How to start Fragment from an Activity

You can either add or replace fragment in your activity. Create a FrameLayout in activity layout xml file.

Then do this in your activity to add fragment:

FragmentManager manager = getFragmentManager();
FragmentTransaction transaction = manager.beginTransaction();
transaction.add(R.id.container,YOUR_FRAGMENT_NAME,YOUR_FRAGMENT_STRING_TAG);
transaction.addToBackStack(null);
transaction.commit();

And to replace fragment do this:

FragmentManager manager = getFragmentManager();
FragmentTransaction transaction = manager.beginTransaction();
transaction.replace(R.id.container,YOUR_FRAGMENT_NAME,YOUR_FRAGMENT_STRING_TAG);
transaction.addToBackStack(null);
transaction.commit();

See Android documentation on adding a fragment to an activity or following related questions on SO:

Difference between add(), replace(), and addToBackStack()

Basic difference between add() and replace() method of Fragment

Difference between add() & replace() with Fragment's lifecycle

Simple way

1. Create a new java class

   public class ActivityName extends FragmentActivity {
   @Override
    public void onCreate(Bundle savedInstanceState) {
      super.onCreate(savedInstanceState);
       if (savedInstanceState == null){
       getSupportFragmentManager().beginTransaction()
               .add(android.R.id.content, new Fragment_name_which_you_wantto_open()).commit();}
    }
   }
   

2. in your activity where u want to call fragment

    Intent i = new Intent(Currentactivityname.this,ActivityName.class);
    startActivity(i);
   

Another Method

1. Place frame layout in your activity where u want to open fragment

     <FrameLayout
           android:id="@+id/frameLayout"
           android:layout_width="match_parent"
           android:layout_height="wrap_content">
       </FrameLayout>
   

2. Paste this code where u want to open fragment

       Fragment mFragment = null;
           mFragment = new Name_of_fragment_which_you_want_to_open();
           FragmentManager fragmentManager = getSupportFragmentManager();
           fragmentManager.beginTransaction()
                   .replace(R.id.frameLayout, mFragment).commit();
   

Firstly, you start Activities and Services with an intent, you start fragments with fragment transactions. Secondly, your transaction isnt doing anything. Change it to something like:

FragmentTransaction transaction = getFragmentManager();
    transaction.beginTransaction()
        .replace(R.layout.container, newFragment) //<---replace a view in your layout (id: container) with the newFragment 
        .commit();

Another ViewGroup:

A fragment is a ViewGroup which can be shown in an Activity. But it needs a Container. The container can be any Layout (FragmeLayout, LinearLayout, etc. It does not matter).

Step 1:

Define Activity Layout:

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

Step 2:

Define Fragment Layout:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center"
    android:orientation="vertical">
    <EditText
       android:id="@+id/user"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"/>
    <EditText
       android:id="@+id/password"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:inputType="textPassword"/>
    <Button
       android:id="@+id/login"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
       android:text="Login"/>
</LinearLayout>

Step 3:

Create Fragment class

public class LoginFragment extends Fragment {


    private Button login;
    private EditText username, password;

    public static LoginFragment getInstance(String username){
       Bundle bundle = new Bundle();
       bundle.putInt("USERNAME", username);
       LoginFragment fragment = new LoginFragment();
       fragment.setArguments(bundle);
       return fragment;
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup parent, Bundle savedInstanceState){
       View view = inflater.inflate(R.layout.login_fragment, parent, false);
       login = view.findViewById(R.id.login);
       username = view.findViewById(R.id.user);
       password = view.findViewById(R.id.password);
       String name = getArguments().getInt("USERNAME");
       username.setText(username);
       return view;
    }

}

Step 4:

Add fragment in Activity

public class ActivityB extends AppCompatActivity{

   private Fragment currentFragment;

   @Override
   protected void onCreate(Bundle savedInstanceState) {

       super.onCreate(savedInstanceState);
       setContentView(R.layout.activity_main);

       currentFragment = LoginFragment.getInstance("Rohit");
       getSupportFragmentManager()
                .beginTransaction()
                .add(R.id.fragmentHolder, currentFragment, "LOGIN_TAG")
                .commit();
   }

}

Demo Project:

This is code is very basic. If you want to learn more advanced topics in Fragment then you can check out these resources:

Woof - Learn the fragment right way
My Android Garage

How to start fragment from Activity in Android Kotlin

Add Fragment:

val manager: FragmentManager = supportFragmentManager
val transaction: FragmentTransaction = manager.beginTransaction()
transaction.add(R.id.container,YOUR_FRAGMENT_NAME,YOUR_FRAGMENT_STRING_TAG)
transaction.addToBackStack(null)
transaction.commit()

Replace Fragment:

val manager: FragmentManager = supportFragmentManager
val transaction = manager.beginTransaction()
transaction.replace(R.id.container, YOUR_FRAGMENT_NAME, YOUR_FRAGMENT_STRING_TAG)
transaction.addToBackStack(null)
transaction.commit()

Here is more on fragment manager: https://developer.android.com/guide/fragments/fragmentmanager