Question

I get this error message in LogCat:

06-25 15:17:34.495: E/AndroidRuntime(5891):
java.lang.RuntimeException: Unable to start activity
ComponentInfo{com.example/com.example.MainActivity}:
java.lang.NullPointerException
06-25 16:07:43.589: E/AndroidRuntime(6587): Caused by: java.lang.NullPointerException
06-25 16:07:43.589: E/AndroidRuntime(6587): at com.example.MyTabsListener.onTabSelected(MyTabListener.java:28)

This is my main activity:

package com.example;

import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;

import com.actionbarsherlock.app.ActionBar;
import com.actionbarsherlock.app.SherlockActivity;

public class MainActivity extends SherlockActivity {    
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        ActionBar actionbar = getSupportActionBar();
        actionbar.setNavigationMode(ActionBar.NAVIGATION_MODE_TABS);

        ActionBar.Tab tab1 = actionbar.newTab().setText("Fragment A");
        SherlockFragment fragment1 = new AFragment();

        tab1.setTabListener(new MyTabsListener(fragment1, this
            .getApplicationContext()));
        actionbar.addTab(tab1);
    }

}

And my simple TabListener:

package com.example;    

import android.content.Context;
import android.support.v4.app.FragmentTransaction;
import android.widget.Toast;  

import com.actionbarsherlock.app.ActionBar;
import com.actionbarsherlock.app.ActionBar.Tab;
import com.actionbarsherlock.app.SherlockFragment;

class MyTabsListener implements ActionBar.TabListener {
    public SherlockFragment fragment;
    private final Context appContext;

    public MyTabsListener(SherlockFragment fragment, Context ctx) {
        this.fragment = fragment;
        this.appContext = ctx;
    }

    @Override
    public void onTabReselected(Tab tab, FragmentTransaction ft) {
        Toast.makeText(this.appContext, "Reselected!", Toast.LENGTH_LONG)
                .show();
    }

    @Override
    public void onTabSelected(Tab tab, FragmentTransaction ft) {
        ft.replace(R.id.fragment_container, fragment);
    }

    @Override
    public void onTabUnselected(Tab tab, FragmentTransaction ft) {
        ft.remove(fragment);
    }
}

And finally my class Fragment A:

package com.example;

import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;

import com.actionbarsherlock.app.SherlockFragment;

public class AFragment extends SherlockFragment {

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
            Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.weekdays, container, false);
    }

}

So this function is causing the error in the TabListener:

@Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
  ft.replace(R.id.fragment_container, fragment);
}

Answer 1

1. MainActivity has to extend SherlockFragmentActivity (credits to JosipPranjic!).

2. The NullPointerException can be fixed with following code from this SO question: Null FragmentTransaction being passed to TabListener.onTabSelected()

   @Override
   public void onTabSelected(Tab tab, FragmentTransaction ignoredFt) {
     FragmentManager fragMgr = ((FragmentActivity) mActivity)
           .getSupportFragmentManager();
     FragmentTransaction ft = fragMgr.beginTransaction();
   
     // Check if the fragment is already initialized
     if (mFragment == null) {
         // If not, instantiate and add it to the activity
         mFragment = Fragment.instantiate(mActivity, mClass.getName());
   
         ft.add(android.R.id.content, mFragment, mTag);
     } else {
         // If it exists, simply attach it in order to show it
         ft.attach(mFragment);
     }
   
     ft.commit();
   }
   
   @Override
   public void onTabUnselected(Tab tab, FragmentTransaction ignoredFt) {
     FragmentManager fragMgr = ((FragmentActivity) mActivity)
             .getSupportFragmentManager();
     FragmentTransaction ft = fragMgr.beginTransaction();
   
     // Check if the fragment is already initialized
     if (mFragment == null) {
         // If not, instantiate and add it to the activity
         mFragment = Fragment.instantiate(mActivity, mClass.getName());
   
         ft.add(android.R.id.content, mFragment, mTag);
     } else {
         // If it exists, simply attach it in order to show it
         ft.detach(mFragment);
     }
   
     ft.commit();
   }