public class MainActivity extends AppCompatActivity {

    private AlertDialog.Builder builder;
    private AlertDialog dialog;
    private EditText editText;
    private Button saveButton;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        final List<MyListData> myListData = new ArrayList<MyListData>();
        final RecyclerView recyclerView = (RecyclerView) findViewById(R.id.recyclerView);

        myListData.add(new MyListData("Email", android.R.drawable.ic_dialog_email));
        myListData.add(new MyListData("Info", android.R.drawable.ic_dialog_info));
        myListData.add(new MyListData("Delete", android.R.drawable.ic_delete));
        myListData.add(new MyListData("Dialer", android.R.drawable.ic_dialog_dialer));
        myListData.add(new MyListData("Alert", android.R.drawable.ic_dialog_alert));
        myListData.add(new MyListData("Map", android.R.drawable.ic_dialog_map));

        final MyListAdapter adapter = new MyListAdapter(myListData);

        recyclerView.setHasFixedSize(true);
        recyclerView.setLayoutManager(new LinearLayoutManager(this));
        recyclerView.setAdapter(adapter);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {

            public void onClick(View v) {

                builder = new AlertDialog.Builder(MainActivity.this);
                View view = getLayoutInflater().inflate(R.layout.popup, null);

                editText = view.findViewById(R.id.editText);
                saveButton = view.findViewById(R.id.saveButton);
                saveButton.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {

                        String s = editText.getText().toString();

                        dialog.dismiss();

                        if (!s.isEmpty()) {

                            myListData.add(new MyListData(s, android.R.drawable.ic_dialog_alert));
                            final MyListAdapter adapter = new MyListAdapter(myListData);
                            recyclerView.setAdapter(adapter);

                        }

                    }
                });

                builder.setView(view);
                dialog = builder.create();
                dialog.show();

            }

        });

    }

}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/layout_id"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <androidx.appcompat.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:background="?attr/colorPrimary"
        android:minHeight="?attr/actionBarSize"
        android:theme="?attr/actionBarTheme">

        <TextView
            android:id="@+id/title"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            android:gravity="center"
            android:text="@string/enter_item"
            android:textAppearance="@style/TextAppearance.AppCompat.Medium"
            android:textSize="18sp" />

    </androidx.appcompat.widget.Toolbar>

    <androidx.cardview.widget.CardView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content">

        <RelativeLayout

            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:padding="24dp">

            <EditText
                android:id="@+id/editText"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_alignParentTop="true"
                android:layout_marginTop="19dp"
                android:autofillHints=""
                android:hint="@string/hint_item"
                android:textColorHighlight="@color/colorPrimary"
                android:textColorLink="@color/colorPrimaryDark"
                android:textCursorDrawable="@color/colorPrimaryDark" />

            <Button
                android:id="@+id/saveButton"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_below="@id/editText"
                android:layout_marginTop="15dp"
                android:layout_marginEnd="5dp"
                android:background="?attr/colorPrimary"
                android:text="@string/save_button"
                android:textColor="@android:color/white"
                android:textSize="14sp"
                android:textStyle="bold" />


        </RelativeLayout>

    </androidx.cardview.widget.CardView>


</LinearLayout>

<resources>
    <string name="app_name">RVPTH</string>
    <string name="enter_item">Toevoegen</string>
    <string name="hint_item">Vul in...</string>
    <string name="save_button">OK</string>
</resources>