Question

这是databaseAdapter中的代码，用于从DB获取值：

public ArrayList<TaskClass> getAllTaskCategoryWise(String categoryName)
{
ArrayList<TaskClass> getTaskListofSelectedCategory = new ArrayList<TaskClass>();
Log.d("query print:","");
Cursor cursor = db.rawQuery("SELECT * FROM " + tb_Name_Task + " WHERE " + KEY_TASKCATEGORY + " = '"+ categoryName.trim()+ "'"+"" + " ORDER BY " + KEY_TASKDATE +"" + " ASC",null);
//Cursor cursor = db.rawQuery("SELECT * FROM " + tb_Name_Task,null);
if(cursor!= null)
{
    Log.d("cursor info:","cursor values:"+cursor);
    cursor.moveToFirst();
    while(!cursor.isAfterLast())
       {
       TaskClass taskClassInstance1 = new TaskClass();
       taskClassInstance1.setTask_id(cursor.getInt(0));
       taskClassInstance1.setTitle(cursor.getString(1));
       taskClassInstance1.setDescription(cursor.getString(2));
       taskClassInstance1.setTaskCategory(cursor.getString(3));
       taskClassInstance1.setTaskDate(cursor.getString(4));
       taskClassInstance1.setStatus(cursor.getInt(5));
     // Adding task class instance to list
       getTaskListofSelectedCategory.add(taskClassInstance1);
       Log.d("task list:","values:+ taskClassInstance.setTitle(cursor.getString(1))");
       cursor.moveToNext();
     }
}
else
{
    Log.d("values of getAllTaskCatgeory","cursor null");
}

Log.d("retrieving all task for selected category","getAllTaskCategoryWise function"+ cursor);
cursor.close();
return getTaskListofSelectedCategory;
}

在函数内调用此函数并返回我的空指针异常，我也使用自定义arrayadapter：custom array adapter code

public void getAllTaskForSelectedCategory(String categorySelection)
{
    Log.d("message for function call","getAllTaskForSelectedCategory function called:"+categorySelection);
    try
    {dataBaseAdapterInstance1  = dataBaseAdapterInstance1.openToRead();

    taskClassListinstance = dataBaseAdapterInstance1.getAllTaskCategoryWise(categorySelection);
    Log.d("LISTVALUES OF TASK TABLE:","VALUES:" + taskClassListinstance);


    listViewInstance.setAdapter(new CustomListAdapter(getApplicationContext(),taskClassListinstance));
    new CustomListAdapter(this,taskClassListinstance).notifyDataSetChanged();
    listViewInstance.setOnItemClickListener(new OnItemClickListener() {

      public void onItemClick(AdapterView<?> parent,View view,int position,long id)
      {
        TaskClass classinstance = (TaskClass) taskClassListinstance.get(position);
        Toast.makeText(getApplicationContext(),"values:" + classinstance,Toast.LENGTH_SHORT).show();
        Toast.makeText(getApplicationContext(),"id values:"+classinstance.getTask_id(),Toast.LENGTH_SHORT).show();

      }

    });
    dataBaseAdapterInstance1.close();
    }
    catch (Exception e) {
        // TODO: handle exception
        Log.d("exception in  getAllTaskForSelectedCategory","error");
        e.printStackTrace();
    }
}

这个“ taskClassListinstance = dataBaseAdapterInstance1.getAllTaskCategoryWise（categorySelection）; ”返回null，这里是logcat：

 06-01 16:39:11.375: E/AndroidRuntime(14691): FATAL EXCEPTION: main
06-01 16:39:11.375: E/AndroidRuntime(14691): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.todolist/com.example.todolist.TaskListCategoryWise}: java.lang.NullPointerException
06-01 16:39:11.375: E/AndroidRuntime(14691):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2180)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2230)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at android.app.ActivityThread.access$600(ActivityThread.java:141)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at android.os.Handler.dispatchMessage(Handler.java:99)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at android.os.Looper.loop(Looper.java:137)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at android.app.ActivityThread.main(ActivityThread.java:5041)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at java.lang.reflect.Method.invokeNative(Native Method)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at java.lang.reflect.Method.invoke(Method.java:511)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at dalvik.system.NativeStart.main(Native Method)
06-01 16:39:11.375: E/AndroidRuntime(14691): Caused by: java.lang.NullPointerException
06-01 16:39:11.375: E/AndroidRuntime(14691):    at com.example.todolist.TaskListCategoryWise.onCreate(TaskListCategoryWise.java:80)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at android.app.Activity.performCreate(Activity.java:5104)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1080)
06-01 16:39:11.375: E/AndroidRuntime(14691):    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2144)
06-01 16:39:11.375: E/AndroidRuntime(14691):    ... 11 more

taskListCategoryWise.java完整代码：

 public class TaskListCategoryWise extends Activity
{  String selectedValueofList;
   DatabaseAdapter dataBaseAdapterInstance1;
   ListView listViewInstance;
   CustomListAdapter instanceCustomAdapter;
   ArrayList<TaskClass> taskClassListinstance;
   CheckBox checkBoxInstance;
    @Override
    public void onCreate(Bundle savedInstanceState)

    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.tasklist_categorywise);
        dataBaseAdapterInstance1 = new DatabaseAdapter(this);
        Button refreshTaskButton = (Button)findViewById(R.id.refreshBtnForTask);
        listViewInstance = (ListView)findViewById(R.id.listViewForTask);
        checkBoxInstance = (CheckBox)findViewById(R.id.checkboxForStatus);

        try
         {
         Intent intent1 = getIntent();
         Log.d("value of intent:","intent values"+intent1);
         selectedValueofList = intent1.getExtras().getString("selectedItem");
         Log.d("retrieving values of selected listItem:","selected value of listitem:"+selectedValueofList);


         }
        catch (Exception e) {
            // TODO: handle exception 
             Log.d("esception of taskLISt","Exception in retrieving vlaues of previous activity");
             e.printStackTrace();
         }
         getAllTaskForSelectedCategory(selectedValueofList);

        refreshTaskButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                getAllTaskForSelectedCategory(selectedValueofList); 
            }
        });

        Button addTaskButton = (Button) findViewById(R.id.addBtnForTask);
        addTaskButton.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub

                Intent openTaskList = new Intent(TaskListCategoryWise.this,TaskList.class);
                openTaskList.putExtra("ListitemValues",selectedValueofList);
                startActivityForResult(openTaskList,0);

            }
        });


        checkBoxInstance.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                if ((checkBoxInstance).isChecked()) {
                    Toast.makeText(getApplicationContext(),
                       "Bro, try Android :)", Toast.LENGTH_LONG).show();
                }
            }
        });
      }
    public void getAllTaskForSelectedCategory(String categorySelection)
    {
        Log.d("message for function call","getAllTaskForSelectedCategory function called:"+categorySelection);
        try
        {dataBaseAdapterInstance1  = dataBaseAdapterInstance1.openToRead();

        taskClassListinstance = dataBaseAdapterInstance1.getAllTaskCategoryWise(categorySelection);
        Log.d("LISTVALUES OF TASK TABLE:","VALUES:" + taskClassListinstance);


        listViewInstance.setAdapter(new CustomListAdapter(getApplicationContext(),taskClassListinstance));
        new CustomListAdapter(this,taskClassListinstance).notifyDataSetChanged();
        listViewInstance.setOnItemClickListener(new OnItemClickListener() {

          public void onItemClick(AdapterView<?> parent,View view,int position,long id)
          {
            TaskClass classinstance = (TaskClass) taskClassListinstance.get(position);
            Toast.makeText(getApplicationContext(),"values:" + classinstance,Toast.LENGTH_SHORT).show();
            Toast.makeText(getApplicationContext(),"id values:"+classinstance.getTask_id(),Toast.LENGTH_SHORT).show();}

        });
        dataBaseAdapterInstance1.close();
    }
        catch (Exception e) {
            // TODO: handle exception
            Log.d("exception in  getAllTaskForSelectedCategory","error");
            e.printStackTrace();
        }
    }



}

Answer 1

您必须初始化checkBoxInstance

checkBoxInstance = (CheckBox) findViewById(R.id.your-checkbox-id-from-xml)

和你的xml

<CheckBox 
   android:id = "@+id/id-here"
   . other properties here
   .
   .
/>

在android中从数据库中检索值时获取“NULL POINTER EXCEPTION”？

1 个答案: