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   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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   * 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.security.acl;
  
  import java.util.Enumeration;
  import java.security.Principal;
  
  /**
   * This is the interface used for representing one entry in an Access
   * Control List (ACL).<p>
   *
   * An ACL can be thought of as a data structure with multiple ACL entry
   * objects. Each ACL entry object contains a set of permissions associated
   * with a particular principal. (A principal represents an entity such as
   * an individual user or a group). Additionally, each ACL entry is specified
   * as being either positive or negative. If positive, the permissions are
   * to be granted to the associated principal. If negative, the permissions
   * are to be denied. Each principal can have at most one positive ACL entry
   * and one negative entry; that is, multiple positive or negative ACL
   * entries are not allowed for any principal.
   *
   * Note: ACL entries are by default positive. An entry becomes a
   * negative entry only if the
   * {@link #setNegativePermissions() setNegativePermissions}
   * method is called on it.
   *
   * @see Acl
   *
   * @author      Satish Dharmaraj
   */
  public interface AclEntry extends Cloneable {
  
      /**
       * Specifies the principal for which permissions are granted or denied
       * by this ACL entry. If a principal was already set for this ACL entry,
       * false is returned, otherwise true is returned.
       *
       * @param user the principal to be set for this entry.
       *
       * @return true if the principal is set, false if there was
       * already a principal set for this entry.
       *
       * @see #getPrincipal
       */
      public boolean setPrincipal(Principal user);
  
      /**
       * Returns the principal for which permissions are granted or denied by
       * this ACL entry. Returns null if there is no principal set for this
       * entry yet.
       *
       * @return the principal associated with this entry.
       *
       * @see #setPrincipal
       */
      public Principal getPrincipal();
  
      /**
       * Sets this ACL entry to be a negative one. That is, the associated
       * principal (e.g., a user or a group) will be denied the permission set
       * specified in the entry.
       *
       * Note: ACL entries are by default positive. An entry becomes a
       * negative entry only if this {@code setNegativePermissions}
       * method is called on it.
       */
      public void setNegativePermissions();
  
      /**
       * Returns true if this is a negative ACL entry (one denying the
       * associated principal the set of permissions in the entry), false
       * otherwise.
       *
       * @return true if this is a negative ACL entry, false if it's not.
       */
      public boolean isNegative();
  
      /**
       * Adds the specified permission to this ACL entry. Note: An entry can
       * have multiple permissions.
       *
       * @param permission the permission to be associated with
       * the principal in this entry.
       *
       * @return true if the permission was added, false if the
       * permission was already part of this entry's permission set.
       */
      public boolean addPermission(Permission permission);
  
      /**
       * Removes the specified permission from this ACL entry.
       *
       * @param permission the permission to be removed from this entry.
       *
       * @return true if the permission is removed, false if the
       * permission was not part of this entry's permission set.
       */
      public boolean removePermission(Permission permission);
  
      /**
       * Checks if the specified permission is part of the
       * permission set in this entry.
       *
       * @param permission the permission to be checked for.
       *
       * @return true if the permission is part of the
       * permission set in this entry, false otherwise.
       */
      public boolean checkPermission(Permission permission);
  
      /**
       * Returns an enumeration of the permissions in this ACL entry.
       *
       * @return an enumeration of the permissions in this ACL entry.
       */
      public Enumeration<Permission> permissions();
  
      /**
       * Returns a string representation of the contents of this ACL entry.
       *
       * @return a string representation of the contents.
       */
      public String toString();
  
      /**
       * Clones this ACL entry.
       *
       * @return a clone of this ACL entry.
       */
      public Object clone();
  }
  