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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
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   * 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.channels;
  
  import java.net.InetAddress;
  import java.net.NetworkInterface;
  import java.io.IOException;
  
  /**
   * A token representing the membership of an Internet Protocol (IP) multicast
   * group.
   *
   * <p> A membership key may represent a membership to receive all datagrams sent
   * to the group, or it may be <em>source-specific</em>, meaning that it
   * represents a membership that receives only datagrams from a specific source
   * address. Whether or not a membership key is source-specific may be determined
   * by invoking its {@link #sourceAddress() sourceAddress} method.
   *
   * <p> A membership key is valid upon creation and remains valid until the
   * membership is dropped by invoking the {@link #drop() drop} method, or
   * the channel is closed. The validity of the membership key may be tested
   * by invoking its {@link #isValid() isValid} method.
   *
   * <p> Where a membership key is not source-specific and the underlying operation
   * system supports source filtering, then the {@link #block block} and {@link
   * #unblock unblock} methods can be used to block or unblock multicast datagrams
   * from particular source addresses.
   *
   * @see MulticastChannel
   *
   * @since 1.7
   */
  public abstract class MembershipKey {
  
      /**
       * Initializes a new instance of this class.
       */
      protected MembershipKey() {
      }
  
      /**
       * Tells whether or not this membership is valid.
       *
       * <p> A multicast group membership is valid upon creation and remains
       * valid until the membership is dropped by invoking the {@link #drop() drop}
       * method, or the channel is closed.
       *
       * @return  {@code true} if this membership key is valid, {@code false}
       *          otherwise
       */
      public abstract boolean isValid();
  
      /**
       * Drop membership.
       *
       * <p> If the membership key represents a membership to receive all datagrams
       * then the membership is dropped and the channel will no longer receive any
       * datagrams sent to the group. If the membership key is source-specific
       * then the channel will no longer receive datagrams sent to the group from
       * that source address.
       *
       * <p> After membership is dropped it may still be possible to receive
       * datagrams sent to the group. This can arise when datagrams are waiting to
       * be received in the socket's receive buffer. After membership is dropped
       * then the channel may {@link MulticastChannel#join join} the group again
       * in which case a new membership key is returned.
       *
       * <p> Upon return, this membership object will be {@link #isValid() invalid}.
       * If the multicast group membership is already invalid then invoking this
       * method has no effect. Once a multicast group membership is invalid,
       * it remains invalid forever.
       */
      public abstract void drop();
  
      /**
       * Block multicast datagrams from the given source address.
       *
       * <p> If this membership key is not source-specific, and the underlying
       * operating system supports source filtering, then this method blocks
       * multicast datagrams from the given source address. If the given source
       * address is already blocked then this method has no effect.
       * After a source address is blocked it may still be possible to receive
       * datagrams from that source. This can arise when datagrams are waiting to
       * be received in the socket's receive buffer.
       *
       * @param   source
       *          The source address to block
       *
       * @return  This membership key
       *
       * @throws  IllegalArgumentException
       *          If the {@code source} parameter is not a unicast address or
       *          is not the same address type as the multicast group
       * @throws  IllegalStateException
       *          If this membership key is source-specific or is no longer valid
       * @throws  UnsupportedOperationException
       *          If the underlying operating system does not support source
       *          filtering
       * @throws  IOException
       *          If an I/O error occurs
       */
      public abstract MembershipKey block(InetAddress source) throws IOException;
  
      /**
       * Unblock multicast datagrams from the given source address that was
       * previously blocked using the {@link #block(InetAddress) block} method.
       *
       * @param   source
       *          The source address to unblock
       *
       * @return  This membership key
       *
       * @throws  IllegalStateException
       *          If the given source address is not currently blocked or the
       *          membership key is no longer valid
       */
      public abstract MembershipKey unblock(InetAddress source);
  
      /**
       * Returns the channel for which this membership key was created. This
       * method will continue to return the channel even after the membership
       * becomes {@link #isValid invalid}.
       *
       * @return  the channel
       */
      public abstract MulticastChannel channel();
  
      /**
       * Returns the multicast group for which this membership key was created.
       * This method will continue to return the group even after the membership
       * becomes {@link #isValid invalid}.
       *
       * @return  the multicast group
       */
      public abstract InetAddress group();
  
      /**
       * Returns the network interface for which this membership key was created.
       * This method will continue to return the network interface even after the
       * membership becomes {@link #isValid invalid}.
       *
       * @return  the network interface
       */
      public abstract NetworkInterface networkInterface();
  
      /**
       * Returns the source address if this membership key is source-specific,
       * or {@code null} if this membership is not source-specific.
       *
       * @return  The source address if this membership key is source-specific,
       *          otherwise {@code null}
       */
      public abstract InetAddress sourceAddress();
  }
  