getWritableDatabase（）导致nullPointerException

Question

I want to open my database with SQLite. But when attempting to take data from a TextInputEditText and put it into the database it throws NPE at db = this.getWritableDatabase();

Seems that most of the time this is caused by the context being null. But it is not null and I checked it with debugger. ( myDB = new DatabaseHelper(this); )

(I have tried all other questions about this problem and my situation must be different because they did not help.)

MainActivity.java

public class MainActivity extends AppCompatActivity {
    DatabaseHelper myDB;
    TextInputEditText inputEmail;
    TextInputEditText inputPass;
    Button btn_login;
    Button btn_signup;

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

        try {
            myDB = new DatabaseHelper(this);
        } catch (Exception e) {
            System.out.print("Exception occurred");
        }
        btn_signup = findViewById(R.id.btn_signup);
        btn_login = findViewById(R.id.btn_login);
        addData();
    }

    public void addData() {
        btn_signup.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                inputEmail = findViewById(R.id.editTextEmail);
                inputPass = findViewById(R.id.editTextPass);
                //try {
                myDB.insertData(inputEmail.getText().toString(), 
                inputPass.getText().toString());

                Toast.makeText(MainActivity.this, "Data Inserted", 
                Toast.LENGTH_LONG).show();

               // } catch (Exception e) {
               //     e.printStackTrace();
               // }
            }
        });
    }

DatabaseHelper.java

public class DatabaseHelper extends SQLiteOpenHelper {

    public static final String DATABASE_NAME = "Login_android.db";
    public static final String TABLE_NAME = "login_table.db";
    public static final String COL1 = "ID";
    public static final String COL2 = "EMAIL";
    public static final String COL3 = "PASSWORD";


    public DatabaseHelper(Context context) {
        super(context, DATABASE_NAME, null, 1);
        //SQLiteDatabase db = this.getWritableDatabase();
    }

    @Override
    public void onCreate(SQLiteDatabase db) {
        db.execSQL("CREATE TABLE " + TABLE_NAME +" (ID INTEGER PRIMARY KEY 
        AUTOINCREMENT, EMAIL TEXT, PASSWORD 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 email, String password) {

        SQLiteDatabase db = null;
        try {
            db = this.getWritableDatabase();
        } catch (Exception e) {
            e.printStackTrace();
        }
        //db.isOpen();
        ContentValues contentValues = new ContentValues();
        contentValues.put(COL2, email);
        contentValues.put(COL3, password);
        long result = db.insert(TABLE_NAME, null, contentValues);
        if (result == -1) {
            return false;
        } else {
            return true;
        }
    }
}

Exception

12-06 23:39:12.799 26304-26304/com.example.ryan.temporaryname E/AndroidRuntime: FATAL EXCEPTION: main Process: com.example.ryan.temporaryname, PID: 26304 java.lang.NullPointerException: Attempt to invoke virtual method 'long android.database.sqlite.SQLiteDatabase.insert(java.lang.String, java.lang.String, android.content.ContentValues)' on a null object reference at com.example.ryan.temporaryname.DatabaseHelper.insertData(DatabaseHelper.java:45) at com.example.ryan.temporaryname.MainActivity$1.onClick(MainActivity.java:64) at android.view.View.performClick(View.java:6294) at android.view.View$PerformClick.run(View.java:24770) at android.os.Handler.handleCallback(Handler.java:790) at android.os.Handler.dispatchMessage(Handler.java:99) at android.os.Looper.loop(Looper.java:164) at android.app.ActivityThread.main(ActivityThread.java:6494) at java.lang.reflect.Method.invoke(Native Method) at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) at com.android.in ternal.os.ZygoteInit.main(ZygoteInit.java:807)

Answer 1

我认为你应该（如果这不是一个测试sqlite应用程序）通过创建一个App（）类来创建一个单例并在那里初始化数据库。尝试也解决你的问题

Answer 2

I think the problem is about TABLE_NAME - "login_table.db" suggests a table named "db" in a database named "login_table", which does not exist, and a null database object is returned.

Please try to change the name.