Hive Generic UDF : array_first

UDFArrayFirst.java

import org.apache.hadoop.hive.ql.exec.UDFArgumentException;
import org.apache.hadoop.hive.ql.metadata.HiveException;
import org.apache.hadoop.hive.ql.udf.generic.GenericUDF;
import org.apache.hadoop.hive.serde2.objectinspector.ListObjectInspector;
import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector;
import org.apache.hadoop.hive.serde2.objectinspector.ObjectInspector.Category;

public class UDFArrayFirst extends GenericUDF {
    ListObjectInspector listInputObjectInspector;

    public ObjectInspector initialize(ObjectInspector[] args) throws UDFArgumentException {
        assert (args.length == 1); // This UDF accepts one argument
        assert(args[0].getCategory() == Category.LIST); // The first argument is a list

        listInputObjectInspector = (ListObjectInspector)args[0];

        /* Here comes the real usage for Object Inspectors : we know that our
         * return type is equal to the type of the elements of the input array.
         * We don't need to know in details what this type is, the
         * ListObjectInspector already has it */
        return listInputObjectInspector.getListElementObjectInspector();
    }

    public Object evaluate(DeferredObject[] args) throws HiveException {
        if (args.length != 1) return null;

        // Access the deferred value. Hive passes the arguments as "deferred" objects 
        // to avoid some computations if we don't actually need some of the values
        Object oin = args[0].get();

        if (oin == null) return null;

        int nbElements = listInputObjectInspector.getListLength(oin);
        if (nbElements > 0) {
            // The list is not empty, return its head
            return  listInputObjectInspector.getListElement(oin, 0);
        } else {
            return null;
        }
    }

    @Override
    public String getDisplayString(String[] args) {
        return "Here, write a nice description";
    }
}

Thanks for sharing! what if I want to add two elements of the list?