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   * Copyright (c) 2007, 2009, Oracle and/or its affiliates. All rights reserved.
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   *
   * This code is free software; you can redistribute it and/or modify it
   * under the terms of the GNU General Public License version 2 only, as
   * published by the Free Software Foundation.  Oracle designates this
   * particular file as subject to the "Classpath" exception as provided
   * by Oracle in the LICENSE file that accompanied this code.
   *
   * This code 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 General Public License
   * version 2 for more details (a copy is included in the LICENSE file that
   * accompanied this code).
   *
   * You should have received a copy of the GNU General Public License version
   * 2 along with this work; if not, write to the Free Software Foundation,
   * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
   *
   * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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   */
  
  package java.nio.file.attribute;
  
  import java.io.IOException;
  
  /**
   * An object to lookup user and group principals by name. A {@link UserPrincipal}
   * represents an identity that may be used to determine access rights to objects
   * in a file system. A {@link GroupPrincipal} represents a <em>group identity</em>.
   * A {@code UserPrincipalLookupService} defines methods to lookup identities by
   * name or group name (which are typically user or account names). Whether names
   * and group names are case sensitive or not depends on the implementation.
   * The exact definition of a group is implementation specific but typically a
   * group represents an identity created for administrative purposes so as to
   * determine the access rights for the members of the group. In particular it is
   * implementation specific if the <em>namespace</em> for names and groups is the
   * same or is distinct. To ensure consistent and correct behavior across
   * platforms it is recommended that this API be used as if the namespaces are
   * distinct. In other words, the {@link #lookupPrincipalByName
   * lookupPrincipalByName} should be used to lookup users, and {@link
   * #lookupPrincipalByGroupName lookupPrincipalByGroupName} should be used to
   * lookup groups.
   *
   * @since 1.7
   *
   * @see java.nio.file.FileSystem#getUserPrincipalLookupService
   */
  
  public abstract class UserPrincipalLookupService {
  
      /**
       * Initializes a new instance of this class.
       */
      protected UserPrincipalLookupService() {
      }
  
      /**
       * Lookup a user principal by name.
       *
       * @param   name
       *          the string representation of the user principal to lookup
       *
       * @return  a user principal
       *
       * @throws  UserPrincipalNotFoundException
       *          the principal does not exist
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, it checks {@link RuntimePermission}<tt>("lookupUserInformation")</tt>
       */
      public abstract UserPrincipal lookupPrincipalByName(String name)
          throws IOException;
  
      /**
       * Lookup a group principal by group name.
       *
       * <p> Where an implementation does not support any notion of group then
       * this method always throws {@link UserPrincipalNotFoundException}. Where
       * the namespace for user accounts and groups is the same, then this method
       * is identical to invoking {@link #lookupPrincipalByName
       * lookupPrincipalByName}.
       *
       * @param   group
       *          the string representation of the group to lookup
       *
       * @return  a group principal
       *
       * @throws  UserPrincipalNotFoundException
       *          the principal does not exist or is not a group
       * @throws  IOException
       *          if an I/O error occurs
       * @throws  SecurityException
       *          In the case of the default provider, and a security manager is
       *          installed, it checks {@link RuntimePermission}<tt>("lookupUserInformation")</tt>
       */
      public abstract GroupPrincipal lookupPrincipalByGroupName(String group)
          throws IOException;
  }
  