MainActivity.java

package com.example.derm_ezy;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;

import com.google.android.gms.auth.api.signin.GoogleSignInClient;
import com.google.firebase.auth.FirebaseAuth;
import com.google.firebase.auth.FirebaseUser;

public class MainActivity extends AppCompatActivity {

    private FirebaseAuth mAuth;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        mAuth = FirebaseAuth.getInstance();
    }
    @Override
    public void onStart() {
        super.onStart();
        // Check if user is signed in (non-null) and update UI accordingly.
        FirebaseUser currentUser = mAuth.getCurrentUser();

        if (currentUser == null){
            Intent intent = new Intent(MainActivity.this,GoogleLogin.class);
            startActivity(intent);
            finish();
        }
    }
}