import org.apache.hadoop.hive.ql.exec.UDAFEvaluator;
import org.apache.hadoop.hive.serde2.io.DoubleWritable;

public class UDAFSum_Sample extends NumericUDAF {
        public static class Evaluator implements UDAFEvaluator {
                private boolean mEmpty;
                private double mSum;
                public Evaluator() {
                        super();
                        init();
                }

public void init() {
                        mSum = 0;
                        mEmpty = true;
                }

public boolean iterate(DoubleWritable o) {
                        if (o != null) {
                                mSum += o.get();
                                mEmpty = false;
                        }
                        return true;
                }

public DoubleWritable terminatePartial() {
                        // This is SQL standard - sum of zero items should be null.
                        return mEmpty ? null : new DoubleWritable(mSum);
                }

public boolean merge(DoubleWritable o) {
                        if (o != null) {
                                mSum += o.get();
                                mEmpty = false;
                        }
                        return true;
                }

public DoubleWritable terminate() {
                        // This is SQL standard - sum of zero items should be null.
                        return mEmpty ? null : new DoubleWritable(mSum);
                }
        }
}

hive> create temporary function sum_test as 'com.hrj.hive.udf.UDAFSum_Sample';

hive> select sum_test(t.num) from t;

hive> drop temporary function sum_test;

hive> quit;