An useful util to handle nested collection in Java

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import java.util.HashMap; import java.util.HashSet; import java.util.Map; import java.util.Set;   public class CommonUtil {       public static <K, V, O extends V> V getAndCreateIfNotExist(K key, Map<K, V> map, Class<O> c) {         V result = map.get(key);         if (result == null) {             synchronized (map) {                 result = map.get(key);                 if (result == null) {                     try {                         result = c.newInstance();                     } catch (Exception e) {                         throw new RuntimeException("Error in getAndCreateIfNotExist", e);                     }                       map.put(key, result);                 }             }         }         return result;     }     // Test     public static void main(String[] args) {         Map<String, Set<String>> phoneNumbersPerName = new HashMap<>();         String personName = "Alex";         Set<String> phoneNumbers = getAndCreateIfNotExist(personName, phoneNumbersPerName, HashSet.class);         phoneNumbers.add("01234567891");         phoneNumbers.add("09090909090");         System.out.println(phoneNumbersPerName);         // {Alex=[09090909090, 01234567891]}     } }

An useful util to separate a list to multiple sublist in Java

Assume that you need to group/separate a List of object to multiple sublist with an specified size, below use should be useful.

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import java.util.LinkedList; import java.util.List;   public class CommonUtil {     public static <T> List<List<T>> group(List<T> input, int bulkSize) {         List<List<T>> result = new LinkedList<>();         int from = 0;         int to = bulkSize;         int size = input.size();           while (to < size) {             result.add(input.subList(from, to));             from = to;             to += bulkSize;         }           if (from < size) {             result.add(input.subList(from, size));         }         return result;     }   }

Java Send HTTP POST Request with x-www-form-urlencoded Format

The key thing is that we need to use the util URLEncoder to encode parameter name and its value.

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import java.io.BufferedReader; import java.io.DataOutputStream; import java.io.IOException; import java.io.InputStreamReader; import java.net.HttpURLConnection; import java.net.URL; import java.net.URLEncoder; import java.util.Map; import java.util.Map.Entry;   public class HttpRequestSender {     public static int requestSender(String url, Map<String, Object> params, int connectionTimeout, int readTimeout, StringBuilder response) throws IOException {         int responseCode = 0;         if (params != null) {             StringBuilder postData = new StringBuilder();             String paramSeparator = "";             for (Entry<String, Object> entry : params.entrySet()) {                 postData.append(paramSeparator);                 postData.append(URLEncoder.encode(entry.getKey(), "UTF-8")).append('=').append(URLEncoder.encode(String.valueOf(entry.getValue()), "UTF-8"));                 paramSeparator = "&";             }             byte[] postDataBytes = postData.toString().getBytes("UTF-8");             URL urlObj = new URL(url);             HttpURLConnection conn = (HttpURLConnection) urlObj.openConnection();             conn.setRequestMethod("POST");             conn.setInstanceFollowRedirects(false);             conn.setRequestProperty("Content-Type", "application/x-www-form-urlencoded");             conn.setRequestProperty("charset", "UTF-8");             conn.setRequestProperty("content-length", String.valueOf(postDataBytes.length));             conn.setDoOutput(true);             conn.setConnectTimeout(connectionTimeout);             conn.setReadTimeout(readTimeout);             try (DataOutputStream wr = new DataOutputStream(conn.getOutputStream())) {                 wr.write(postDataBytes);                 wr.flush();             }             responseCode = conn.getResponseCode();             if (response != null) {                 try (BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()))) {                     String line = reader.readLine();                     while (line != null) {                         response.append(line);                         line = reader.readLine();                     }                 }             }         }         return responseCode;     } }