title: How can I do a fragmentTransaction in other class?
body: <p>I want create a class whose funtion is only do a fragmentTransaction but I have an error. My class is the next:</p>
  
  <pre><code>package net.elinformaticoenganchado.sergio.crossfighters.common;
  
  import android.support.v4.app.Fragment;
  import android.support.v4.app.FragmentActivity;
  import android.support.v4.app.FragmentTransaction;
  
  import net.elinformaticoenganchado.sergio.crossfighters.R;
  import net.elinformaticoenganchado.sergio.crossfighters.statics.Main;
  
  /**
   * @author Sergio Herrero Cruz &lt;sergioherre96@gmail.com&gt;
   * @package net.elinformaticoenganchado.sergio.crossfighters.common
   */
  public class FragmentsGoTo extends FragmentActivity{
  
  
      public void goToMainFragment() {
          Fragment fragment = new Main();
          FragmentTransaction ftConfig = getSupportFragmentManager().beginTransaction();
          ftConfig.replace(R.id.main_frame, fragment);
          ftConfig.commit();
      }
  }
  </code></pre>
  
  <p>And I call to this function here:</p>
  
  <pre><code>package net.elinformaticoenganchado.sergio.crossfighters.configuration;
  
  import android.os.Bundle;
  import android.support.annotation.Nullable;
  import android.support.v4.app.Fragment;
  import android.view.LayoutInflater;
  import android.view.View;
  import android.view.ViewGroup;
  import android.widget.Button;
  import android.widget.EditText;
  import net.elinformaticoenganchado.sergio.crossfighters.R;
  import net.elinformaticoenganchado.sergio.crossfighters.common.FragmentsGoTo;
  import net.elinformaticoenganchado.sergio.crossfighters.entity.Configuration;
  
  /**
   * @author Sergio Herrero Cruz &lt;sergioherre96@gmail.com&gt;
   */
  public class ConfigurationEdit extends Fragment implements View.OnClickListener{
  
      EditText name;
  
      /**
       * @param inflater           The LayoutInflater object that can be used to inflate
       *                           any views in the fragment,
       * @param container          If non-null, this is the parent view that the fragment's
       *                           UI should be attached to.  The fragment should not add the view itself,
       *                           but this can be used to generate the LayoutParams of the view.
       * @param savedInstanceState If non-null, this fragment is being re-constructed
       *                           from a previous saved state as given here.
       * @return Return the View for the fragment's UI, or null.
       */
      @Nullable
      @Override
      public View onCreateView(LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) {
          View view = inflater.inflate(R.layout.configuration_edit, container, false);
  
          name = (EditText) view.findViewById(R.id.configuration_edit_name);
          Button save = (Button) view.findViewById(R.id.configuration_edit_button);
  
          save.setOnClickListener(this);
  
          return view;
      }
  
      /**
       * Called when a view has been clicked.
       *
       * @param v The view that was clicked.
       */
      @Override
      public void onClick(View v) {
          if (v.getId() == R.id.configuration_edit_button) {
              Configuration config = new Configuration(name.getText().toString());
              config.save();
              FragmentsGoTo fragmentsGoTo = new FragmentsGoTo();
              fragmentsGoTo.goToMainFragment();
          }
      }
  
  }
  </code></pre>
  
  <p>They are simple class. But I have an error:</p>
  
  <blockquote>
    <p>Process: net.elinformaticoenganchado.sergio.crossfighters, PID: 5116
    java.lang.RuntimeException: Unable to start activity
    ComponentInfo{net.elinformaticoenganchado.sergio.crossfighters/net.elinformaticoenganchado.sergio.crossfighters.MainActivity}:
    java.lang.NullPointerException: Attempt to invoke virtual method
    'android.support.v4.app.FragmentManager
    android.support.v4.app.FragmentActivity.getSupportFragmentManager()'
    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$H.handleMessage(ActivityThread.java:1344)
                                                                                                        at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                                                        at android.os.Looper.loop(Looper.java:148)
                                                                                                        at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                                                        at java.lang.reflect.Method.invoke(Native Method)
                                                                                                        at
    com.android.internal.os.ZygoteInit$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 'android.support.v4.app.FragmentManager
    android.support.v4.app.FragmentActivity.getSupportFragmentManager()'
    on a null object reference
                                                                                                        at
    net.elinformaticoenganchado.sergio.crossfighters.common.FragmentsGoTo.goToMainFragment(FragmentsGoTo.java:19)
                                                                                                        at
    net.elinformaticoenganchado.sergio.crossfighters.MainActivity.onCreate(MainActivity.java:42)
                                                                                                        at android.app.Activity.performCreate(Activity.java:6237)
                                                                                                        at
    android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                                                                                                        at
    android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                                                                                                        at
    android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                                                                                                        at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                                                                        at
    android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                                                                                                        at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                                                        at android.os.Looper.loop(Looper.java:148) 
                                                                                                        at android.app.ActivityThread.main(ActivityThread.java:5417) 
                                                                                                        at java.lang.reflect.Method.invoke(Native Method) 
                                                                                                        at
    com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                                                                                                        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)</p>
  </blockquote>
  
  <p>So how can i this in java without errors.</p>
  
  <p>Thank you.</p>
  
  <p>Edit: 
  Add mainActivity class:</p>
  
  <pre><code>package net.elinformaticoenganchado.sergio.crossfighters;
  
  import android.os.Bundle;
  import android.support.design.widget.FloatingActionButton;
  import android.support.design.widget.Snackbar;
  import android.support.v4.app.Fragment;
  import android.support.v4.app.FragmentTransaction;
  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 net.elinformaticoenganchado.sergio.crossfighters.common.FragmentsGoTo;
  import net.elinformaticoenganchado.sergio.crossfighters.configuration.ConfigurationEdit;
  import net.elinformaticoenganchado.sergio.crossfighters.entity.Configuration;
  
  public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {
  
      @Override
      protected void onCreate(Bundle savedInstanceState) {
          super.onCreate(savedInstanceState);
  
          setContentView(R.layout.activity_main);
  
          FragmentsGoTo fragmentsGoTo = new FragmentsGoTo();
          //@Todo change for start
          //Configuration.deleteAll(Configuration.class);
  
          if (Configuration.count(Configuration.class) == 0) {
              Fragment fragment = new ConfigurationEdit();
              FragmentTransaction ftConfig = getSupportFragmentManager().beginTransaction();
              ftConfig.replace(R.id.main_frame, fragment);
              ftConfig.commit();
          } else {
              fragmentsGoTo.goToMainFragment();
          }
  
          Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
          setSupportActionBar(toolbar);
  
          FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
          fab.setOnClickListener(new View.OnClickListener() {
              @Override
              public void onClick(View view) {
                  Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                          .setAction("Action", null).show();
              }
          });
  
          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.addDrawerListener(toggle);
          toggle.syncState();
  
          NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
          navigationView.setNavigationItemSelectedListener(this);
      }
  
      @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();
  
          if (id == R.id.nav_camera) {
              // Handle the camera action
          } else if (id == R.id.nav_gallery) {
  
          } else if (id == R.id.nav_slideshow) {
  
          } else if (id == R.id.nav_manage) {
  
          } else if (id == R.id.nav_share) {
  
          } else if (id == R.id.nav_send) {
  
          }
  
          DrawerLayout drawer = (DrawerLayout) findViewById(R.id.drawer_layout);
          drawer.closeDrawer(GravityCompat.START);
          return true;
      }
  }
  </code></pre>