001    /**
002     * Licensed to the Apache Software Foundation (ASF) under one
003     * or more contributor license agreements.  See the NOTICE file
004     * distributed with this work for additional information
005     * regarding copyright ownership.  The ASF licenses this file
006     * to you under the Apache License, Version 2.0 (the
007     * "License"); you may not use this file except in compliance
008     * with the License.  You may obtain a copy of the License at
009     *
010     *     http://www.apache.org/licenses/LICENSE-2.0
011     *
012     * Unless required by applicable law or agreed to in writing, software
013     * distributed under the License is distributed on an "AS IS" BASIS,
014     * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
015     * See the License for the specific language governing permissions and
016     * limitations under the License.
017     */
018    package org.apache.hadoop.fs;
019    
020    import org.apache.hadoop.classification.InterfaceAudience;
021    import org.apache.hadoop.classification.InterfaceStability;
022    
023    /**
024     * Opaque interface that identifies a disk location. Subclasses
025     * should implement {@link Comparable} and override both equals and hashCode.
026     */
027    @InterfaceStability.Unstable
028    @InterfaceAudience.Public
029    public interface VolumeId extends Comparable<VolumeId> {
030    
031      /**
032       * Represents an invalid Volume ID (ID for unknown content).
033       */
034      public static final VolumeId INVALID_VOLUME_ID = new VolumeId() {
035        
036        @Override
037        public int compareTo(VolumeId arg0) {
038          // This object is equal only to itself;
039          // It is greater than null, and
040          // is always less than any other VolumeId:
041          if (arg0 == null) {
042            return 1;
043          }
044          if (arg0 == this) {
045            return 0;
046          } else {
047            return -1;
048          }
049        }
050        
051        @Override
052        public boolean equals(Object obj) {
053          // this object is equal only to itself:
054          return (obj == this);
055        }
056        
057        @Override
058        public int hashCode() {
059          return Integer.MIN_VALUE;
060        }
061        
062        @Override
063        public boolean isValid() {
064          return false;
065        }
066        
067        @Override
068        public String toString() {
069          return "Invalid VolumeId";
070        }
071      };
072      
073      /**
074       * Indicates if the disk identifier is valid. Invalid identifiers indicate
075       * that the block was not present, or the location could otherwise not be
076       * determined.
077       * 
078       * @return true if the disk identifier is valid
079       */
080      public boolean isValid();
081    
082      @Override
083      abstract public int compareTo(VolumeId arg0);
084    
085      @Override
086      abstract public int hashCode();
087    
088      @Override
089      abstract public boolean equals(Object obj);
090    
091    }