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@Override
public boolean onCreateOptionsMenu(Menu menu) {
    getMenuInflater().inflate(R.menu.menu_main, menu);
    return super.onCreateOptionsMenu(menu);
}

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

    <TextView
        android:id="@+id/txt_name"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"

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...
<android.support.design.widget.TextInputLayout
    android:id="@+id/input_name"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_below="@+id/toolbar">

    <EditText
        android:id="@+id/name"
        android:layout_width="match_parent"

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<ListView
    android:id="@+id/list_data"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_below="@+id/input_email">
</ListView>

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...
<ProgressBar
    android:id="@+id/circular_progress"
    android:visibility="invisible"
    android:layout_centerInParent="true"
    android:theme="@style/CircularProgress"
    style="@style/Widget.AppCompat.ProgressBar"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" />
...

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public class User {
    String uid, name, email;

    public User() {
    }

    public User(String uid, String name, String email) {
        this.uid = uid;
        this.name = name;
        this.email = email;

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public class ListViewAdapter extends BaseAdapter {

    Activity activity;
    List<User> listUsers;
    LayoutInflater inflater;
    
    public ListViewAdapter(Activity activity, List<User> listUsers) {
        this.activity = activity;
        this.listUsers = listUsers;
    }

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public class MainActivity extends AppCompatActivity {

    EditText input_name, input_email;
    private ListView list_data;
    private ProgressBar circular_progress;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        ...

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private FirebaseDatabase mFirebaseDatabase;
private DatabaseReference mDatabaseReference;

@Override
protected void onCreate(Bundle savedInstanceState) {

    ...
    
    //Firebase
    initFirebase(); 

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...
//добавляем List, который будет хранить записи из БД
private List<User> list_users = new ArrayList<>();

@Override
protected void onCreate(Bundle savedInstanceState) {

    ...
    
    //Firebase