First professional project I took on as an android developer. The use of paypal library, firebase backend server, internal SQLite database and google maps library.

app/src/debug/res/values/google_maps_api.xml

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+<resources>
+    <!--
+    TODO: Before you run your application, you need a Google Maps API key.
+
+    To get one, follow this link, follow the directions and press "Create" at the end:
+
+    https://console.developers.google.com/flows/enableapi?apiid=maps_android_backend&keyType=CLIENT_SIDE_ANDROID&r=00:C6:8D:18:DC:29:57:E8:E4:FB:FF:21:BD:13:7E:9F:1B:DA:97:00%3Bcom.example.h_mal.mochee
+
+    You can also add your credentials to an existing key, using these values:
+
+    Package name:
+    00:C6:8D:18:DC:29:57:E8:E4:FB:FF:21:BD:13:7E:9F:1B:DA:97:00
+
+    SHA-1 certificate fingerprint:
+    00:C6:8D:18:DC:29:57:E8:E4:FB:FF:21:BD:13:7E:9F:1B:DA:97:00
+
+    Alternatively, follow the directions here:
+    https://developers.google.com/maps/documentation/android/start#get-key
+
+    Once you have your key (it starts with "AIza"), replace the "google_maps_key"
+    string in this file.
+    -->
+    <string name="google_maps_key" templateMergeStrategy="preserve" translatable="false">AIzaSyCgLZP1YMHv_-Sri0H6_xE6pRKl9he2Lw8</string>
+</resources>

app/src/main/java/com/example/h_mal/mochee/Config.java

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+package com.example.h_mal.mochee;
+
+import android.view.View;
+import android.view.ViewGroup;
+import android.widget.EditText;
+
+/**
+ * Created by h_mal on 14/01/2018.
+ */
+
+public class Config {
+
+    private Config() {}
+
+    public static final String PAYPAL_CLIENT_ID = "AXp9Ak3gihwYll_W1ny-jee4FQfCZ4dknR7aSCKPSpQWi4HRYg2CHCcJ7zvUL2dBLEulTke67-msQUwL";
+
+    public static final void clearForm(ViewGroup group) {
+        for (int i = 0, count = group.getChildCount(); i < count; ++i) {
+            View view = group.getChildAt(i);
+            if (view instanceof EditText) {
+                ((EditText)view).setText("");
+            }
+
+            if(view instanceof ViewGroup && (((ViewGroup)view).getChildCount() > 0))
+                clearForm((ViewGroup)view);
+        }
+    }
+}

app/src/main/res/drawable/line.xml

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+<?xml version="1.0" encoding="utf-8"?>
+<selector xmlns:android="http://schemas.android.com/apk/res/android">
+    <stroke android:width="1dp" android:color="#0000FF"/>
+    <size android:height="50dp" />
+</selector>

app/src/release/res/values/google_maps_api.xml

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+<resources>
+    <!--
+    TODO: Before you release your application, you need a Google Maps API key.
+
+    To do this, you can either add your release key credentials to your existing
+    key, or create a new key.
+
+    Note that this file specifies the API key for the release build target.
+    If you have previously set up a key for the debug target with the debug signing certificate,
+    you will also need to set up a key for your release certificate.
+
+    Follow the directions here:
+
+    https://developers.google.com/maps/documentation/android/signup
+
+    Once you have your key (it starts with "AIza"), replace the "google_maps_key"
+    string in this file.
+    -->
+    <string name="google_maps_key" templateMergeStrategy="preserve" translatable="false">YOUR_KEY_HERE</string>
+</resources>