数据库错误:"NullPointerException:尝试在空对象引用上调用虚拟方法getApplicationInfo()..."

问题描述

我正在构建一个简单的多活动列表应用程序以与SQLite一起练习.我试图从数据库中获取字符串，以在启动时显示在 listview 中.我遇到了错误

I'm building a simple multi-activity list app to practice with SQLite. I was trying to get the strings from the database to display in the listview when this started. I'm getting the error

代码在这里

public class MainActivity extends AppCompatActivity {
    SharedPreferences sharedPreferences;
    ArrayList<String> listItem;
    ArrayAdapter adapter;
    ListView lv = (ListView) findViewById(R.id.list_view);
    DatabaseHelper db = new DatabaseHelper(this);
    Context context;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        listItem = new ArrayList<>();
        viewData();
        context = this;

        FloatingActionButton fab = findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                startActivity(new Intent(MainActivity.this, textInput.class));
            }
        });
    }

    // cast from sql database to the listview
    private void viewData() {
        Cursor res = db.viewData();

        if (res.getCount() == 0 ){
            Toast.makeText(this,"No Data!",Toast.LENGTH_SHORT).show();
        }
        else{
            while (res.moveToNext()){
                listItem.add(res.getString(1));
            }
            adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1,listItem);
            lv.setAdapter(adapter);
        }
    }
}

这是数据库助手:

public class DatabaseHelper extends SQLiteOpenHelper {
public static final String DATABASE_NAME = "item.db";
public static final String TABLE_NAME = "item_table";
public static final String ID = "item_id";
public static final String NAME = "item_name";
public static final String PRICE = "item_price";
public static final String URL = "item_url";

public DatabaseHelper(@Nullable Context context) {
    super(context, DATABASE_NAME, null, 1);
}

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL("create table " + TABLE_NAME +" ( item_id INTEGER PRIMARY
KEY AUTOINCREMENT,item_name TEXT,item_price INTEGER,item_url TEXT) ");
}

@Override
public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    db.execSQL("DROP TABLE IF EXISTS "+TABLE_NAME);
    onCreate(db);
}

public boolean insertData(String name, String price, String url){
    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(NAME, name);
    contentValues.put(PRICE, price);
    contentValues.put(URL, url);
    long result = db.insert(TABLE_NAME,null,contentValues);
    if(result == -1)
        return false;
    else
        return true;
}


public Cursor viewData(){
    SQLiteDatabase db  = this.getReadableDatabase();
    Cursor res = db.rawQuery("select * from "+TABLE_NAME,null);
    return res;
}

这是textInput活动

This is the textInput Activity

public class textInput extends AppCompatActivity {
SharedPreferences sharedPreferences;
//    Editor editor;
//    Context _context;
//    int PRIVATE_MODE = 0;
//    private static final String PREF_NAME = "Items";
//    public static final String NAME_LIST = "name";


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_text_input);
    //DatabaseHelper db;

    //return's to main activity on press
    Button addButton = (Button) findViewById(R.id.addButton);
    final EditText name = (EditText) findViewById(R.id.itemName);
    final EditText url = (EditText) findViewById(R.id.itemURL);
    final EditText price = (EditText) findViewById(R.id.itemPrice);
 //   final SharedPreferences sharedPreferences =
getApplicationContext().getSharedPreferences("items", 0);
    final DatabaseHelper db = new DatabaseHelper(this);
    addButton.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            //Checks to make sure that all fields are filled out, if yes
then returns to MainActivity
            if (name.length() != 0 && price.length() != 0 && url.length()
 !=0){
                boolean isInserted =
db.insertData(name.getText().toString(),
                        price.getText().toString(),
                        url.getText().toString());
                if(isInserted == true)
                    Toast.makeText(textInput.this, "New Item Added",
 Toast.LENGTH_SHORT).show();
                else
                    Toast.makeText(textInput.this, "Failed to Add",
Toast.LENGTH_SHORT).show();
                //The graphic and the return to MainActivity
                Intent returnToMain = new Intent(getApplicationContext(),
MainActivity.class);
                startActivity(returnToMain);
            }
            else{
                Toast.makeText(textInput.this, "Please fill out all
fields!", Toast.LENGTH_SHORT).show();
            }

        }
    });

调试(错误?)日志

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.xmaswishlist, PID: 25470
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.xmaswishlist/com.example.xmaswishlist.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:2841)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032)
        at android.app.ActivityThread.-wrap11(Unknown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
        at android.os.Handler.dispatchMessage(Handler.java:105)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6944)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
     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:163)
        at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:157)
        at android.content.Context.obtainStyledAttributes(Context.java:655)
        at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:692)
        at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:659)
        at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:479)
        at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:214)
        at com.example.xmaswishlist.MainActivity.<init>(MainActivity.java:36)
        at java.lang.Class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1180)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2831)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032) 
        at android.app.ActivityThread.-wrap11(Unknown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696) 
        at android.os.Handler.dispatchMessage(Handler.java:105) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6944) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)

我尝试使用以下方法解决此问题:

I have tried to fix this using the following:

1. Android Studio尝试在空对象引用上调用虚拟方法android.database.Cursor"问题
2. 数据库错误:尝试在空对象引用上调用虚拟方法'Cursor .getScene()'
3. 'android.content.pm.ApplicationInfo android.content.Context通知Android通知时在空对象引用上添加.getApplicationInfo()"

推荐答案

您尝试在设置 setContentView 中的内容之前获取视图，这会导致 exception .在 onCreate 内实例化 ListView 和 DatabaseHelper .

You try to get view before setting the content in setContentView, which cause the exception. Instantiate ListView and DatabaseHelper inside onCreate.

ListView lv;
DatabaseHelper db;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);

    ...

    lv = (ListView) findViewById(R.id.list_view);
    db = new DatabaseHelper(this);
    ...
}

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