我在Android Studio上有一个项目，我遇到了这种情况。主要想法，我的应用程序显示使用向导模型的订单步骤。我使用viewpager和FragmentStatePagerAdapter。我有一些片段，一个片段（第6个片段）是用户的联系人，在用户未登录时显示。
用户可以登录此片段（使用Facebook）。登录成功后，应删除此片段。我试图删除此片段，但发生错误。它告诉我在尝试获取getActivity（）值时（在第7个片段）有一个空指针异常。我不知道为什么会出现这个错误，因为当用户没有登录第6个片段时，可以正常使用。
这是我的代码
MyAcitivity.java
public class MyActivity extends AppCompatActivity {
ClassName className = new ClassName ();
ProgressDialog progressDialog;
ProgressBar progressBar;
ViewPager viewPager;
MyActivity.PagerAdapter adapter;
String type;
ArrayList<Addons> listAddons = new ArrayList<Addons>();
boolean changed = false;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.my_activity);
progressDialog = new ProgressDialog(this);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
getSupportActionBar().setDisplayHomeAsUpEnabled(true);
progressBar = (ProgressBar) findViewById(R.id.progress_booking);
viewPager = (ViewPager) findViewById(R.id.pager);
viewPager.setPagingEnabled(false);
adapter = new PagerAdapter(this, getSupportFragmentManager());
viewPager.setAdapter(adapter);
viewPager.addOnPageChangeListener(new android.support.v4.view.ViewPager.SimpleOnPageChangeListener(){
@Override
public void onPageSelected(int position) {
updateToolbar();
}
});
updateToolbar();
}
@Override
protected void onResume() {
super.onResume();
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
switch(item.getItemId()){
case android.R.id.home:
onBackPressed();
break;
}
return true;
}
@Override
public void onBackPressed() {
}
public void updateToolbar() {
int position = viewPager.getCurrentItem();
getSupportActionBar().setTitle(adapter.getTitle(position));
bookingProgressBar.setVisibility(View.VISIBLE);
bookingProgressBar.setMax(adapter.getCount());
bookingProgressBar.setProgress(position+1);
getSupportActionBar().setHomeAsUpIndicator(R.drawable.close);
}
public class PagerAdapter extends FragmentStatePagerAdapter {
List<Fragment> items = new ArrayList<>();
List<String> titles = new ArrayList<>();
List<Boolean> instantiated = new ArrayList<>();
public PagerAdapter(Activity activity, FragmentManager fm) {
super(fm);
items.add(new 1stFragment());
items.add(new 2ndFragment());
items.add(new 3rdFragment());
items.add(new 4thFragment());
items.add(new 5thFragment());
items.add(new 6thFragment());
items.add(new 7thFragment());
items.add(new 8thFragment());
titles.add(activity.getString(R.string.1));
titles.add(activity.getString(R.string.2));
titles.add(activity.getString(R.string.3));
titles.add(activity.getString(R.string.4));
titles.add(activity.getString(R.string.5));
titles.add(activity.getString(R.string.6));
titles.add(activity.getString(R.string.7));
titles.add(activity.getString(R.string.8));
instantiated.add(false);
instantiated.add(false);
instantiated.add(false);
instantiated.add(false);
instantiated.add(false);
instantiated.add(false);
instantiated.add(false);
instantiated.add(false);
}
@Override
public void destroyItem(ViewGroup container, int position, Object object) {
super.destroyItem(container, position, object);
instantiated.set(position, false);
}
@Override
public void finishUpdate(ViewGroup container) {
super.finishUpdate(container);
if(!instantiated.get(viewPager.getCurrentItem())){
((MyBaseFragment) items.get(viewPager.getCurrentItem())).setListAddons(listAddons);
((MyBaseFragment) items.get(viewPager.getCurrentItem())).setClassName(className);
((MyBaseFragment) items.get(viewPager.getCurrentItem())).instantiate();
instantiated.set(viewPager.getCurrentItem(), true);
}
}
@Override
public int getItemPosition(Object object) {
return POSITION_NONE;
}
@Override
public Fragment getItem(int position) {
return items.get(position);
}
public String getTitle(int position){
return titles.get(position);
}
@Override
public int getCount() {
return items.size();
}
public void removeContact(){
items.remove(5);
titles.remove(5);
instantiated.remove(5);
notifyDataSetChanged();
updateToolbar();
((MyBaseFragment) items.get(viewPager.getCurrentItem())).setListAddons(listAddons);
((MyBaseFragment) items.get(viewPager.getCurrentItem())).setClassName(className);
((MyBaseFragment) items.get(viewPager.getCurrentItem())).instantiate();
}
}
}
6thFragment 请首先关注onCompleted回调..
public class 6thFragment extends MyBaseFragment {
EditText emailEditText;
private CallbackManager callbackManager;
public 6thFragment() {
// Required empty public constructor
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
final View view = inflater.inflate(R.layout.6fragment_layout, container, false);
emailEditText = (EditText) view.findViewById(R.id.editText_email);
if((new SessionPreference(getActivity())).isLoggedIn()){
((MyActivity.PagerAdapter)((ViewPager) view.getParent()).getAdapter()).removeContact();
}
callbackManager = CallbackManager.Factory.create();
LoginButton loginButton = (LoginButton) view.findViewById(R.id.login_button);
loginButton.setCompoundDrawablesWithIntrinsicBounds(0, 0, 0, 0);
loginButton.setFragment(this);
loginButton.setReadPermissions(Arrays.asList("public_profile, email"));
loginButton.registerCallback(callbackManager, new FacebookCallback<LoginResult>() {
@Override
public void onSuccess(LoginResult loginResult) {
GraphRequest request = GraphRequest.newMeRequest(
loginResult.getAccessToken(),
new GraphRequest.GraphJSONObjectCallback() {
@Override
public void onCompleted(JSONObject object, GraphResponse response) {
//**HERE THE MAIN QUESTION, AFTER USER SUCCESS LOGIN USING FACEBOOK**//
// **6th fragment should be removed**
((MyActivity.PagerAdapter)((ViewPager) view.getParent()).getAdapter()).removeContact();
}
});
Bundle parameters = new Bundle();
parameters.putString("fields", "id,name,email");
request.setParameters(parameters);
request.executeAsync();
}
@Override
public void onCancel() {
Toast.makeText(getActivity(), "Login is canceled", Toast.LENGTH_SHORT).show();
}
@Override
public void onError(FacebookException exception) {
Toast.makeText(getActivity(), "Login is error", Toast.LENGTH_SHORT).show();
}
});
return view;
}
@Override
public void instantiate() {
//somecode
}
@Override
public boolean isValid() {
//some code
}
@Override
public boolean isChanged() {
return emailEditText.getText().toString().trim().equals(generalCleaning.getEmail());
}
@Override
public ClassName getClassName() {
///some code
}
@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
super.onActivityResult(requestCode, resultCode, data);
callbackManager.onActivityResult(requestCode, resultCode, data);
}
}
然后是 7thFragment 请首先关注setPrice方法
public class 7thFragment extends MyBaseFragment implements View.OnClickListener{
String planStr, planStrOld;
LinearLayout weeklyLinearLayout, biweeklyLinearLayout, onetimeLinearLayout;
RadioButton weeklyRadioButton, biweeklyRadioButton, onetimeRadioButton;
RadioButton prevChecked;
ProgressDialog progressDialog;
TextView priceTextView, dateTextView, timeTextView;
SimpleDateFormat dateFormater = new SimpleDateFormat("EEEE, MMM d", Locale.getDefault()),
timeFormater = new SimpleDateFormat("HH:mm", Locale.getDefault());
public 7thFragment() {
// Required empty public constructor
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.7th_layout, container, false);
progressDialog = new ProgressDialog(getActivity());
//codes
return view;
}
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if(isVisibleToUser){
setPrice();
setRadioButton();
}
}
@Override
public void instantiate() {
if(isAdded()){
setPrice();
setRadioButton();
}
}
@Override
public boolean isValid() {
//codes
}
@Override
public boolean isChanged() {
if(planStr.equalsIgnoreCase(planStrOld)){
return true;
}else {
return false;
}
}
@Override
public ClassName getClassName() {
//code
}
@Override
public void onClick(View view) {
prevChecked.setChecked(false);
if(view instanceof RadioButton) {
((RadioButton) view).setChecked(true);
prevChecked = ((RadioButton) view);
}else {
LinearLayout linearLayout = (LinearLayout) view;
((RadioButton) linearLayout.getChildAt(0)).setChecked(true);
prevChecked = ((RadioButton) linearLayout.getChildAt(0));
}
String plan;
if(prevChecked.getId()==R.id.plan_weekly){
plan = getString(R.string.weekly);
}else if(prevChecked.getId()==R.id.plan_biweekly){
plan = getString(R.string.biweekly);
}else{
plan = getString(R.string.one_time);
}
getSideBar(plan);
}
private void setPrice(){
try {
SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss", Locale.getDefault());
String date = dateFormater.format(sdf.parse(className.getBooked_date()));
String time = timeFormater.format(sdf.parse(className.getBooked_date()));
dateTextView.setText(date);
//HERE THE MAIN ERROR, I GET NULL POINTER EXCEPTION (at getActivity())
timeTextView.setText(time+" - "+getActivity().getString(R.string.general_cleaning_duration_value, generalCleaning.getDuration()));
priceTextView.setText(className.getPrice());
} catch (ParseException pe) {
pe.printStackTrace();
}
}
private void setRadioButton(){
//codes
}
}
希望大家都理解我的问题，谢谢你的帮助。
答案 0 :(得分：0)
我不知道为什么mu问题得到这么多的支持。
我找到了答案，我将此代码添加到我的7thFragment中，并将myActivity替换为getActivity（）。一切正常。
@Override
public void onAttach(Context context) {
super.onAttach(context);
myActivity = (MyActivity) context;
}
@SuppressWarnings("deprecation")
@Override
public void onAttach(Activity activity) {
super.onAttach(activity);
if (Build.VERSION.SDK_INT < Build.VERSION_CODES.M) {
myActivity = (MyActivity) activity;
}
}
希望这能帮到你