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;
  
  import java.util.Map;
  import java.util.Properties;
  import java.util.function.Function;
  import java.util.stream.Collectors;
  
  /**
   * Resolves external properties from an
   * underlying {@code Properties} object.
   *
   */
  public final class PropertiesExpander
      implements PropertyResolver {
  
      /** The underlying values. */
      private final Map<String, String> values;
  
      /**
       * Creates a new PropertiesExpander.
       *
       * @param properties the underlying properties to use for
       *     property resolution.
       * @throws IllegalArgumentException when properties argument is null
       */
      public PropertiesExpander(Properties properties) {
          if (properties == null) {
              throw new IllegalArgumentException("cannot pass null");
          }
          values = properties.stringPropertyNames()
                  .stream()
                  .collect(Collectors.toMap(Function.identity(), properties::getProperty));
      }
  
      @Override
      public String resolve(String name) {
          return values.get(name);
      }
  
  }
  