main.java

```java
package br.com.firstsoft.escolasbrasil.activitiy;

import android.content.Intent;
import android.support.annotation.NonNull;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
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;

import java.io.Serializable;

import br.com.firstsoft.escolasbrasil.R;

public class MainActivity extends AppCompatActivity {

    private Button btnEntrar;
    private Button btnCadastrar;

    private EditText textEmail;
    private EditText textSenha;

    private FirebaseAuth firebaseAuth;
    private FirebaseAuth.AuthStateListener firebaseAuthListener;

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

        firebaseAuth = FirebaseAuth.getInstance();
        configureAuthListener();

        btnEntrar = (Button) findViewById(R.id.btn_entrar);
        btnCadastrar = (Button) findViewById(R.id.btn_cadastrar);
        textEmail = (EditText) findViewById(R.id.input_login_email);
        textSenha = (EditText) findViewById(R.id.input_login_senha);
    }

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

        firebaseAuth.addAuthStateListener(firebaseAuthListener);

        btnEntrar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if ( !(textSenha.toString().isEmpty() && textEmail.toString().isEmpty()) ) {
                    doLogin(textEmail.getText().toString(), textSenha.getText().toString());
                }
            }
        });

        btnCadastrar.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity.this, CadastrarActivity.class);
                startActivity(intent);
            }
        });
    }

    @Override
    public void onStop() {
        super.onStop();
        if (firebaseAuthListener != null) {
            firebaseAuth.removeAuthStateListener(firebaseAuthListener);
        }
    }

    public void configureAuthListener() {
        firebaseAuthListener = new FirebaseAuth.AuthStateListener() {
            @Override
            public void onAuthStateChanged(@NonNull FirebaseAuth firebaseAuth) {
                FirebaseUser user = firebaseAuth.getCurrentUser();
                if (user != null) {
                    Intent intent = new Intent(MainActivity.this, HomeActivity.class);
                    startActivity(intent);
                } else {
                    Log.d("FIREBASE AUTH", "onAuthStateChanged:signed_out");
                }
            }
        };
    }

    public void doLogin(String email, String senha) {
        firebaseAuth.signInWithEmailAndPassword(email, senha).addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if (!task.isSuccessful()) {
                    Toast.makeText(MainActivity.this, task.getException().getMessage(), Toast.LENGTH_LONG).show();
                }
            }
        });
    }
}
```