Implemented basic version.

.gitignore

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+gradlew*
+.gradle/*
+gradle/*
+.idea/*
+out/*

build.gradle

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+plugins {
+    id 'java'
+}
+
+group 'com.github.prominence'
+version '1.0-SNAPSHOT'
+
+sourceCompatibility = 1.8
+
+repositories {
+    mavenCentral()
+}
+
+dependencies {
+    compile "org.telegram:telegrambots:4.1.2"
+    compile group: 'org.apache.logging.log4j', name: 'log4j-api', version: '2.11.2'
+    compile group: 'org.apache.logging.log4j', name: 'log4j-core', version: '2.11.2'
+    compile group: 'com.alibaba', name: 'fastjson', version: '1.2.44'
+}

settings.gradle

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+rootProject.name = 'randomkitties-telegram-bot'
+

src/main/java/com/github/prominence/randomkitties/bot/Main.java

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+/*
+ * Copyright (c) 2019 Alexey Zinchenko
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ */
+
+package com.github.prominence.randomkitties.bot;
+
+import com.github.prominence.randomkitties.bot.handlers.RandomKittiesBot;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.telegram.telegrambots.ApiContextInitializer;
+import org.telegram.telegrambots.meta.TelegramBotsApi;
+import org.telegram.telegrambots.meta.exceptions.TelegramApiException;
+
+public class Main {
+
+    public static final Logger log = LogManager.getLogger(Main.class);
+
+    public static void main(String[] args) {
+        ApiContextInitializer.init();
+        log.info("API context was initialized.");
+
+        TelegramBotsApi telegramBotsApi = new TelegramBotsApi();
+        try {
+            telegramBotsApi.registerBot(new RandomKittiesBot());
+        } catch (TelegramApiException ex) {
+            ex.printStackTrace();
+        }
+    }
+}

src/main/java/com/github/prominence/randomkitties/bot/handlers/RandomKittiesBot.java

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+/*
+ * Copyright (c) 2019 Alexey Zinchenko
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ */
+
+package com.github.prominence.randomkitties.bot.handlers;
+
+import com.github.prominence.randomkitties.bot.model.Kitty;
+import com.github.prominence.randomkitties.bot.util.CatUtils;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+import org.telegram.telegrambots.bots.TelegramLongPollingBot;
+import org.telegram.telegrambots.meta.api.methods.send.SendMediaGroup;
+import org.telegram.telegrambots.meta.api.methods.send.SendMessage;
+import org.telegram.telegrambots.meta.api.objects.Message;
+import org.telegram.telegrambots.meta.api.objects.Update;
+import org.telegram.telegrambots.meta.api.objects.media.InputMedia;
+import org.telegram.telegrambots.meta.api.objects.media.InputMediaPhoto;
+import org.telegram.telegrambots.meta.exceptions.TelegramApiException;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class RandomKittiesBot extends TelegramLongPollingBot {
+
+    private static final Logger log = LogManager.getLogger(RandomKittiesBot.class);
+
+    @Override
+    public void onUpdateReceived(Update update) {
+        if (update.hasMessage()) {
+            final Message message = update.getMessage();
+            if (message.hasText()) {
+
+                final String requestText = message.getText();
+                final String sender = message.getFrom().toString();
+
+                final Long chatId = message.getChatId();
+
+                logCommunication(sender, requestText);
+
+                try {
+                    switch (requestText) {
+                        case "/kitty":
+                        case "/kitty@RandomKittiesBot":
+                            sendPhotosResponse(chatId, 1);
+                            break;
+                        case "/more_kitties":
+                        case "/more_kitties@RandomKittiesBot":
+                            sendPhotosResponse(chatId, 3);
+                            break;
+                        case "/moooooore_kitties":
+                        case "/moooooore_kitties@RandomKittiesBot":
+                            sendPhotosResponse(chatId, 5);
+                            break;
+                        case "/help":
+                        case "/help@RandomKittiesBot":
+                        default:
+                            sendHelp(chatId);
+                            break;
+                    }
+                } catch (TelegramApiException ex) {
+                    ex.printStackTrace();
+                    log.error(ex);
+                }
+            }
+        }
+    }
+
+    @Override
+    public String getBotUsername() {
+        return "RandomKittiesBot";
+    }
+
+    @Override
+    public String getBotToken() {
+        return "<YOUR BOT API KEY FROM BotFather>";
+    }
+
+    private void sendHelp(Long chatId) throws TelegramApiException {
+        execute(new SendMessage(chatId, "Please, use on of the next commands: /kitty, /more_kitties, /moooooore_kitties"));
+    }
+
+    private void sendPhotosResponse(Long chatId, int amount) throws TelegramApiException {
+        SendMediaGroup mediaGroup = new SendMediaGroup().setChatId(chatId);
+        List<InputMedia> mediaList = new ArrayList<>(amount);
+
+        final List<Kitty> kittiesList = CatUtils.getRandomCatsList(amount);
+
+        kittiesList.forEach(kitty -> mediaList.add(new InputMediaPhoto(kitty.getUrl(), null)));
+
+        mediaGroup.setMedia(mediaList);
+
+        execute(mediaGroup);
+    }
+
+    private void logCommunication(String sender, String requestText) {
+        StringBuilder builder = new StringBuilder();
+        builder.append("\n=========== REQUEST ===========\n");
+        builder.append("Sender : ");
+        builder.append(sender);
+        builder.append('\n');
+        builder.append("Request text: ");
+        builder.append(requestText);
+        builder.append('\n');
+
+        log.debug(builder.toString());
+    }
+
+}

src/main/java/com/github/prominence/randomkitties/bot/model/Kitty.java

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+/*
+ * Copyright (c) 2019 Alexey Zinchenko
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ */
+
+package com.github.prominence.randomkitties.bot.model;
+
+public class Kitty {
+
+    private String id;
+    private String url;
+
+    public Kitty() {
+    }
+
+    public Kitty(String id, String url) {
+        this.id = id;
+        this.url = url;
+    }
+
+    public String getId() {
+        return id;
+    }
+
+    public void setId(String id) {
+        this.id = id;
+    }
+
+    public String getUrl() {
+        return url;
+    }
+
+    public void setUrl(String url) {
+        this.url = url;
+    }
+}

src/main/java/com/github/prominence/randomkitties/bot/util/CatUtils.java

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+/*
+ * Copyright (c) 2019 Alexey Zinchenko
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ *
+ */
+
+package com.github.prominence.randomkitties.bot.util;
+
+import com.github.prominence.randomkitties.bot.model.Kitty;
+import org.apache.logging.log4j.LogManager;
+import org.apache.logging.log4j.Logger;
+
+import java.io.IOException;
+import java.io.InputStream;
+import java.net.HttpURLConnection;
+import java.net.URL;
+import java.util.List;
+
+public class CatUtils {
+
+    public static final Logger log = LogManager.getLogger(CatUtils.class);
+
+    private static final String API_URL = "https://api.thecatapi.com/v1/images/search?api_key=<YOUR API KEY FROM THECATAPI.COM>";
+
+    public static List<Kitty> getRandomCatsList() {
+        return getRandomCatsList(1);
+    }
+
+    public static List<Kitty> getRandomCatsList(int amount) {
+        try {
+            final String spec = API_URL + "&limit=" + amount;
+
+            log.debug("Making request: " + spec);
+
+            HttpURLConnection connection = (HttpURLConnection) new URL(spec).openConnection();
+            connection.setRequestMethod("GET");
+            final int responseCode = connection.getResponseCode();
+            if (responseCode != HttpURLConnection.HTTP_OK) {
+                throw new RuntimeException("Bad response.");
+            }
+
+            final InputStream inputStream = connection.getInputStream();
+            return JSONUtils.parseJSON(inputStream);
+        } catch (IOException e) {
+            throw new RuntimeException(e);
+        }
+    }
+}

src/main/java/com/github/prominence/randomkitties/bot/util/JSONUtils.java

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+/*
+ * Copyright (c) 2019 Alexey Zinchenko
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+package com.github.prominence.randomkitties.bot.util;
+
+import com.alibaba.fastjson.JSON;
+import com.alibaba.fastjson.TypeReference;
+import com.github.prominence.randomkitties.bot.model.Kitty;
+
+import java.io.BufferedReader;
+import java.io.IOException;
+import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.util.List;
+
+public final class JSONUtils {
+
+    private JSONUtils() {}
+
+    public static List<Kitty> parseJSON(InputStream inputStream) throws IOException {
+        TypeReference<List<Kitty>> typeRef = new TypeReference<List<Kitty>>() {};
+        return JSON.parseObject(getStringFromStream(inputStream), typeRef);
+    }
+
+    private static String getStringFromStream(InputStream inputStream) throws IOException {
+        BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream));
+
+        StringBuilder result = new StringBuilder();
+        String line;
+        while ((line = reader.readLine()) != null) {
+            result.append(line);
+        }
+
+        reader.close();
+
+        return result.toString();
+    }
+}

src/main/resources/log4j2.properties

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+status = error
+name = PropertiesConfig
+
+filters = threshold
+
+filter.threshold.type = ThresholdFilter
+filter.threshold.level = debug
+
+appenders = console
+
+appender.console.type = Console
+appender.console.name = STDOUT
+appender.console.layout.type = PatternLayout
+appender.console.layout.pattern = %d{yyyy-MM-dd HH:mm:ss} %-5p %c{1}:%L - %m%n
+
+rootLogger.level = debug
+rootLogger.appenderRefs = stdout
+rootLogger.appenderRef.stdout.ref = STDOUT