How do I return a list of users if I use the Firebase simple username & password authentication

When using email / password authentication in Firebase Authentication (previously known as Firebase SimpleLogin), your user's email and password combination is securely stored separately from the data actually stored in your Firebase.

This barrier between the data in your Firebase and your users' email / password hash combinations is by design: we want to make it easier for you to (1) develop your application, (2) prevent any accidental user credential leaks, and (3) still give you total flexibility with how to store your user data in Firebase.

That means that we only store the email address / password hash combination and nothing else, so it is up to you to decide how to store actual user data in your Firebase. As you suggested, you should be taking the user id and storing that data in your Firebase in a location such as /users/$id, and using the Firebase Security Rules Language to determine read / write access to that data. Your user's unique id and email are already in the auth variable you'll use when writing rules.

Here i created an Android program to do what Rob said for firebase beginner(like me) out there.this program first store the username of the signedUp or signedIn user and then display them in a listView

SignInActivity.java

public class SignInActivity extends BaseActivity implements View.OnClickListener,View.OnKeyListener{

    private DatabaseReference mDatabase;
    public static FirebaseAuth mAuth;
    private static final String TAG = "MainActivity";

    EditText usernameField;
    EditText passwordField;
    TextView changeSignUpModeTextView;
    Button signUpButton;
    ImageView logo;
    RelativeLayout relativeLayout;

    Boolean signUpModeActive;
    static ArrayList<String> userList = new ArrayList<>();

    @Override
    public void onStart() {
        super.onStart();

        // Check auth on Activity start
        if (mAuth.getCurrentUser() != null) {
            onAuthSuccess(mAuth.getCurrentUser());
        }
    }
    @Override
    public boolean onKey(View view, int i, KeyEvent keyEvent) {

        if(i == keyEvent.KEYCODE_ENTER && keyEvent.getAction() == keyEvent.ACTION_DOWN){
            signUpOrLogIn(view);
        }
         return false;
    }

    @Override
    public void onClick(View view) {

        if(view.getId() == R.id.changeSignUpMode){

            if (signUpModeActive == true){

                signUpModeActive = false;
                changeSignUpModeTextView.setText("Sign Up");
                signUpButton.setText("Log In");

            }else{

                signUpModeActive = true;
                changeSignUpModeTextView.setText("Log In");
                signUpButton.setText("Sign Up");
            }

        }else if(view.getId() == R.id.logo || view.getId() == R.id.relativeLayout){

            InputMethodManager inm = (InputMethodManager) getSystemService(INPUT_METHOD_SERVICE);
            inm.hideSoftInputFromWindow(getCurrentFocus().getWindowToken(),0);

        }

    }


    public void signUpOrLogIn(View view) {

        showProgressDialog();
        String email = usernameField.getText().toString().trim();
        String password = passwordField.getText().toString().trim();

        if (signUpModeActive == true) {
            mAuth.createUserWithEmailAndPassword(email,password)
                    .addOnCompleteListener(MainActivity.this, new OnCompleteListener<AuthResult>() {
                        @Override
                        public void onComplete(@NonNull Task<AuthResult> task) {
                            hideProgressDialog();
                            Toast.makeText(MainActivity.this, "createUserWithEmail:onComplete:" + task.isSuccessful(), Toast.LENGTH_SHORT).show();
                            // If sign in fails, display a message to the user. If sign in succeeds
                            // the auth state listener will be notified and logic to handle the
                            // signed in user can be handled in the listener.
                            if (!task.isSuccessful()) {
                                Toast.makeText(MainActivity.this, "Authentication failed." + task.getException().toString().substring(task.getException().toString().indexOf(" ")),
                                        Toast.LENGTH_SHORT).show();
                                Log.i("Error", task.getException().toString());
                            } else {
                                onAuthSuccess(task.getResult().getUser());
                                showUserList();
                            }
                        }
                    });
        } else {
            mAuth.signInWithEmailAndPassword(email,password)
                    .addOnCompleteListener(MainActivity.this, new OnCompleteListener<AuthResult>() {
                        @Override
                        public void onComplete(@NonNull Task<AuthResult> task) {
                            hideProgressDialog();
                            // If sign in fails, display a message to the user. If sign in succeeds
                            // the auth state listener will be notified and logic to handle the
                            // signed in user can be handled in the listener.
                            if (!task.isSuccessful()) {
                                // there was an error

                                Toast.makeText(MainActivity.this, task.getException().toString(),
                                        Toast.LENGTH_LONG).show();
                            } else

                            {
                                onAuthSuccess(task.getResult().getUser());
                                showUserList();
                            }
                        }
                    });
        }
    }

    public void showUserList(){
        startActivity(new Intent(getApplicationContext(), UserList.class));
        finish();
    }
    private void onAuthSuccess(FirebaseUser user) {
        String username = usernameFromEmail(user.getEmail());

        // Write new user
        writeNewUser(user.getUid(), username, user.getEmail());

        // Go to MainActivity

    }
    private String usernameFromEmail(String email) {
        if (email.contains("@")) {
            return email.split("@")[0];
        } else {
            return email;
        }
    }

    private void writeNewUser(String userId, String name, String email) {
        User user = new User(name, email);

        mDatabase.child("users").child(userId).setValue(user);
        ArrayList<String> userNames = new ArrayList<>();
        userNames.add(name);
        mDatabase.child("usernamelist").setValue(userNames);
    }


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

        mAuth = FirebaseAuth.getInstance();
        mDatabase = FirebaseDatabase.getInstance().getReference();


        if(mAuth.getCurrentUser()!=null){
            showUserList();
        }

        usernameField = (EditText) findViewById(R.id.username);
        passwordField = (EditText) findViewById(R.id.password);
        changeSignUpModeTextView = (TextView) findViewById(R.id.changeSignUpMode);
        signUpButton = (Button) findViewById(R.id.signupbutton);
        logo = (ImageView)findViewById(R.id.logo);
        relativeLayout= (RelativeLayout)findViewById(R.id.relativeLayout);

        signUpModeActive = true;

        changeSignUpModeTextView.setOnClickListener(this);

        usernameField.setOnKeyListener(this);
        passwordField.setOnKeyListener(this);

        logo.setOnClickListener(this);
        relativeLayout.setOnClickListener(this);



    }
}

UserList.java

public class UserList extends AppCompatActivity {

    private static final String TAG = "UserList" ;
    private DatabaseReference userlistReference;
    private ValueEventListener mUserListListener;
    ArrayList<String> usernamelist = new ArrayList<>();
    ArrayAdapter arrayAdapter;;

    ListView userListView;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_user_list);
        userlistReference = FirebaseDatabase.getInstance().getReference().child("usernamelist");
        onStart();
        userListView = (ListView) findViewById(R.id.userlistview);


    }

    @Override
    protected void onStart() {
        super.onStart();
        final ValueEventListener userListener = new ValueEventListener() {
            @Override
            public void onDataChange(DataSnapshot dataSnapshot) {
                usernamelist = new ArrayList<>((ArrayList) dataSnapshot.getValue());
                usernamelist.remove(usernameOfCurrentUser());
                Log.i(TAG, "onDataChange: "+usernamelist.toString());
                arrayAdapter = new ArrayAdapter(UserList.this,android.R.layout.simple_list_item_1,usernamelist);
                userListView.setAdapter(arrayAdapter);
            }

            @Override
            public void onCancelled(DatabaseError databaseError) {
                Log.w(TAG, "onCancelled: ",databaseError.toException());
                Toast.makeText(UserList.this, "Failed to load User list.",
                        Toast.LENGTH_SHORT).show();
            }
        };
        userlistReference.addValueEventListener(userListener);

        mUserListListener = userListener;
    }
    public String usernameOfCurrentUser()
    {
        String email = MainActivity.mAuth.getCurrentUser().getEmail();
        if (email.contains("@")) {
            return email.split("@")[0];
        } else {
            return email;
        }
    }
    @Override
    public void onStop() {
        super.onStop();

        // Remove post value event listener
        if (mUserListListener != null) {
            userlistReference.removeEventListener(mUserListListener);
        }

    }

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

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        switch(item.getItemId()) {
            case R.id.action_logout:
                FirebaseAuth.getInstance().signOut();
                startActivity(new Intent(this, MainActivity.class));
                finish();
                return true;
            default:
                return super.onOptionsItemSelected(item);
        }
    }
}

You can use Google Identity Toolkit API to get a list of all registered users in your Firebase project, this API is used by the Firebase CLI which can be accessed by running firebase auth:export results-file

Make sure Identity Toolkit API is enabled

firebase-users-list.js

const serviceAccount = require('path/to/firebase-sdk-json-service-account');

const googleapis = require('googleapis');
const identitytoolkit = googleapis.identitytoolkit('v3');

const authClient = new googleapis.auth.JWT(
    serviceAccount.client_email,
    null,
    serviceAccount.private_key,
    ['https://www.googleapis.com/auth/firebase'],
    null
);

authClient.authorize((err) => {
    if (err) {
        return console.error(err);
    }

    let nextPageToken = undefined;
    let users = [];
    const getAccounts = () => identitytoolkit.relyingparty.downloadAccount({
        auth: authClient,
        resource: {
            targetProjectId: serviceAccount.project_id,
            maxResults: 200,
            nextPageToken: nextPageToken
        }
    }, (e, results) => {
        if (e) {
            return console.error(err);
        }

        users = users.concat(results.users);
        if (results.nextPageToken) {
            nextPageToken = results.nextPageToken;
            return getAccounts();
        } else {
            console.log(users);
        }
    });
    getAccounts();
});