简明易懂的简单UDAF范例（非GenericUDAF）

/**
 * This is a simple UDAF that calculates average.
 * 
 * It should be very easy to follow and can be used as an example for writing
 * new UDAFs.
 * 
 * Note that Hive internally uses a different mechanism (called GenericUDAF) to
 * implement built-in aggregation functions, which are harder to program but
 * more efficient.
 * 
 */
public final class UdafShiftCount extends UDAF {

	/**
	 * The internal state of an aggregation for average.
	 * 
	 * Note that this is only needed if the internal state cannot be represented
	 * by a primitive.
	 * 
	 * The internal state can also contains fields with types like
	 * ArrayList and HashMap if needed.
	 */
	public static class UDAFAvgStateStruct {
		private long mCount;
		private double mSum;
	}

	/**
	 * The actual class for doing the aggregation. Hive will automatically look
	 * for all internal classes of the UDAF that implements UDAFEvaluator.
	 */
	public static class UDAFExampleAvgEvaluator implements UDAFEvaluator {

		UDAFAvgStateStruct state;

		public UDAFExampleAvgEvaluator() {
			super();
			state = new UDAFAvgStateStruct();
			init();
		}

		/**
		 * Reset the state of the aggregation.
		 */
		public void init() {
			state.mSum = 0;
			state.mCount = 0;
		}

		/**
		 * Iterate through one row of original data.
		 * 
		 * The number and type of arguments need to the same as we call this
		 * UDAF from Hive command line.
		 * 
		 * This function should always return true.
		 */
		public boolean iterate(Double o) {
			if (o != null) {
				state.mSum += o;
				state.mCount++;
			}
			return true;
		}

		/**
		 * Terminate a partial aggregation and return the state. If the state is
		 * a primitive, just return primitive Java classes like Integer or
		 * String.
		 */
		public UDAFAvgStateStruct terminatePartial() {
			// This is SQL standard - average of zero items should be null.
			return state.mCount == 0 ? null : state;
		}

		/**
		 * Merge with a partial aggregation.
		 * 
		 * This function should always have a single argument which has the same
		 * type as the return value of terminatePartial().
		 */
		public boolean merge(UDAFAvgStateStruct o) {
			if (o != null) {
				state.mSum += o.mSum;
				state.mCount += o.mCount;
			}
			return true;
		}

		/**
		 * Terminates the aggregation and return the final result.
		 */
		public Double terminate() {
			// This is SQL standard - average of zero items should be null.
			return state.mCount == 0 ? null : Double.valueOf(state.mSum
					/ state.mCount);
		}
	}

	private UDAFExampleAvg() {
		// prevent instantiation
	}

}