android - Activity 中的 NullPointerException

Question

我对 Android 有点陌生，正在尝试开发我的第一个生产应用。它是一个用户定义的数据存储应用程序，由一个列表 Activity 和一个添加对话框 Activity 以及一个数据库适配器组成。我所有的“CRUD”(创建、读取、更新、删除)功能似乎都运行良好。但是当我在数据尚未保存到数据库时更改添加对话框的方向时遇到以下错误。换句话说，当我将方向从纵向更改为横向或反之时，我的 onSavedInstanceState() 函数失败。

02-16 11:42:05.321: E/AndroidRuntime(347): java.lang.RuntimeException: 无法暂停 Activity {net.learn2develop.Homework/net.learn2develop.Homework.HomeworkEditActivity}: java.lang.NullPointerException

这是我的代码:

public class HomeworkEditActivity extends Activity{
          private Long mRowId;

       // SQLite Db Helper
            private HomeworkDbAdapter mDbHelper ;




// declarations for SQLitedatabase actions
private EditText mSubject;
private EditText mDueText;
private EditText mDescText;



private Button btnSave = null;

/** 
 * Called when the activity 
 *  is first created
 */ 
@Override
protected void onCreate(Bundle savedInstanceState){

    // called to display the reminder edit class
    super.onCreate(savedInstanceState);

    // open the database
    mDbHelper = new HomeworkDbAdapter(this);

    // set the content view by the xml layout
    setContentView(R.layout.homeworkedit);


    // initialize the save button
    btnSave     = (Button) findViewById(R.id.btnSave);

    // initiate the edit text fields
    mSubject = (EditText) findViewById(R.id.txtSubject);
    mDueText  = (EditText) findViewById(R.id.txtDueDate);
    mDescText = (EditText)  findViewById(R.id.txtDesc);


    mRowId = savedInstanceState != null
        ? savedInstanceState.getLong(HomeworkDbAdapter.KEY_ROWID)
        : null;

    // register listeners and set the text of the buttons to their default
    registerButtonListenersAndSetDefaultText();

}
private void setRowIdFromIntent(){
    if(mRowId == null){
        Bundle extras = getIntent().getExtras();
        mRowId = extras != null
        ? extras.getLong(HomeworkDbAdapter.KEY_ROWID)
        : null;
    }
}
@Override
protected void onPause(){

    mDbHelper.close();
    super.onPause();
}

@Override
protected void onResume(){
    super.onResume();
    mDbHelper.open();
    setRowIdFromIntent();

        populateFields();

}

private void registerButtonListenersAndSetDefaultText() {
    btnSave.setOnClickListener(new View.OnClickListener(){
        @Override
        public void onClick (View v){
            mSubject.getText();
            mDescText.getText();
            mDueText.getText();

            saveState();
            setResult(RESULT_OK);
            Toast.makeText(HomeworkEditActivity.this, 
                    getString(R.string.task_saved_message), 
                    Toast.LENGTH_SHORT).show();
            finish();                                               // return to list activity
        }   



            });

}

private void populateFields()  {
    if(mRowId != null){
        Cursor reminder = mDbHelper.fetchHomework(mRowId);
            startManagingCursor(reminder);
            mSubject.setText(reminder.getString(
                         reminder.getColumnIndexOrThrow(HomeworkDbAdapter.KEY_SUBJECT)));
            mDueText.setText(reminder.getString(
                    reminder.getColumnIndexOrThrow(HomeworkDbAdapter.KEY_DUE)));
            mDescText.setText(reminder.getString(
                    reminder.getColumnIndexOrThrow(HomeworkDbAdapter.KEY_DESC)));
    }
    if(mRowId == null){
        onCreate(null);
    }
}

@Override
protected void onSaveInstanceState(Bundle outState){

    try{
    outState.putLong(HomeworkDbAdapter.KEY_ROWID, mRowId);
    }catch(NullPointerException npe){
        npe.printStackTrace();
    }
    super.onSaveInstanceState(outState);
}



private void saveState() {
    // add the save of the text reference here
    String subject = mSubject.getText().toString();
    String due_date = mDueText.getText().toString();
    String desc  = mDescText.getText().toString();



    if(mRowId == null){ 
        long id = mDbHelper.createHomework(subject, due_date, desc);
        if(id > 0){
            mRowId = id;
        }
    }else{
        mDbHelper.updateHomework(mRowId, subject, due_date, desc);
    }
}

Answer 1

NullPointerException 消息包含“无法暂停 Activity ”这一事实意味着问题出在您的 onPause(..) 覆盖上。我假设当 mDbHelper 为 null 时会发生这种情况，尽管查看您的代码，但我不明白为什么会这样。