 文件中调整“context”，如下所示:

private static final String DB_NAME = "pharmacyDemo.db";
private static final int DB_VER = 1;

public Database(Context context) {
super(context, DB_NAME, null, DB_VER);
context = this;
}