code: ///////////////////////////////////////////////////////////////////////////////////////////////
  // checkstyle: Checks Java source code and other text files for adherence to a set of rules.
  // Copyright (C) 2001-2023 the original author or authors.
  //
  // This library is free software; you can redistribute it and/or
  // modify it under the terms of the GNU Lesser General Public
  // License as published by the Free Software Foundation; either
  // version 2.1 of the License, or (at your option) any later version.
  //
  // This library is distributed in the hope that it will be useful,
  // but WITHOUT ANY WARRANTY; without even the implied warranty of
  // MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  // Lesser General Public License for more details.
  //
  // You should have received a copy of the GNU Lesser General Public
  // License along with this library; if not, write to the Free Software
  // Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
  ///////////////////////////////////////////////////////////////////////////////////////////////
  
  package com.puppycrawl.tools.checkstyle.checks;
  
  import java.io.File;
  import java.io.IOException;
  import java.io.InputStream;
  import java.nio.file.Files;
  import java.util.HashMap;
  import java.util.Map;
  import java.util.Map.Entry;
  import java.util.Properties;
  import java.util.concurrent.atomic.AtomicInteger;
  import java.util.regex.Matcher;
  import java.util.regex.Pattern;
  
  import com.puppycrawl.tools.checkstyle.StatelessCheck;
  import com.puppycrawl.tools.checkstyle.api.AbstractFileSetCheck;
  import com.puppycrawl.tools.checkstyle.api.FileText;
  
  /**
   * <p>
   * Detects duplicated keys in properties files.
   * </p>
   * <p>
   * Rationale: Multiple property keys usually appear after merge or rebase of
   * several branches. While there are no problems in runtime, there can be a confusion
   * due to having different values for the duplicated properties.
   * </p>
   * <ul>
   * <li>
   * Property {@code fileExtensions} - Specify file type extension of the files to check.
   * Type is {@code java.lang.String[]}.
   * Default value is {@code .properties}.
   * </li>
   * </ul>
   * <p>
   * To configure the check:
   * </p>
   * <pre>
   * &lt;module name=&quot;UniqueProperties&quot;/&gt;
   * </pre>
   * <p>
   * Example: in foo.properties file
   * </p>
   * <pre>
   * key.one=44
   * key.two=32 // OK
   * key.one=54 // violation
   * </pre>
   * <p>
   * To configure the check to scan custom file extensions:
   * </p>
   * <pre>
   * &lt;module name=&quot;UniqueProperties&quot;&gt;
   *  &lt;property name=&quot;fileExtensions&quot; value=&quot;customProperties&quot;/&gt;
   * &lt;/module&gt;
   * </pre>
   * <p>
   * Example: in foo.customProperties file
   * </p>
   * <pre>
   * key.one=44
   * key.two=32 // OK
   * key.one=54 // violation
   * </pre>
   * <p>
   * Example: in foo.properties file
   * </p>
   * <pre>
   * key.one=44
   * key.two=32 // OK
   * key.one=54 // OK, file is not checked
   * </pre>
   * <p>
   * Parent is {@code com.puppycrawl.tools.checkstyle.Checker}
   * </p>
   * <p>
   * Violation Message Keys:
   * </p>
   * <ul>
   * <li>
   * {@code properties.duplicate.property}
   * </li>
   * <li>
   * {@code unable.open.cause}
   * </li>
   * </ul>
   *
   * @since 5.7
   */
  @StatelessCheck
  public class UniquePropertiesCheck extends AbstractFileSetCheck {
  
      /**
       * Localization key for check violation.
       */
      public static final String MSG_KEY = "properties.duplicate.property";
      /**
       * Localization key for IO exception occurred on file open.
       */
      public static final String MSG_IO_EXCEPTION_KEY = "unable.open.cause";
  
      /**
       * Pattern matching single space.
       */
      private static final Pattern SPACE_PATTERN = Pattern.compile(" ");
  
      /**
       * Construct the check with default values.
       */
      public UniquePropertiesCheck() {
          setFileExtensions("properties");
      }
  
      @Override
      protected void processFiltered(File file, FileText fileText) {
          final UniqueProperties properties = new UniqueProperties();
          try (InputStream inputStream = Files.newInputStream(file.toPath())) {
              properties.load(inputStream);
          }
          catch (IOException ex) {
              log(1, MSG_IO_EXCEPTION_KEY, file.getPath(),
                      ex.getLocalizedMessage());
          }
  
          for (Entry<String, AtomicInteger> duplication : properties
                  .getDuplicatedKeys().entrySet()) {
              final String keyName = duplication.getKey();
              final int lineNumber = getLineNumber(fileText, keyName);
              // Number of occurrences is number of duplications + 1
              log(lineNumber, MSG_KEY, keyName, duplication.getValue().get() + 1);
          }
      }
  
      /**
       * Method returns line number the key is detected in the checked properties
       * files first.
       *
       * @param fileText
       *            {@link FileText} object contains the lines to process
       * @param keyName
       *            key name to look for
       * @return line number of first occurrence. If no key found in properties
       *         file, 1 is returned
       */
      private static int getLineNumber(FileText fileText, String keyName) {
          final Pattern keyPattern = getKeyPattern(keyName);
          int lineNumber = 1;
          final Matcher matcher = keyPattern.matcher("");
          for (int index = 0; index < fileText.size(); index++) {
              final String line = fileText.get(index);
              matcher.reset(line);
              if (matcher.matches()) {
                  break;
              }
              ++lineNumber;
          }
          // -1 as check seeks for the first duplicate occurrence in file,
          // so it cannot be the last line.
          if (lineNumber > fileText.size() - 1) {
              lineNumber = 1;
          }
          return lineNumber;
      }
  
      /**
       * Method returns regular expression pattern given key name.
       *
       * @param keyName
       *            key name to look for
       * @return regular expression pattern given key name
       */
      private static Pattern getKeyPattern(String keyName) {
          final String keyPatternString = "^" + SPACE_PATTERN.matcher(keyName)
                  .replaceAll(Matcher.quoteReplacement("\\\\ ")) + "[\\s:=].*$";
          return Pattern.compile(keyPatternString);
      }
  
      /**
       * Properties subclass to store duplicated property keys in a separate map.
       *
       * @noinspection ClassExtendsConcreteCollection
       * @noinspectionreason ClassExtendsConcreteCollection - we require custom
       *      {@code put} method to find duplicate keys
       */
      private static final class UniqueProperties extends Properties {
  
          /** A unique serial version identifier. */
          private static final long serialVersionUID = 1L;
          /**
           * Map, holding duplicated keys and their count. Keys are added here only if they
           * already exist in Properties' inner map.
           */
          private final Map<String, AtomicInteger> duplicatedKeys = new HashMap<>();
  
          /**
           * Puts the value into properties by the key specified.
           */
          @Override
          public synchronized Object put(Object key, Object value) {
              final Object oldValue = super.put(key, value);
              if (oldValue != null && key instanceof String) {
                  final String keyString = (String) key;
  
                  duplicatedKeys.computeIfAbsent(keyString, empty -> new AtomicInteger(0))
                          .incrementAndGet();
              }
              return oldValue;
          }
  
          /**
           * Retrieves a collections of duplicated properties keys.
           *
           * @return A collection of duplicated keys.
           */
          public Map<String, AtomicInteger> getDuplicatedKeys() {
              return new HashMap<>(duplicatedKeys);
          }
  
      }
  
  }
  