public class MainActivity extends AppCompatActivity {

    MaterialEditText edtNewUser, edtNewPassword, edtNewEmail;       //sign up
    MaterialEditText edtUser, edtPassword;  //for sign in

    Button btnSignUp, btnSignIn;


    FirebaseDatabase database;
    DatabaseReference users;


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

        //Firebase
        database= FirebaseDatabase.getInstance();
        if(){
            users = database.getReference("Students");
        }
        else {
            users = database.getReference("Teachers");
        }


        edtUser = (MaterialEditText)findViewById(R.id.edtUser);
        edtPassword = (MaterialEditText)findViewById(R.id.edtPassword);

        btnSignIn = (Button)findViewById(R.id.btn_sign_in);
        btnSignUp = (Button)findViewById(R.id.btn_sign_up);

        btnSignUp.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                showSignUpDialog();
            }
        });

        btnSignIn.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                signIn(edtUser.getText().toString(), edtPassword.getText().toString());
            }
        });

    }

    private void signIn(final String user,final String pwd) {
        users.addListenerForSingleValueEvent(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                if(dataSnapshot.child(user).exists())
                {
                    if(!user.isEmpty())
                    {
                        User login = dataSnapshot.child(user).getValue(User.class);
                        if(login.getPassword().equals(pwd))
                        {
                            Toast.makeText(MainActivity.this, "Login ok!", Toast.LENGTH_SHORT).show();
                        }
                        else
                        {
                            Toast.makeText(MainActivity.this, "Wrong password", Toast.LENGTH_SHORT).show();
                        }
                    }
                    else
                    {
                        Toast.makeText(MainActivity.this, "Please enter your user name", Toast.LENGTH_SHORT).show();
                    }
                }
                else
                {
                    Toast.makeText(MainActivity.this, "User is not existing!", Toast.LENGTH_SHORT).show();
                }
            }

            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
    }

    private void showSignUpDialog() {
        AlertDialog.Builder alertDialog = new AlertDialog.Builder(MainActivity.this);
        alertDialog.setTitle("Sign up");
        alertDialog.setMessage("Please fill full information");

        LayoutInflater inflater = this.getLayoutInflater();
        View sign_up_layout = inflater.inflate(R.layout.sign_up_layout, null);

        edtNewUser = (MaterialEditText)sign_up_layout.findViewById(R.id.edtNewUserName);
        edtNewPassword = (MaterialEditText)sign_up_layout.findViewById(R.id.edtNewPassword);
        edtNewEmail = (MaterialEditText)sign_up_layout.findViewById(R.id.edtNewEmail);

        alertDialog.setView(sign_up_layout);


        alertDialog.setNegativeButton("NO", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialogInterface, int i) {
                dialogInterface.dismiss();
            }
        });

        alertDialog.setPositiveButton("YES", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialogInterface, int i) {
               final User user = new User(edtNewUser.getText().toString(), edtNewPassword.getText().toString(), edtNewEmail.getText().toString());

                users.addListenerForSingleValueEvent(new ValueEventListener() {
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                        if(dataSnapshot.child(user.getUserName()).exists()){
                            Toast.makeText(MainActivity.this, "User already exists!", Toast.LENGTH_SHORT).show();
                        }
                        else {
                           users.child(user.getUserName()).setValue(user);
                            Toast.makeText(MainActivity.this, "User registration success!", Toast.LENGTH_SHORT).show();
                        }
                    }

                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) {

                    }
                });
                dialogInterface.dismiss();
            }
        });
        alertDialog.show();
    }
}


****************************************

package com.cristi.logintest.Model;

public class User {

    private String userName;
    private String password;
    private String email;

    public User(){
    }

    public User(String userName, String password, String email) {
        this.userName = userName;
        this.password = password;
        this.email = email;
    }

    public String getUserName() {
        return userName;
    }

    public void setUserName(String userName) {
        this.userName = userName;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }
}


*************************************

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@color/colorPrimary"
    tools:context=".MainActivity">


        <RelativeLayout

            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:cardElevation="4dp"
            android:layout_margin="8dp"
            app:layout_constraintBottom_toBottomOf="parent"
            app:layout_constraintLeft_toLeftOf="parent"
            app:layout_constraintRight_toRightOf="parent"
            app:layout_constraintTop_toTopOf="parent" >

            <android.support.v7.widget.CardView
                android:id="@+id/info_login"
                xmlns:android="http://schemas.android.com/apk/res/android"
                xmlns:app="http://schemas.android.com/apk/res-auto"
                android:layout_width="match_parent"
                android:layout_height="wrap_content"
                android:layout_margin="8dp"
                app:cardElevation="4dp">
            <LinearLayout
                android:padding="16dp"
                android:orientation="vertical"
                android:layout_width="match_parent"
                android:layout_height="wrap_content">

                <com.rengwuxian.materialedittext.MaterialEditText
                    android:id="@+id/edtUser"
                    android:hint="User name"
                    android:textColorHint="@color/colorPrimary"
                    android:textColor="@color/colorPrimary"
                    android:textSize="24sp"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    app:met_baseColor="@color/colorPrimary"
                    app:met_floatingLabel="highlight"
                    app:met_primaryColor="@color/colorPrimary"
                    app:met_singleLineEllipsis="true"
                    />

                <com.rengwuxian.materialedittext.MaterialEditText
                    android:id="@+id/edtPassword"
                    android:hint="Password"
                    android:textColorHint="@color/colorPrimary"
                    android:textColor="@color/colorPrimary"
                    android:textSize="24sp"
                    android:inputType="textPassword"
                    android:layout_width="match_parent"
                    android:layout_height="wrap_content"
                    app:met_baseColor="@color/colorPrimary"
                    app:met_floatingLabel="highlight"
                    app:met_primaryColor="@color/colorPrimary"
                    app:met_singleLineEllipsis="true"
                    />




            </LinearLayout>
            </android.support.v7.widget.CardView>
            <LinearLayout
                android:layout_below="@id/info_login"
                android:orientation="horizontal"
                android:weightSum="2"
                android:layout_margin="8dp"
                android:layout_width="match_parent"
                android:layout_height="wrap_content">

                <Button
                    android:id="@+id/btn_sign_up"
                    android:text="Sign up"
                    style="@style/Widget.AppCompat.Button.Colored"
                    android:layout_weight="1"
                    android:layout_width="0dp"
                    android:layout_height="wrap_content" />

                <Button
                    android:id="@+id/btn_sign_in"
                    android:text="Sign in"
                    style="@style/Widget.AppCompat.Button.Colored"
                    android:layout_weight="1"
                    android:layout_width="0dp"
                    android:layout_height="wrap_content" />


            </LinearLayout>

        </RelativeLayout>



</android.support.constraint.ConstraintLayout>


**************************************

<?xml version="1.0" encoding="utf-8"?>
<android.support.v7.widget.CardView
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_margin="8dp"
    app:cardElevation="4dp">

    <LinearLayout
        android:orientation="vertical"
        android:layout_width="match_parent"
        android:layout_height="wrap_content">

        <com.rengwuxian.materialedittext.MaterialEditText
            android:id="@+id/edtNewUserName"
            android:hint="User name"
            android:textColorHint="@color/colorPrimary"
            android:textColor="@color/colorPrimary"
            android:textSize="24sp"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:met_baseColor="@color/colorPrimary"
            app:met_floatingLabel="highlight"
            app:met_primaryColor="@color/colorPrimary"
            app:met_singleLineEllipsis="true"
            />

        <com.rengwuxian.materialedittext.MaterialEditText
            android:id="@+id/edtNewPassword"
            android:hint="Password"
            android:textColorHint="@color/colorPrimary"
            android:textColor="@color/colorPrimary"
            android:textSize="24sp"
            android:inputType="textPassword"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:met_baseColor="@color/colorPrimary"
            app:met_floatingLabel="highlight"
            app:met_primaryColor="@color/colorPrimary"
            app:met_singleLineEllipsis="true"
            />

        <com.rengwuxian.materialedittext.MaterialEditText
            android:id="@+id/edtNewEmail"
            android:hint="Email"
            android:textColorHint="@color/colorPrimary"
            android:textColor="@color/colorPrimary"
            android:textSize="24sp"
            android:inputType="textEmailAddress"
            android:layout_width="match_parent"
            android:layout_height="wrap_content"
            app:met_baseColor="@color/colorPrimary"
            app:met_floatingLabel="highlight"
            app:met_primaryColor="@color/colorPrimary"
            app:met_singleLineEllipsis="true"
            />


    </LinearLayout>

</android.support.v7.widget.CardView>