API Key on BuildConfig

build.gradle

apply plugin: 'com.android.application'

android {
    compileSdkVersion 23
    buildToolsVersion "25.0.2"
    defaultConfig {
        applicationId "com.udacity.learn.android"
        minSdkVersion 21
        targetSdkVersion 23
        versionCode 1
        versionName "1.0"
        testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
        
        //Here, we are declaring our API Key!
        buildConfigField 'String', 'MY_API_KEY', "\"my_api_key_content\""
    }
    buildTypes {
        release {
            minifyEnabled false
            proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'

        }
    }
}

MainActivity.java

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.widget.Toast;

import com.udacity.learn.android.BuildConfig;
import com.udacity.learn.android.R;

public class MainActivity extends AppCompatActivity {

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

        //at this point, we are using the api key that we have defined in the build config!
        Toast.makeText(this, BuildConfig.MY_API_KEY, Toast.LENGTH_LONG).show();
    }
}