我正在开发一个允许用户将项目添加到列表（待办事项列表）的应用程序。用户可以选择项目将进入的列表。该应用程序使用选项卡式视图（Simular to Skype），我需要能够从数据库（SQLite）填充列表。
我的MainActivity.java：
import android.content.ContentValues;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.database.Cursor;
import android.speech.RecognizerIntent;
import android.support.design.widget.TabLayout;
import android.support.design.widget.FloatingActionButton;
import android.support.v7.app.AlertDialog;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import static com.example.todo.Constants.*;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.os.Bundle;
import android.text.Editable;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ListView;
import android.widget.SimpleCursorAdapter;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import java.util.List;
public class MainActivity extends AppCompatActivity {
private static String[] FROM = { _ID, TODO_TEXT, LOCATION, };
private static int[] TO = { R.id.todoTextView };
private static String ORDER_BY = ORDER + " DESC";
ListView taskListView;
/**
* The {@link android.support.v4.view.PagerAdapter} that will provide
* fragments for each of the sections. We use a
* {@link FragmentPagerAdapter} derivative, which will keep every
* loaded fragment in memory. If this becomes too memory intensive, it
* may be best to switch to a
* {@link android.support.v4.app.FragmentStatePagerAdapter}.
*/
private SectionsPagerAdapter mSectionsPagerAdapter;
public Button speakButton;
/**
* The {@link ViewPager} that will host the section contents.
*/
private ViewPager mViewPager;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
// Create the adapter that will return a fragment for each of the three
// primary sections of the activity.
mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());
// Set up the ViewPager with the sections adapter.
mViewPager = (ViewPager) findViewById(R.id.container);
mViewPager.setAdapter(mSectionsPagerAdapter);
TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
tabLayout.setupWithViewPager(mViewPager);
Context context = this;
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab2);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
showAlert();
}
});
Cursor cursor = getEvents();
showEvents(cursor);
}
private void addEvent(String string) {
// Insert a new record into the Events data source.
// You would do something similar for delete and update.
ContentValues values = new ContentValues();
values.put(ORDER, System.currentTimeMillis());
values.put(REMINDER_TEXT, string);
getContentResolver().insert(CONTENT_URI, values);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
void showAlert(){
final EditText reminderinput = (EditText) findViewById(R.id.reminderText);
AlertDialog.Builder adb = new AlertDialog.Builder(MainActivity.this);
LayoutInflater inflater = MainActivity.this.getLayoutInflater();
adb.setView(inflater.inflate(R.layout.dialog_new_reminder, null))
.setPositiveButton(R.string.ok, new DialogInterface.OnClickListener(){
@Override
public void onClick(DialogInterface dialog, int id) {
// save reminder here
Editable reminderText = reminderinput.getText();
String reminderTextString = String.valueOf(reminderText);
addEvent(reminderTextString);
}
})
.setNegativeButton(R.string.cancel, new DialogInterface.OnClickListener() {
public void onClick(DialogInterface dialog, int id) {
//it already cancels
}
});
adb.setTitle("Set a reminder");
AlertDialog ad = adb.create();
ad.show();
}
private Cursor getEvents() {
// Perform a managed query. The Activity will handle closing
// and re-querying the cursor when needed.
return managedQuery(CONTENT_URI, FROM, null, null, ORDER_BY);
}
private void showEvents(Cursor cursor) {
// Set up data binding
ListView mListView = (ListView) findViewById(R.id.listview);
SimpleCursorAdapter adapter = new SimpleCursorAdapter(this,
R.layout.task_item, cursor, FROM, TO);
mListView.setAdapter(adapter);
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
Intent intent = new Intent(this, AppSettings.class);
startActivity(intent);
}
return super.onOptionsItemSelected(item);
}
/**
* A placeholder fragment containing a simple view.
*/
public static class PlaceholderFragment extends Fragment {
/**
* The fragment argument representing the section number for this
* fragment.
*/
private static final String ARG_SECTION_NUMBER = "section_number";
public PlaceholderFragment() {
}
/**
* Returns a new instance of this fragment for the given section
* number.
*/
public static PlaceholderFragment newInstance(int sectionNumber) {
PlaceholderFragment fragment = new PlaceholderFragment();
Bundle args = new Bundle();
args.putInt(ARG_SECTION_NUMBER, sectionNumber);
fragment.setArguments(args);
return fragment;
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container, false);
return rootView;
}
}
/**
* A {@link FragmentPagerAdapter} that returns a fragment corresponding to
* one of the sections/tabs/pages.
*/
public class SectionsPagerAdapter extends FragmentPagerAdapter {
public SectionsPagerAdapter(FragmentManager fm) {
super(fm);
}
@Override
public Fragment getItem(int position) {
// getItem is called to instantiate the fragment for the given page.
// Return a PlaceholderFragment (defined as a static inner class below).
return PlaceholderFragment.newInstance(position + 1);
}
@Override
public int getCount() {
// Show 3 total pages.
return 3;
}
@Override
public CharSequence getPageTitle(int position) {
switch (position) {
case 0:
return "Important";
case 1:
return "Kinda Important";
case 2:
return "Not very Important";
}
return null;
}
}
}
显然，我有一个Constants.java以及项目中的一些布局文件。 不幸的是，该应用程序抛出错误说：
Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
HELP！
答案 0 :(得分：0)
所以你的片段中有listviews，而你正在做
ListView mListView = (ListView) findViewById(R.id.listview);
这会在activity_main中找到listview而不是片段。 您应该填充片段中的列表视图。 在片段中调用它。
ListView mListView = rootview.findViewById(R.id.listview);
Cursor cursor = getEvents();
showEvents(cursor);