Question

我正在用SQLite制作一个简单的笔记应用程序。它有两个活动和一个SQLite的帮助器类。但是，当我在MainActivity.java中单击示例注释（Akhilesh Chobey）并尝试在Main2Activity.java中编辑它时，应用程序崩溃。

主要活动：

    public class MainActivity extends AppCompatActivity {



    ListView notesListView;
    static ArrayList<String> notesArrayList;
    ArrayAdapter<String> adapter;

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

        notesListView = (ListView) findViewById(R.id.notesListView);
        notesArrayList = new ArrayList<String>();
        notesArrayList.add("Akhilesh Chobey");
        adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, notesArrayList);
        notesListView.setAdapter(adapter);

        notesListView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> adapterView, View view, int i, long l) {
                Intent intent = new Intent(getApplicationContext(), Main2Activity.class);
                intent.putExtra("notePosition", i);
                startActivity(intent);
            }
        });



    }
}

Main2Activity：

public class Main2Activity extends AppCompatActivity implements TextWatcher {

DatabaseOperations myDB;
EditText editNote;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main2);
    editNote = (EditText) findViewById(R.id.noteEditText);

    myDb = new DatabaseOperations(Main2Activity.this);

    Intent intent = getIntent();
    int position = intent.getIntExtra("notePosition", -1);
    if(position != -1){

        editNote.setText(MainActivity.notesArrayList.get(position));

    }
    editNote.addTextChangedListener(this);
}
@Override
public void onTextChanged(CharSequence charSequence, int i, int i1, int i2) {
    if (myDB == null){ 
    boolean isInserted = myDB.insertData(editNote.getText().toString());
        if(isInserted){
            Toast.makeText(getApplicationContext(), "Inserted", Toast.LENGTH_SHORT).show();
        }else {
            Toast.makeText(getApplicationContext(), "Error", Toast.LENGTH_SHORT).show();
    }
}
}

助手班级：

public class DatabaseOperations extends SQLiteOpenHelper {

public static final String DatabaseName = "notes.db";
public static final String TableName = "notes";
public static final String Col1 = "text";

public DatabaseOperations(Context context) {
    super(context, DatabaseName, null, 1);
}

@Override
public void onCreate(SQLiteDatabase db) {
    db.execSQL("create table " + TableName + " (text TEXT PRIMARY KEY AUTOINCREMENT) ");
}

@Override
public void onUpgrade(SQLiteDatabase db, int i, int i1) {

    db.execSQL("DROP TABLE IF EXISTS " + TableName);
    onCreate(db);

}

public boolean insertData(String note){

    SQLiteDatabase db = this.getWritableDatabase();
    ContentValues contentValues = new ContentValues();
    contentValues.put(Col1, note);

    long result = db.insert(TableName, null, contentValues);
    if(result == -1){
        return false;
    }else {
        return true;
    }

}

}

日志：

06-29 17:23:34.996 2231-2231/com.akhileshchobey.mynotes E/AndroidRuntime: FATAL EXCEPTION: main
                                                                      Process: com.akhileshchobey.mynotes, PID: 2231
                                                                      java.lang.NullPointerException: Attempt to invoke virtual method 'boolean com.akhileshchobey.mynotes.DatabaseOperations.insertData(java.lang.String)' on a null object reference
                                                                          at com.akhileshchobey.mynotes.Main2Activity.onTextChanged(Main2Activity.java:39)
                                                                          at android.widget.TextView.sendOnTextChanged(TextView.java:7746)
                                                                          at android.widget.TextView.handleTextChanged(TextView.java:7806)
                                                                          at android.widget.TextView$ChangeWatcher.onTextChanged(TextView.java:9724)
                                                                          at android.text.SpannableStringBuilder.sendTextChanged(SpannableStringBuilder.java:964)
                                                                          at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:515)
                                                                          at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:454)
                                                                          at android.text.SpannableStringBuilder.replace(SpannableStringBuilder.java:33)
                                                                          at android.view.inputmethod.BaseInputConnection.replaceText(BaseInputConnection.java:685)
                                                                          at android.view.inputmethod.BaseInputConnection.setComposingText(BaseInputConnection.java:445)
                                                                          at com.android.internal.view.IInputConnectionWrapper.executeMessage(IInputConnectionWrapper.java:340)
                                                                          at com.android.internal.view.IInputConnectionWrapper$MyHandler.handleMessage(IInputConnectionWrapper.java:78)
                                                                          at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                          at android.os.Looper.loop(Looper.java:135)
                                                                          at android.app.ActivityThread.main(ActivityThread.java:5264)
                                                                          at java.lang.reflect.Method.invoke(Native Method)
                                                                          at java.lang.reflect.Method.invoke(Method.java:372)
                                                                          at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:900)
                                                                          at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:695)

Answer 1

不要初始化myDb，

在您onCreate()的{{1}}

中初始化Main2Activity

myDb = new DatabaseOperations(Main2Activity.this);

修改

您现在已经初始化了myDb，它不会为空。因此，您必须在onTextChanged()方法中进行如下更改。

if (myDB == null)

到

if (myDB != null)

Answer 2

使用安全空检查更改行boolean isInserted = myDB.insertData(editNote.getText().toString());，如下所示。

if (myDB == null)
   myDB = new DatabaseOperations(getApplicationContext() ); //Initialization
boolean isInserted = myDB.insertData(editNote.getText().toString());

Answer 3

您尚未初始化DatabaseOperations myDB = null;

以下行必须给出空指针异常;

boolean isInserted = myDB.insertData(editNote.getText().toString());

因为您尚未初始化myDB

Answer 4

myDB未初始化。您需要在使用前初始化它。在null对象上调用insertData是您的应用崩溃的原因。

应用程序在onTextChanged（）期间崩溃

4 个答案: