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A module that is everything required to understands Druid Segments
There is a newer version: 30.0.0
/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */

package org.apache.druid.data.input;

import com.fasterxml.jackson.annotation.JsonSubTypes;
import com.fasterxml.jackson.annotation.JsonTypeInfo;
import org.apache.druid.guice.annotations.PublicApi;
import org.joda.time.DateTime;

import javax.annotation.Nullable;
import java.util.List;

/**
 * A Row of data.  This can be used for both input and output into various parts of the system.  It assumes
 * that the user already knows the schema of the row and can query for the parts that they care about.
 */
@PublicApi
@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, property = "version", defaultImpl = MapBasedRow.class)
@JsonSubTypes(value = {
    @JsonSubTypes.Type(name = "v1", value = MapBasedRow.class)
})
public interface Row extends Comparable
{
  /**
   * Returns the timestamp from the epoch in milliseconds.  If the event happened _right now_, this would return the
   * same thing as System.currentTimeMillis();
   *
   * @return the timestamp from the epoch in milliseconds.
   */
  long getTimestampFromEpoch();

  /**
   * Returns the timestamp from the epoch as an org.joda.time.DateTime.  If the event happened _right now_, this would return the
   * same thing as new DateTime();
   *
   * @return the timestamp from the epoch as an org.joda.time.DateTime object.
   */
  DateTime getTimestamp();

  /**
   * Returns the list of dimension values for the given column name.
   * 
   *
   * @param dimension the column name of the dimension requested
   *
   * @return the list of values for the provided column name
   */
  List getDimension(String dimension);

  /**
   * Returns the raw dimension value for the given column name. This is different from {@link #getDimension} which
   * converts all values to strings before returning them.
   *
   * @param dimension the column name of the dimension requested
   *
   * @return the value of the provided column name
   */
  @Nullable
  Object getRaw(String dimension);

  /**
   * Returns the metric column value for the given column name. This method is different from {@link #getRaw} in two
   * aspects:
   *  1. If the column is absent in the row, either numeric zero or null will be returned, depending on
   *     the value of druid.generic.useDefaultValueForNull.
   *  2. If the column has string value, an attempt is made to parse this value as a number.
   */
  @Nullable
  Number getMetric(String metric);
}