.java

import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;

import com.google.android.gms.tasks.OnCompleteListener;
import com.google.android.gms.tasks.Task;
import com.google.firebase.auth.AuthResult;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;

public class LoginActivity extends AppCompatActivity implements View.OnClickListener{

    private FirebaseAuth mAuth;
    private FirebaseAuth.AuthStateListener authStateListener;

    private EditText email;
    private EditText password;

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

        mAuth = FirebaseAuth.getInstance();
        authStateListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();
                if (user!=null){

                }
            }
        };

        email = (EditText)findViewById(R.id.email);
        password = (EditText)findViewById(R.id.password);

        findViewById(R.id.auth).setOnClickListener(this);
       findViewById(R.id.reg).setOnClickListener(this);


    }

    @Override
    public void onClick(View v){
        if (v.getId() ==  R.id.reg){
            createAccount(email.getText().toString(),password.getText().toString());
        }
        else if(v.getId() ==  R.id.auth){
            signIn(email.getText().toString(),password.getText().toString());
        }
    }

    private void createAccount(String email, String password){
        mAuth.createUserWithEmailAndPassword(email, password)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (task.isSuccessful()) {
                            Toast.makeText(getApplicationContext(),"Вы успешно зарегистрировались",Toast.LENGTH_SHORT).show();
                        } else {
                            Toast.makeText(getApplicationContext(),"Регистрация провалена",Toast.LENGTH_SHORT).show();
                        }

                    }
                });
    }

    private void signIn(String email, String password){
        mAuth.signInWithEmailAndPassword(email, password)
                .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                    @Override
                    public void onComplete(@NonNull Task<AuthResult> task) {
                        if (task.isSuccessful()) {
                            Toast.makeText(getApplicationContext(),"Вы успешно вошли",Toast.LENGTH_SHORT).show();
                            Intent intent = new Intent(getApplicationContext(),MainActivity.class);
                            startActivity(intent);
                        } else {
                            Toast.makeText(getApplicationContext(),"Пароль или логин неверные",Toast.LENGTH_SHORT).show();
                        }
                    }
                });
    }
}