嗨，我是Android的新手并原谅我，因为这是我第一次为我的学校作业开发移动应用程序。我有一个错误，当我尝试启动应用程序时，我的应用程序崩溃了，我的猜测是检索是罪魁祸首。
我正在创建一个活动，该活动应该显示一个空列表，允许用户输入CRUD。我确实理解错误是由于null，但我如何使它允许显示空列表？对于如何解决这个问题，我将不胜感激，谢谢！
这是logcat：
02-11 18:41:53.803: W/dalvikvm(18334): threadid=1: thread exiting with uncaught exception (group=0x41763da0)
02-11 18:41:53.803: E/AndroidRuntime(18334): FATAL EXCEPTION: main
02-11 18:41:53.803: E/AndroidRuntime(18334): Process: com.example.sgrecipe, PID: 18334
02-11 18:41:53.803: E/AndroidRuntime(18334): java.lang.NullPointerException
02-11 18:41:53.803: E/AndroidRuntime(18334): at com.example.sgrecipe.FavouriteFragment.fetchData(FavouriteFragment.java:174)
02-11 18:41:53.803: E/AndroidRuntime(18334): at com.example.sgrecipe.FavouriteFragment.initControls(FavouriteFragment.java:92)
02-11 18:41:53.803: E/AndroidRuntime(18334): at com.example.sgrecipe.FavouriteFragment.onStart(FavouriteFragment.java:50)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.support.v4.app.Fragment.performStart(Fragment.java:1484)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.support.v4.app.FragmentManagerImpl.moveToState(FragmentManager.java:941)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.support.v4.app.FragmentManagerImpl.performPendingDeferredStart(FragmentManager.java:807)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.support.v4.app.FragmentManagerImpl.startPendingDeferredFragments(FragmentManager.java:1112)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.support.v4.app.FragmentManagerImpl.execPendingActions(FragmentManager.java:1461)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.support.v4.app.FragmentManagerImpl.executePendingTransactions(FragmentManager.java:461)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.support.v4.app.FragmentPagerAdapter.finishUpdate(FragmentPagerAdapter.java:141)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.support.v4.view.ViewPager.populate(ViewPager.java:1064)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.support.v4.view.ViewPager.populate(ViewPager.java:911)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.support.v4.view.ViewPager.onMeasure(ViewPager.java:1432)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.View.measure(View.java:17396)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5365)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.View.measure(View.java:17396)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5365)
02-11 18:41:53.803: E/AndroidRuntime(18334): at com.android.internal.widget.ActionBarOverlayLayout.onMeasure(ActionBarOverlayLayout.java:382)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.View.measure(View.java:17396)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.ViewGroup.measureChildWithMargins(ViewGroup.java:5365)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.widget.FrameLayout.onMeasure(FrameLayout.java:310)
02-11 18:41:53.803: E/AndroidRuntime(18334): at com.android.internal.policy.impl.PhoneWindow$DecorView.onMeasure(PhoneWindow.java:2505)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.View.measure(View.java:17396)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.ViewRootImpl.performMeasure(ViewRootImpl.java:2175)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.ViewRootImpl.measureHierarchy(ViewRootImpl.java:1316)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:1513)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1200)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:6401)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.Choreographer$CallbackRecord.run(Choreographer.java:803)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.Choreographer.doCallbacks(Choreographer.java:603)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.Choreographer.doFrame(Choreographer.java:573)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:789)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.os.Handler.handleCallback(Handler.java:733)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.os.Handler.dispatchMessage(Handler.java:95)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.os.Looper.loop(Looper.java:157)
02-11 18:41:53.803: E/AndroidRuntime(18334): at android.app.ActivityThread.main(ActivityThread.java:5335)
02-11 18:41:53.803: E/AndroidRuntime(18334): at java.lang.reflect.Method.invokeNative(Native Method)
02-11 18:41:53.803: E/AndroidRuntime(18334): at java.lang.reflect.Method.invoke(Method.java:515)
02-11 18:41:53.803: E/AndroidRuntime(18334): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1265)
02-11 18:41:53.803: E/AndroidRuntime(18334): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1081)
02-11 18:41:53.803: E/AndroidRuntime(18334): at dalvik.system.NativeStart.main(Native Method)
FavouriteFragment.java：
package com.example.sgrecipe;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.view.View.OnClickListener;
import android.content.ContentValues;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import android.widget.Toast;
public class FavouriteFragment extends Fragment implements OnClickListener{
// Primitive Variables
String selected_ID = "";
// Widget GUI Declare
EditText txtTitle, txtDesc, txtSalary;
Button btnAdd, btnUpdate, btnDelete;
ListView lvNotes;
// DB Objects
DBHelper helper;
SQLiteDatabase db;
// Adapter Object
SimpleCursorAdapter adapter;
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.activity_favourite_fragment, container, false);
return rootView;
}
@Override
public void onStart() {
super.onStart();
initControls();
// Init DB Objects
helper = new DBHelper(getActivity());}
// Widget GUI Init
private void initControls(){
txtTitle = (EditText)getView().findViewById(R.id.txtTitle);
txtDesc = (EditText)getView().findViewById(R.id.txtDes);
txtSalary = (EditText)getView().findViewById(R.id.txtSalary);
lvNotes = (ListView)getView().findViewById(R.id.lvNotes);
btnAdd = (Button)getView().findViewById(R.id.btnAdd);
btnUpdate = (Button)getView().findViewById(R.id.btnUpdate);
btnDelete = (Button)getView().findViewById(R.id.btnDelete);
// Attached Listener
btnAdd.setOnClickListener(this);
btnUpdate.setOnClickListener(this);
btnDelete.setOnClickListener(this);
lvNotes.setOnItemClickListener(new OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapter, View v,
int position, long id) {
String title, desc, salary;
// Display Selected Row of Listview into EditText widget
Cursor row = (Cursor) adapter.getItemAtPosition(position);
selected_ID = row.getString(0);
title = row.getString(1);
desc = row.getString(2);
salary = row.getString(3);
txtTitle.setText(title);
txtDesc.setText(desc);
txtSalary.setText(salary);
}
});
// Fetch Data from database
fetchData();
}
@Override
public void onClick(View v) {
// Perform CRUD Operation
if (v == btnAdd) {
// Add Record with help of ContentValues and DBHelper class object
ContentValues values = new ContentValues();
values.put(DBHelper.C_TITLE, txtTitle.getText().toString());
values.put(DBHelper.C_DESCRIPTION, txtDesc.getText().toString());
values.put(DBHelper.C_SALARY, txtSalary.getText().toString());
// Call insert method of SQLiteDatabase Class and close after
// performing task
db = helper.getWritableDatabase();
db.insert(DBHelper.TABLE, null, values);
db.close();
clearFields();
Toast.makeText(this.getActivity(), "Successfully Added",
Toast.LENGTH_LONG).show();
// Fetch Data from database and display into listview
fetchData();
}
if (v == btnUpdate) {
// Update Record with help of ContentValues and DBHelper class
// object
ContentValues values = new ContentValues();
values.put(DBHelper.C_TITLE, txtTitle.getText().toString());
values.put(DBHelper.C_DESCRIPTION, txtDesc.getText().toString());
values.put(DBHelper.C_SALARY, txtSalary.getText().toString());
// Call update method of SQLiteDatabase Class and close after
// performing task
db = helper.getWritableDatabase();
db.update(DBHelper.TABLE, values, DBHelper.C_ID + "=?",
new String[] { selected_ID });
db.close();
// Fetch Data from database and display into listview
fetchData();
Toast.makeText(this.getActivity(), "Record Updated Successfully",
Toast.LENGTH_LONG).show();
clearFields();
}
if (v == btnDelete) {
// Call delete method of SQLiteDatabase Class to delete record and
// close after performing task
db = helper.getWritableDatabase();
db.delete(DBHelper.TABLE, DBHelper.C_ID + "=?",
new String[] { selected_ID });
db.close();
// Fetch Data from database and display into listview
fetchData();
Toast.makeText(this.getActivity(), "Record Deleted Successfully",
Toast.LENGTH_LONG).show();
clearFields();
}
}
// Clear Fields
private void clearFields() {
txtTitle.setText("");
txtDesc.setText("");
txtSalary.setText("");
}
// Fetch Fresh data from database and display into listview
private void fetchData() {
db = helper.getReadableDatabase();
Cursor c = db.query(DBHelper.TABLE, null, null, null, null, null, null);
adapter = new SimpleCursorAdapter(
getActivity(),
R.layout.row,
c,
new String[] { DBHelper.C_TITLE, DBHelper.C_SALARY,
DBHelper.C_DESCRIPTION },
new int[] { R.id.lblTitle, R.id.lblSalary, R.id.lblDescription });
lvNotes.setAdapter(adapter);
}
}
activity_favourite_fragment.xml：
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:focusable="true"
android:focusableInTouchMode="true"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<TextView
android:id="@+id/textView1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_gravity="center"
android:text="Notes"
android:textAppearance="?android:attr/textAppearanceLarge" >
</TextView>
<EditText
android:id="@+id/txtTitle"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="Title" >
<requestFocus>
</requestFocus>
</EditText>
<EditText
android:id="@+id/txtDes"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="Description" >
</EditText>
<EditText
android:id="@+id/txtSalary"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:hint="Enter Salary" >
</EditText>
<LinearLayout
android:id="@+id/linearLayout1"
android:layout_width="match_parent"
android:layout_height="wrap_content" >
<Button
android:id="@+id/btnAdd"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Add" >
</Button>
<Button
android:id="@+id/btnUpdate"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Update Record" >
</Button>
<Button
android:id="@+id/btnDelete"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="Delete" >
</Button>
</LinearLayout>
<ListView
android:id="@+id/lvNotes"
android:layout_width="match_parent"
android:layout_height="match_parent" >
</ListView>
我刚刚开始使用android开发，任何帮助都会受到赞赏！
答案 0 :(得分：3)
helper未初始化。
在您的onStart()中，您只是在致电initControls()并调用使用fetchData()的{{1}}后对其进行初始化。在helper来电之前移动helper初始化。
答案 1 :(得分：3)
您的问题出在onStart方法：
public void onStart()
{
super.onStart();
initControls();
// Init DB Objects
helper = new DBHelper(getActivity());
}
请注意，您在调用initControls()之后创建了帮助程序对象，然后调用fetchData()。
在fetchData中你有这一行：
db = helper.getWritableDatabase();
这是导致空指针异常的行，因为helper为null。
更改onStart方法，如下所示：
public void onStart()
{
super.onStart();
// Init DB Objects
helper = new DBHelper(getActivity());
initControls();
}
这可以解决您的问题。
答案 2 :(得分：0)
更好的解决方案，移动代码行：
helper = new DBHelper(getActivity());
从onStart()到onCreate()，这里是发起对象的正确位置