问题
I want to call another fragment from the current fragment on the click of the button in the current fragment.Based on answers I got here, calling a fragment from fragment. My revised code is as follows :
Here is my Mainactivity :
package com.kibitz4college.k4c;
import android.app.FragmentManager;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.view.View;
import android.support.design.widget.NavigationView;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import com.asd.fragments.RecommendationsFragment;
import com.asd.fragments.SearchCoachingFragment;
import com.asd.fragments.SearchCollegesFragment;
import com.asd.fragments.MainFragment;
import com.asd.fragments.SearchConsultanciesFragment;
import com.asd.fragments.SearchResultsFragment;
import com.asd.fragments.TrendingFragment;
public class MainActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener, SearchCoachingFragment.searchbtnclickedlistner {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
ActionBarDrawerToggle toggle = new ActionBarDrawerToggle(
this, drawer, toolbar, R.string.navigation_drawer_open, R.string.navigation_drawer_close);
drawer.setDrawerListener(toggle);
toggle.syncState();
NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
navigationView.setNavigationItemSelectedListener(this);
FragmentManager fm = getFragmentManager();
fm.beginTransaction().replace(R.id.content_frame,new MainFragment()).commit();
}
@Override
public void onBackPressed() {
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
if (drawer.isDrawerOpen(GravityCompat.START)) {
drawer.closeDrawer(GravityCompat.START);
} else {
super.onBackPressed();
}
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.main, menu);
return true;
}
@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);
}
@SuppressWarnings("StatementWithEmptyBody")
@Override
public boolean onNavigationItemSelected(MenuItem item) {
// Handle navigation view item clicks here.
int id = item.getItemId();
FragmentManager fm = getFragmentManager();
fm.beginTransaction().replace(R.id.content_frame,new MainFragment()).commit();
if (id == R.id.search_colleges) {
// Handle the Search Colleges action
fm.beginTransaction().replace(R.id.content_frame,new SearchCollegesFragment()).commit();
}
else if (id == R.id.search_consultancies) {
fm.beginTransaction().replace(R.id.content_frame,new SearchConsultanciesFragment()).commit();
}
else if (id == R.id.search_coaching) {
fm.beginTransaction().replace(R.id.content_frame,new SearchCoachingFragment()).commit();
}
else if (id == R.id.my_recommendations) {
fm.beginTransaction().replace(R.id.content_frame, new RecommendationsFragment()).commit();
}
else if (id == R.id.trending) {
fm.beginTransaction().replace(R.id.content_frame, new TrendingFragment()).commit();
} else if (id == R.id.profile) {
} else if (id == R.id.logout) {
}
DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
drawer.closeDrawer(GravityCompat.START);
return true;
}
@Override
public void showresults() {
FragmentManager fm = getFragmentManager();
fm.beginTransaction().replace(R.id.content_frame,new SearchResultsFragment()).commit();
}
}
Here is one of my fragment from which I'm trying to call another :
import android.app.Activity;
import android.app.Fragment;
import android.content.Context;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.TextView;
import android.widget.Toast;
import com.kibitz4college.k4c.R;
public class SearchCoachingFragment extends Fragment {
Button search_coll_btn;
searchbtnclickedlistner searchbtnclickedlistner_var;
@Nullable
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
final View rootview = inflater.inflate(R.layout.fragment_search_coaching, container, false);
search_coll_btn = (Button) rootview.findViewById(R.id.Search_coaching_btn );
search_coll_btn.setOnClickListener(new View.OnClickListener() {
TextView lite=(TextView) rootview.findViewById(R.id.textView3);
@Override
public void onClick(View v) {
searchbtnclickedlistner_var.showresults();
}
});
return rootview;
}
public interface searchbtnclickedlistner
{
public void showresults();
}
@Override
public void onAttach(Context context) {
super.onAttach(context);
try{
searchbtnclickedlistner_var=(searchbtnclickedlistner)context;
}
catch(Exception e){
throw new RuntimeException(context.toString()+ " must implement OnMyFragmentInteractionListener");
}
}
}
I've implemented interface & Overridden in the mainactivity, now I get a nullpointer exception as follows:
java.lang.NullPointerException: Attempt to invoke interface method
void com.asd.fragments.SearchCoachingFragment$searchbtnclickedlistner.showresults() on a null object reference.
can someone please tell me what I'm doing wrong?
回答1:
You are calling a method from the interface and that interface is null. You should implement that interface in another class for the callback. Also you should create the interface outside the fragment for a better structure.
回答2:
The problem could be caused from the fact, you don't put replaced/removed Fragment in the backstack, calling the method addToBackStack().
If you do not call addToBackStack() when you perform a transaction that removes a fragment, then that fragment is destroyed when the transaction is committed and the user cannot navigate back to it. Whereas, if you do call addToBackStack() when removing a fragment, then the fragment is stopped and will be resumed if the user navigates back.
So when you try to use a method of a replaced/removed Fragment you have NullPointerException. This is the link.
来源：https://stackoverflow.com/questions/35707076/null-pointer-exception-calling-fragments