App crash if I try to open a fragment in an activity

Question

every time I try to open my fragment in my MainActivity.class my app crashs.

MainActivity.class

/* Menu */
@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    switch (item.getItemId()) {
        case R.id.add_server:
            Intent intent = new Intent(getApplicationContext(), AddServerFragment.class);
            startActivity(intent);
            return true;
        case R.id.menu_refresh:
            myWebView.reload();
            return true;
        default:
            return true;
    }        
...
 public class AddServerFragment extends Fragment {

    public AddServerFragment(){

    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState){
        View rootView = inflater.inflate(R.layout.add_ip, container, false);

        Button btn_back, btn_add;
        final EditText server_ip, server_name;

        server_ip = (EditText) findViewById(R.id.edit_server_address);
        server_name = (EditText) findViewById(R.id.edit_server_name);

         /* Back Button */
        btn_back = (Button) findViewById(R.id.btn_back);
        btn_back.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View view) {
                Intent intent = new Intent(getApplicationContext(), MainActivity.class);
                startActivity(intent);
            }
        });

        /* Add IP Button */
        btn_add = (Button) findViewById(R.id.btn_add);
        btn_add.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View view) {
                String new_server_ip = null, new_server_name = null;

                ArrayList<String> server_name_list = new ArrayList<String>();
                ArrayList<String> server_ip_list = new ArrayList<String>();

                new_server_ip = server_ip.getText().toString();
                server_ip_list.add(new_server_ip);

                new_server_name = server_name.getText().toString();
                server_name_list.add(new_server_name);
            }
        });

        return rootView;
    }
}

My errorlog only say, I have to add the Fragment in my Manifest.xml as activity. But I know, I don't have to add it there.

Errorlog

11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime: FATAL EXCEPTION: main
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime: Process: de.kwietzorek.fulcrumwebview, PID: 18345
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime: android.content.ActivityNotFoundException: Unable to find explicit activity class {de.kwietzorek.fulcrumwebview/de.kwietzorek.fulcrumwebview.MainActivity$AddServerFragment}; have you declared this activity in your AndroidManifest.xml?
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.app.Instrumentation.checkStartActivityResult(Instrumentation.java:1794)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.app.Instrumentation.execStartActivity(Instrumentation.java:1512)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.app.Activity.startActivityForResult(Activity.java:3917)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.app.Activity.startActivityForResult(Activity.java:3877)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:784)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.app.Activity.startActivity(Activity.java:4200)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.app.Activity.startActivity(Activity.java:4168)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at de.kwietzorek.fulcrumwebview.MainActivity.onOptionsItemSelected(MainActivity.java:90)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.app.Activity.onMenuItemSelected(Activity.java:2908)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.support.v4.app.FragmentActivity.onMenuItemSelected(FragmentActivity.java:361)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.support.v7.app.AppCompatActivity.onMenuItemSelected(AppCompatActivity.java:147)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.support.v7.internal.view.WindowCallbackWrapper.onMenuItemSelected(WindowCallbackWrapper.java:100)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.support.v7.app.AppCompatDelegateImplV7.onMenuItemSelected(AppCompatDelegateImplV7.java:621)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.support.v7.internal.view.menu.MenuBuilder.dispatchMenuItemSelected(MenuBuilder.java:811)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.support.v7.internal.view.menu.MenuItemImpl.invoke(MenuItemImpl.java:153)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.support.v7.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:958)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.support.v7.internal.view.menu.MenuBuilder.performItemAction(MenuBuilder.java:948)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.support.v7.internal.view.menu.MenuPopupHelper.onItemClick(MenuPopupHelper.java:191)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.widget.AdapterView.performItemClick(AdapterView.java:310)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.widget.AbsListView.performItemClick(AbsListView.java:1145)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.widget.AbsListView$PerformClick.run(AbsListView.java:3042)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.widget.AbsListView$3.run(AbsListView.java:3879)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.os.Handler.handleCallback(Handler.java:739)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:95)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:148)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5417)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Native Method)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
11-14 15:22:05.935 18345-18345/de.kwietzorek.fulcrumwebview E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)

Edit

My Manifest.xml

<?xml version="1.0" encoding="utf-8"?>

<uses-permission android:name="android.permission.INTERNET" />

<application
    android:allowBackup="true"
    android:icon="@mipmap/ic_launcher"
    android:label="@string/app_name"
    android:supportsRtl="true"
    android:theme="@style/Theme.AppCompat" >
    <activity android:name=".MainActivity" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />

            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>
</application>

Answer 1

Wrong with the above code:

ERROR Intent intent = new Intent(getApplicationContext(), AddServerFragment.class); startActivity(intent);

    Use the correct method for using fragment, you have used method for starting activity.

AddServerFragment is Fragment not activity.

For fragment to add in activity, you have to use the following approach:

In your activity,

XML code part in activity_main.xml:

<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
     android:id="@+id/fragment_container"
     android:layout_width="match_parent"
     android:layout_height="match_parent" />

Java Code Part in MainActivity.class:

    if (findViewById(R.id.fragment_container) != null) {

        if (savedInstanceState != null) {
            return;
        }

        // Create an instance of AddServerFragment
        AddServerFragment firstFragment = new AddServerFragment();
        // if there are any extras
        firstFragment.setArguments(getIntent().getExtras());

        // Add the fragment to the 'fragment_container' FrameLayout
        getSupportFragmentManager().beginTransaction()
                .add(R.id.fragment_container, firstFragment).commit();
    }

Answer 2

You cant use intent when you want open fragment. You can try FragmentManager.

Example:

public void getFragment(Fragment fragment) {
    FragmentManager fm = getSupportFragmentManager();
    Fragment mFragment = fm.findFragmentById(R.id.fragment_container);

    if (mFragment == null) {
        mFragment = fragment;
        fm.beginTransaction()
                .add(R.id.fragment_container, mFragment)
                .commit();
    }
    if (mFragment != null) {
        mFragment = fragment;
        fm.beginTransaction().addToBackStack(null)
                .replace(R.id.fragment_container, mFragment)
                .commit();
    }
}

Answer 3

You need to add the activity declaration to your manifest file:

<activity android:name=".MainActivity" >
</activity>