我使用工具栏作为所有活动的标题栏。现在我的一个班级需要使用getActionBar().setDisplayHomeAsUpEnabled(true);，这就出现了错误。问题是如何将getActionBar().setDisplayHomeAsUpEnabled(true);替换为其他选项。我试过了getSupportActionBar()，但是它出错了。这是logcat错误：
/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.foo.isfoo, PID: 2426
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.foo.isfoo/com.example.foo.isfoo.ListOfFriends}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.app.ActionBar.setDisplayHomeAsUpEnabled(boolean)' on a null object reference
这是我的代码的一部分，我缩短了我的代码：
public class ListOfFriends extends ListActivity{
private Manager imService = null;
private FriendListAdapter friendAdapter;
public String ownusername = new String();
private class FriendListAdapter extends BaseAdapter {
class ViewHolder {
TextView text;
ImageView icon;
}
private LayoutInflater mInflater;
private Bitmap mOnlineIcon;
private Bitmap mOfflineIcon;
private InfoOfFriend[] friends = null;
public FriendListAdapter(Context context) {
super();
mInflater = LayoutInflater.from(context);
mOnlineIcon = BitmapFactory.decodeResource(context.getResources(), R.drawable.online);
mOfflineIcon = BitmapFactory.decodeResource(context.getResources(), R.drawable.offline);
}
public void setFriendList(InfoOfFriend[] friends) {
this.friends = friends;
}
public int getCount() {
return friends.length;
}
public InfoOfFriend getItem(int position) {
return friends[position];
}
public long getItemId(int position) {
return 0;
}
@SuppressLint("InflateParams")
public View getView(int position, View convertView, ViewGroup parent) {
// A ViewHolder keeps references to children views to avoid
// unneccessary calls
// to findViewById() on each row.
ViewHolder holder;
// When convertView is not null, we can reuse it directly, there is
// no need
// to reinflate it. We only inflate a new View when the convertView
// supplied
// by ListView is null.
if (convertView == null) {
convertView = mInflater.inflate(R.layout.friend_list_screen, null);
// Creates a ViewHolder and store references to the two children
// views
// we want to bind data to.
holder = new ViewHolder();
holder.text = (TextView) convertView.findViewById(R.id.text);
holder.icon = (ImageView) convertView.findViewById(R.id.icon);
convertView.setTag(holder);
} else {
// Get the ViewHolder back to get fast access to the TextView
// and the ImageView.
holder = (ViewHolder) convertView.getTag();
}
// Bind the data efficiently with the holder.
holder.text.setText(friends[position].userName);
holder.icon.setImageBitmap(friends[position].status == InfoStatus.ONLINE ? mOnlineIcon : mOfflineIcon);
return convertView;
}
}
public class MessageReceiver extends BroadcastReceiver {
@Override
public void onReceive(Context context, Intent intent) {
Log.i("Broadcast receiver ", "received a message");
Bundle extra = intent.getExtras();
if (extra != null) {
String action = intent.getAction();
if (action.equals(MessagingService.FRIEND_LIST_UPDATED)) {
// taking friend List from broadcast
// String rawFriendList =
// extra.getString(FriendInfo.FRIEND_LIST);
// FriendList.this.parseFriendInfo(rawFriendList);
ListOfFriends.this.updateData(ControllerOfFriend.getFriendsInfo(),
ControllerOfFriend.getUnapprovedFriendsInfo());
}
}
}
};
public MessageReceiver messageReceiver = new MessageReceiver();
private ServiceConnection mConnection = new ServiceConnection() {
public void onServiceConnected(ComponentName className, IBinder service) {
imService = ((MessagingService.IMBinder) service).getService();
InfoOfFriend[] friends = ControllerOfFriend.getFriendsInfo(); // imService.getLastRawFriendList();
if (friends != null) {
ListOfFriends.this.updateData(friends, null); // parseFriendInfo(friendList);
}
setTitle(imService.getUsername() + "'s friend list");
ownusername = imService.getUsername();
}
public void onServiceDisconnected(ComponentName className) {
imService = null;
Toast.makeText(ListOfFriends.this, R.string.local_service_stopped, Toast.LENGTH_SHORT).show();
}
};
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.list_friends);
//error here
getActionBar().setDisplayHomeAsUpEnabled(true);
friendAdapter = new FriendListAdapter(this);
}
public void updateData(InfoOfFriend[] friends, InfoOfFriend[] unApprovedFriends) {
if (friends != null) {
friendAdapter.setFriendList(friends);
setListAdapter(friendAdapter);
}
if (unApprovedFriends != null) {
NotificationManager NM = (NotificationManager) getSystemService(NOTIFICATION_SERVICE);
if (unApprovedFriends.length > 0) {
String tmp = new String();
for (int j = 0; j < unApprovedFriends.length; j++) {
tmp = tmp.concat(unApprovedFriends[j].userName).concat(",");
}
NotificationCompat.Builder mBuilder = new NotificationCompat.Builder(this)
.setSmallIcon(R.drawable.notification)
.setContentTitle(getText(R.string.new_friend_request_exist));
/*
* Notification notification = new
* Notification(R.drawable.stat_sample,
* getText(R.string.new_friend_request_exist),
* System.currentTimeMillis());
*/
Intent i = new Intent(this, WaitingListFriends.class);
i.putExtra(InfoOfFriend.FRIEND_LIST, tmp);
PendingIntent contentIntent = PendingIntent.getActivity(this, 0, i, 0);
mBuilder.setContentText("You have new friend request(s)");
/*
* notification.setLatestEventInfo(this,
* getText(R.string.new_friend_request_exist),
* "You have new friend request(s)", contentIntent);
*/
mBuilder.setContentIntent(contentIntent);
NM.notify(R.string.new_friend_request_exist, mBuilder.build());
} else {
// if any request exists, then cancel it
NM.cancel(R.string.new_friend_request_exist);
}
}
}
@Override
protected void onListItemClick(ListView l, View v, int position, long id) {
super.onListItemClick(l, v, position, id);
Intent i = new Intent(this, PerformingMessaging.class);
InfoOfFriend friend = friendAdapter.getItem(position);
i.putExtra(InfoOfFriend.USERNAME, friend.userName);
i.putExtra(InfoOfFriend.PORT, friend.port);
i.putExtra(InfoOfFriend.IP, friend.ip);
startActivity(i);
}
@Override
protected void onPause() {
unregisterReceiver(messageReceiver);
unbindService(mConnection);
super.onPause();
}
@Override
protected void onResume() {
super.onResume();
bindService(new Intent(ListOfFriends.this, MessagingService.class), mConnection,
Context.BIND_AUTO_CREATE);
IntentFilter i = new IntentFilter();
// i.addAction(IMService.TAKE_MESSAGE);
i.addAction(MessagingService.FRIEND_LIST_UPDATED);
registerReceiver(messageReceiver, i);
}
@Override
public boolean onCreateOptionsMenu(android.view.Menu menu) {
// TODO Auto-generated method stub
getMenuInflater().inflate(R.menu.addfriend_menu, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// TODO Auto-generated method stub
switch (item.getItemId()) {
case android.R.id.home:
onBackPressed();
return true;
case R.id.add_friend:
Intent i = new Intent(ListOfFriends.this, FriendAdder.class);
startActivity(i);
return true;
}
return super.onOptionsItemSelected(item);
}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
}
@Override
public void onBackPressed() {
// TODO Auto-generated method stub
imService.exit();
Intent i = new Intent(ListOfFriends.this, stu_homepage.class);
startActivity(i);
finish();
}
}
答案 0 :(得分：1)
修改
ListActivity不延长AppCompatActivity因此您必须使用getActionBar()，setActionBar()和android.widget.Toolbar。此外，您必须使用minSdkVersion 21文件中的build.gradle使用最低api级别21。
OLD答案：
在使用getActionBar()之前，您应将工具栏设置为操作栏。
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setActionBar(toolbar);
//now you can use the action bar.
getActionBar().setDisplayHomeAsUpEnabled(true);;
注意：如果您的活动扩展了AppCompatActivity，您应该使用方法setSupportActionBar和getSupportActionBar
答案 1 :(得分：0)
ListActivity在这里是错误的。您应该使用AppCompatActivity来访问此API。对于建议使用RecyclerView的项目列表。
对于您提供的特定代码，Android Studio提供了一个提示＆＃34;可能的NullPointer＆＃34;所以你应该在调用某些方法之前检查null。