java - NullPointerException - 尝试调用虚拟方法 `notifyDataSetChanged()`

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 7 年前。

在标记此重复项之前，我确实查看了其他类似问题的解决方案，但它不起作用。仅当我第一次运行我的应用程序时，我才收到此错误。我不知道为什么会发生这种情况。我打电话MainActivity.adapter.notifyDataSetChanged在 Main2Activity (编辑文本 Activity )。安装后在 EditText Activity 中按后退按钮时应用程序崩溃，即 Main2Activity.java .

主要 Activity :

public class MainActivity extends AppCompatActivity {

ListView notesListView;
static ArrayList<String> notesArrayList;
static 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>();

        adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, notesArrayList);
        notesListView.setAdapter(adapter);
        Main2Activity.myDb = new DatabaseOperations(this);

    if (Main2Activity.myDb != null) {
        notesArrayList.clear();
        Cursor result = Main2Activity.myDb.getData();
        if (result.getCount() == 0) {
            Log.i("Error", "error");
            return;
        }
        while (result.moveToNext()) {
            notesArrayList.add(result.getString(result.getColumnIndex("text")));
        }
    }

    //notesArrayList.add("Akhilesh Chobey");      

    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);
        }
    });

    notesListView.setOnItemLongClickListener(new AdapterView.OnItemLongClickListener() {
        @Override
        public boolean onItemLongClick(AdapterView<?> adapterView, View view, int i, long l) {


            return false;
        }
    });
}

@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {

    int id = item.getItemId();
    if (id == R.id.add_button) {
        notesArrayList.add("");
    }
    Intent intent = new Intent(getApplicationContext(), Main2Activity.class);
    intent.putExtra("notePosition", notesArrayList.size() - 1);
    startActivity(intent);


    return true;

}
}

Main2Activity(EditText Activity ):

public class Main2Activity extends AppCompatActivity implements TextWatcher {

static DatabaseOperations myDb;
EditText editNote;
int position;

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

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

    }
    editNote.addTextChangedListener(this);
}

@Override
public void beforeTextChanged(CharSequence charSequence, int i, int i1, int i2) {

}

@Override
public void onTextChanged(CharSequence charSequence, int i, int i1, int i2) {
    if (myDb != null) {   

    }

}

@Override
public void afterTextChanged(Editable editable) {

}

@Override
public void onBackPressed(){
    myDb.insertData(editNote.getText().toString());
    MainActivity.notesArrayList.set(position, editNote.getText().toString());
    MainActivity.adapter.notifyDataSetChanged();
    this.finish();
}
}

日志:

07-02 18:45:40.402 17500-17500/com.akhileshchobey.mynotes E/MessageQueue-JNI: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ArrayAdapter.notifyDataSetChanged()' on a null object reference at com.akhileshchobey.mynotes.Main2Activity.onBackPressed(Main2Activity.java:70) at android.app.Activity.onKeyUp(Activity.java:2495) at android.view.KeyEvent.dispatch(KeyEvent.java:2695) at android.app.Activity.dispatchKeyEvent(Activity.java:2746) at android.support.v7.app.AppCompatActivity.dispatchKeyEvent(AppCompatActivity.java:533) at android.support.v7.view.WindowCallbackWrapper.dispatchKeyEvent(WindowCallbackWrapper.java:53) at android.support.v7.app.AppCompatDelegateImplBase$AppCompatWindowCallbackBase.dispatchKeyEvent(AppCompatDelegateImplBase.java:254) at com.android.internal.policy.impl.PhoneWindow$DecorView.dispatchKeyEvent(PhoneWindow.java:2221) at android.view.ViewRootImpl$ViewPostImeInputStage.processKeyEvent(ViewRootImpl.java:3918) at android.view.ViewRootImpl$ViewPostImeInputStage.onProcess(ViewRootImpl.java:3880) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3449) at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3502) at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3468) at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3578) at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3476) at android.view.ViewRootImpl$AsyncInputStage.apply(ViewRootImpl.java:3635) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3449) at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3502) at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3468) at android.view.ViewRootImpl$InputStage.apply(ViewRootImpl.java:3476) at android.view.ViewRootImpl$InputStage.deliver(ViewRootImpl.java:3449) at android.view.ViewRootImpl$InputStage.onDeliverToNext(ViewRootImpl.java:3502) at android.view.ViewRootImpl$InputStage.forward(ViewRootImpl.java:3468) at android.view.ViewRootImpl$AsyncInputStage.forward(ViewRootImpl.java:3611) at android.view.ViewRootImpl$ImeInputStage.onFinishedInputEvent(ViewRootImpl.java:3772) at android.view.inputmethod.InputMethodManager$PendingEvent.run(InputMethodManager.java:2244) at android.view.inputmethod.InputMethodManager.invokeFinishedInputEventCallback(InputMethodManager.java:1885) at android.view.inputmethod.InputMethodManager.finishedInputEvent(InputMethodManager.java:1876) at android.view.inputmethod.InputMethodManager$ImeInputEventSender.onInputEventFinished(InputMethodManager.java:2221) at android.view.InputEventSender.dispatchInputEventFinished(InputEventSender.java:141) at android.os.MessageQueue.nativePollOnce(Native Method) at android.os.MessageQueue.next(MessageQueue.java:143) at android.os.Looper.loop(Looper.java:122) at android.app.ActivityThread.main(ActivityThread.java:5264) at java.lang.reflect.Method.invoke(Native Method) at java.lang.reflect.Method.invoke(Method.java:372) at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:900) at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:695)

Answer 1

ArrayAdapter.notifyDataSetChanged() 已被调用，但 ArrayAdapter 实例指向空引用(未初始化或错误初始化...)

在你的情况下，看起来第二个选项是原因......

代码中有这样的内容:

adapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, notesArrayList);

但需要检查构造函数中的参数实例:这个布局中的资源:android.R.layout.simple_list_item_1 或列表:notesArrayList

其中至少有一个破坏了适配器的正确初始化，这意味着，在该线路适配器之后仍然是null

结果是:调用adapter.notifyDataSetChanged()会抛出NPE