我在一个单独的项目中创建了这个listView，一切正常。 当我试图把它带到主应用程序我开发时我遇到了这个错误（等等）：
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ListView.setAdapter(android.widget.ListAdapter)' on a null object reference
at com.example.tiago.newapponthego.MainActivity.initialVocabList(MainActivity.java:148)
at com.example.tiago.newapponthego.MainActivity.onCreate(MainActivity.java:110)
指向这一行：
vocabListView.setAdapter(itemsAdapter);
这是我的MainActivity：
public class MainActivity extends ActionBarActivity
implements NavigationDrawerFragment.NavigationDrawerCallbacks {
//initialize timer
private TextView timerTextView;
//from tutorial
private long startTime = 0L;
private Handler customHandler = new Handler();
long timeInMilliseconds = 0L;
long timeSwapBuff = 0L;
long updatedTime = 0L;
//initialize points
private TextView pointsTextView;
private boolean initialBonus = true;
private int totalPoints = 0;
//initialize vocab list
private ArrayList<String> vocabsList;
private ArrayAdapter<String> itemsAdapter;
private ListView vocabListView;
private String clickedVocab;
static Random randomBackground = new Random();
/**
* Fragment managing the behaviors, interactions and presentation of the navigation drawer.
*/
private NavigationDrawerFragment mNavigationDrawerFragment;
/**
* Used to store the last screen title. For use in {@link #restoreActionBar()}.
*/
private CharSequence mTitle;
public static int randomRgb(int min, int max) {
//Random randomBackground = new Random(); //already done in the main scope
// nextInt is normally exclusive of the top value,
// so add 1 to make it inclusive
int rgbReturn = randomBackground.nextInt((max - min) + 1) + min;
return rgbReturn;
}
@Override
protected void onPostCreate(Bundle savedInstanceState) {
super.onPostCreate(savedInstanceState);
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main); // this is the original one
//Retrieve values for Timer and points
SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
timeInMilliseconds = preferences.getLong("Time", 0);
totalPoints = preferences.getInt("Points", 0);
//starts timer and
timerTextView = (TextView) findViewById(R.id.timeTV);
startTime = SystemClock.uptimeMillis();
customHandler.postDelayed(updateTimerThread, 0);
timeSwapBuff += timeInMilliseconds;
//points
initialPoints();
//constructor vocabulary list
initialVocabList(); // <---- THE PROBLEM IS TRIGGERED HERE
//native initialization
mNavigationDrawerFragment = (NavigationDrawerFragment)
getSupportFragmentManager().findFragmentById(R.id.navigation_drawer);
mTitle = getTitle();
// Set up the drawer.
mNavigationDrawerFragment.setUp(
R.id.navigation_drawer,
(DrawerLayout) findViewById(R.id.drawer_layout));
}
public void initialVocabList() {
// ADD HERE
vocabListView = (ListView) findViewById(R.id.vocabList);
//vocabsList = new ArrayList<String>();
//retrieve from SharedPreferences
TinyDB tinydb = new TinyDB(getBaseContext());
vocabsList = tinydb.getListString("vocabs");
//itemsAdapter = new ArrayAdapter<String>(this, R.layout.vocab_notes_item, items);
itemsAdapter = new ArrayAdapter<String>(getApplicationContext(), R.layout.vocab_notes_item, vocabsList);
vocabListView.setAdapter(itemsAdapter); // <---- HERE IS THE PROBLEM
// Setup remove listener method call
setupListViewListener();
// Setup Onclick method call
setOnClickListener();
}
private void setupListViewListener() {
vocabListView.setOnItemLongClickListener(
new AdapterView.OnItemLongClickListener() {
@Override
public boolean onItemLongClick(AdapterView<?> adapter,
View item, int pos, long id) {
// Remove the item within array at position
/* createDelVocDialog();
if (confirmDelete){ //if boolean is true
items.remove(pos);
}*/
noteDeletedToast(vocabsList.get(pos));
vocabsList.remove(pos);
// Refresh the adapter
itemsAdapter.notifyDataSetChanged();
// Return true consumes the long click event (marks it handled)
return true;
}
});
}
private void setOnClickListener() {
vocabListView.setOnItemClickListener(
new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> adapter,
View view, int pos, long id) {
//System.out.println("entrei"); //anderson's test "dividir para conquistar"
Context context = view.getContext();
clickedVocab = vocabsList.get(pos);
Toast.makeText(context, "Item: " + clickedVocab, Toast.LENGTH_SHORT).show();
}
});
}
private void noteDeletedToast(String note) {
Toast.makeText(this, "Vocab note deleted: " + note, Toast.LENGTH_SHORT).show();
}
;
public void onAddVocab(View v) {
EditText vocabTextBox = (EditText) findViewById(R.id.edTextVocab);
if (vocabTextBox.getText().toString().equals("")) { //Check if the EditText is not empty
Toast.makeText(this, "You forgot to write in the box!", Toast.LENGTH_SHORT).show();
return;
} else {
String itemText = vocabTextBox.getText().toString();
itemsAdapter.add(itemText.trim()); //This also performs "trim" on text
vocabTextBox.setText("");
Toast.makeText(this, "Note added", Toast.LENGTH_SHORT).show();
//save to SharedPreferences
TinyDB tinydb = new TinyDB(getBaseContext());
tinydb.putListString("vocabs", vocabsList);
hideKeyboard();
}
}
private void hideKeyboard() {
// Check if no view has focus:
View view = this.getCurrentFocus();
if (view != null) {
InputMethodManager inputManager = (InputMethodManager) this.getSystemService(Context.INPUT_METHOD_SERVICE);
inputManager.hideSoftInputFromWindow(view.getWindowToken(), InputMethodManager.HIDE_NOT_ALWAYS);
}
}
public void initialPoints() {
pointsTextView = (TextView) findViewById(R.id.pointsTV);
pointsTextView.setText(String.valueOf(totalPoints));
}
@Override //native
public void onNavigationDrawerItemSelected(int position) {
Fragment objFragment = null;
switch (position) {
case 0:
objFragment = new home_fragment();
//findViewById(com.example.tiago.newapponthego.R.id.mainContainer).setBackgroundColor(Color.rgb(255, 255, 255));
break;
case 1:
objFragment = new proverbs_fragment();
findViewById(com.example.tiago.newapponthego.R.id.mainContainer).setBackgroundColor(Color.rgb(255, 255, 255));
break;
case 2:
objFragment = new jokes_fragment();
findViewById(com.example.tiago.newapponthego.R.id.mainContainer).setBackgroundColor(Color.rgb(255, 255, 255));
break;
case 3:
objFragment = new VocabularyFragment();
//findViewById(com.example.tiago.newapponthego.R.id.mainContainer).setBackgroundColor(Color.rgb(255, 255, 255));
break;
}
// update the main content by replacing fragments
FragmentManager fragmentManager = getSupportFragmentManager();
fragmentManager.beginTransaction()
.replace(R.id.mainContainer, objFragment)
.commit();
}
public void onSectionAttached(int number) {
switch (number) {
case 1:
mTitle = getString(R.string.nav1home);
break;
case 2:
mTitle = getString(R.string.nav2prov);
break;
case 3:
mTitle = getString(R.string.nav3joke);
break;
}
}
public void restoreActionBar() {
ActionBar actionBar = getSupportActionBar();
actionBar.setDisplayShowTitleEnabled(true);
actionBar.setTitle(mTitle);
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
if (!mNavigationDrawerFragment.isDrawerOpen()) {
// Only show items in the action bar relevant to this screen
// if the drawer is not showing. Otherwise, let the drawer
// decide what to show in the action bar.
getMenuInflater().inflate(R.menu.main_activity_navigation, menu);
restoreActionBar();
return true;
}
return super.onCreateOptionsMenu(menu);
}
@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) {
return true;
}
return super.onOptionsItemSelected(item);
}
public void randomJoke(View y) {
//String chosenJoke = new String();
randomBground();
Button button = (Button) y;
MyArrayLists.pickJoke();
String chosenJoke = MyArrayLists.jokeTxt.toString();
TextView theTextView = (TextView) findViewById(R.id.thisTextView);
theTextView.setText(chosenJoke);
addPoints(1);
// theTextView.setOnClickListener();
}
public void randomProv(View y) {
randomBground();
Button button = (Button) y;
MyArrayLists.pickProv();
String chosenProv = MyArrayLists.provTxt.toString();
TextView theTextView = (TextView) findViewById(R.id.thisTextView);
theTextView.setText(chosenProv);
addPoints(1);
}
public void randomBground() {
Integer R = randomRgb(225, 250);
Integer G = randomRgb(225, 250);
Integer B = randomRgb(225, 250);
findViewById(com.example.tiago.newapponthego.R.id.mainContainer).setBackgroundColor(Color.rgb(R, G, B));
}
public void whiteBackground() {
findViewById(com.example.tiago.newapponthego.R.id.mainContainer).setBackgroundColor(Color.rgb(255, 255, 255));
}
public void saveTime() {
SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = preferences.edit();
editor.putLong("Time", timeInMilliseconds);
editor.apply();
}
public void savePoints() {
SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(this);
SharedPreferences.Editor editor = preferences.edit();
editor.putInt("Points", totalPoints);
editor.apply();
}
/**
* 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;
}
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
((MainActivity) activity).onSectionAttached(
getArguments().getInt(ARG_SECTION_NUMBER));
}
}
public void addPoints(int pointsToAdd) {
totalPoints = totalPoints + pointsToAdd;
// run the mUpdateUITimerTask's run() method in 1000 milliseconds from now
mHandler.postDelayed(mUpdateUITimerTask, 100);
savePoints();
}
private final Runnable mUpdateUITimerTask = new Runnable() {
public void run() {
// do whatever you want to change here, like:
pointsTextView.setText(String.valueOf(totalPoints));
}
};
private final Handler mHandler = new Handler();
private Runnable updateTimerThread = new Runnable() {
public void run() {
timeInMilliseconds = SystemClock.uptimeMillis() - startTime;
updatedTime = timeSwapBuff + timeInMilliseconds;
int secs = (int) (updatedTime / 1000);
int mins = secs / 60;
//int upTime = (int) updatedTime;
if (secs == 120 && initialBonus == true) {
addPoints(10);
initialBonus = false;
}
secs = secs % 60;
//int milliseconds = (int) (updatedTime % 1000); // Show milliseconds
timerTextView.setText("" + mins + ":"
+ String.format("%02d", secs)
// + ":" + String.format("%03d", milliseconds) // Show milliseconds
);
customHandler.postDelayed(this, 0);
saveTime();
}
};
可能相关的片段：
public class VocabularyFragment extends Fragment {
View rootview;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
rootview = inflater.inflate(R.layout.fragment_vocab_list,container, false);
return rootview;
}
此片段布局：
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools" android:layout_width="match_parent"
android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/console_height"
android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".ListViewActivity">
<LinearLayout
android:orientation="horizontal"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:id="@+id/linearLayout"
android:weightSum="1">
<EditText
android:layout_width="258dp"
android:layout_height="wrap_content"
android:id="@+id/edTextVocab"
android:layout_weight="0.98"
android:paddingRight="5dp"
android:paddingTop="5dp"
android:singleLine="true"
android:hint="Write a note here" />
<Button
android:layout_width="50dp"
android:layout_height="50dp"
android:id="@+id/BtnAddVocab"
android:onClick="onAddVocab"
android:background="@mipmap/ic_add" />
</LinearLayout>
<include
android:layout_width="wrap_content"
android:layout_height="wrap_content"
layout="@layout/vocab_notes_list"
android:layout_centerHorizontal="true"
android:layout_below="@+id/linearLayout"
android:id="@+id/include"
android:layout_above="@+id/textView" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="Tap and hold to delete"
android:id="@+id/textView"
android:layout_centerHorizontal="true"
android:layout_alignParentBottom="true"
android:textColor="#ffb3b3b3"
android:textSize="@dimen/abc_text_size_medium_material" />
主要布局：
<android.support.v4.widget.DrawerLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/drawer_layout"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context=".MainActivity">
<FrameLayout android:id="@+id/mainContainer" android:layout_width="match_parent"
android:layout_height="match_parent"
android:background="#ffffffff">
<LinearLayout
android:orientation="horizontal"
android:layout_width="fill_parent"
android:id="@+id/backgroundMain"
android:layout_height="wrap_content"
android:layout_gravity="start"
android:layout_marginTop="2dp"
android:layout_marginBottom="2dp"
android:background="#ffe6e2e6"
android:gravity="center_vertical">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="0"
android:id="@+id/pointsTV"
android:layout_gravity="center_vertical"
android:layout_marginLeft="10dp"
android:layout_marginStart="10dp"
android:layout_marginRight="8dp"
android:layout_marginEnd="8dp"/>
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="?android:attr/textAppearanceLarge"
android:text="points"
android:id="@+id/points"
android:layout_gravity="center_vertical"
android:layout_marginRight="10dp"
android:layout_marginEnd="10dp"/>
<!-- timer icon-->
<ImageView
android:layout_width="30dp"
android:layout_height="30dp"
android:id="@+id/imageView"
android:src="@mipmap/ic_timer_gr"
android:layout_gravity="center_vertical"
android:contentDescription="timerIcon" />
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:textAppearance="@android:style/TextAppearance.DeviceDefault.Medium"
android:text="@string/timerVal"
android:id="@+id/timeTV"
android:layout_marginLeft="8dp"
android:layout_marginRight="8dp"
android:visibility="visible"
android:textColor="@android:color/background_dark" />
</LinearLayout>
</FrameLayout>
<!-- android:layout_gravity="start" tells DrawerLayout to treat
this as a sliding drawer on the left side for left-to-right
languages and on the right side for right-to-left languages.
If you're not building against API 17 or higher, use
android:layout_gravity="left" instead. -->
<fragment android:id="@+id/navigation_drawer"
android:layout_width="@dimen/navigation_drawer_width"
android:layout_height="match_parent"
android:layout_gravity="start"
android:name="com.example.tiago.newapponthego.NavigationDrawerFragment"
tools:layout="@layout/fragment_navigation_drawer" />
还有更多的java文件和布局，如果需要，我可以在一秒钟内添加它们。
提前致谢
编辑：这是缺少的xml文件：vocab_notes_list
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:id="@+id/rlLayout"
android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="${packageName}.${activityClass}" >
<ListView
android:id="@+id/vocabList"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentBottom="true"
android:layout_alignParentTop="true" >
</ListView>
</RelativeLayout>
答案 0 :(得分：4)
vocabListView = (ListView) findViewById(R.id.vocabList);
在此之后，vocabListView仍然为空。因此findViewById()返回null，因此找不到视图。
您尝试获得此属性的视图：
android:id="@+id/vocabList"
答案 1 :(得分：2)
从我发现的代码中，您在listView vocabList布局文件中没有任何activity_main。
vocabListView = (ListView) findViewById(R.id.vocabList);
因此，这一行会在vocabListView中为您提供 null MainActivity。
如果这部分fragment_vocab_list布局文件，
你的代码是......
public class VocabularyFragment extends Fragment {
View rootview;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
rootview = inflater.inflate(R.layout.fragment_vocab_list,container, false);
initialVocabList(rootview);
return rootview;
}
}
功能应该是，
public void initialVocabList(View view) {
// ADD HERE
vocabListView = (ListView)view.findViewById(R.id.vocabList);
//vocabsList = new ArrayList<String>();
//retrieve from SharedPreferences
TinyDB tinydb = new TinyDB(getBaseContext());
vocabsList = tinydb.getListString("vocabs");
//itemsAdapter = new ArrayAdapter<String>(this, R.layout.vocab_notes_item, items);
itemsAdapter = new ArrayAdapter<String>(getApplicationContext(), R.layout.vocab_notes_item, vocabsList);
vocabListView.setAdapter(itemsAdapter);
// Setup remove listener method call
setupListViewListener();
// Setup Onclick method call
setOnClickListener();
}