java - KDB : Inappropriate type casting for aggregation query result

Question

我在 KDB 中运行了以下查询以创建表结构并在其中插入数据:-

n:1000000;item:`apple`banana`orange`pear;city:`beijing`chicago`london`paris;tab:([]time:asc n?0D0;n?item;amount:n?100;n?city);

发布后，我运行了一个 Java 程序来使用聚合进行选择查询。代码如下:-

import java.io.IOException;
import java.lang.reflect.Array;
import java.util.logging.Level;
import java.util.logging.Logger;

import kx.c;

public class Aggregate {

    public static class KxTableModel {
        private c.Flip flip;
        public void setFlip(c.Flip data) {
            this.flip = data;
        }

        public int getRowCount() {
            return Array.getLength(flip.y[0]);
        }

        public int getColumnCount() {
            return flip.y.length;
        }

        public Object getValueAt(int rowIndex, int columnIndex) {
            return c.at(flip.y[columnIndex], rowIndex);
        }

        public String getColumnName(int columnIndex) {
            return flip.x[columnIndex];
        }
    };

    public static void main(String[] args) {
        KxTableModel model = new KxTableModel();
        c c = null;
        try {
            c = new c("localhost", 5001,":");
            String query="select sum amount by city from tab";
//          String query="0!select last price by sym from trade where date=last date";
            model.setFlip((c.Flip) c.k(query));
        } catch (Exception ex) {
            Logger.getLogger(Main.class.getName()).log(Level.SEVERE, null, ex);
        } finally {
            if (c != null) {try{c.close();} catch (IOException ex) {}
          }
        }
        System.out.println(model.getRowCount());
        System.out.println(model.getValueAt(1, 1));

    }
}

上面的代码在执行后给了我以下错误:-

Apr 14, 2016 9:30:10 AM Aggregate main
SEVERE: null
java.lang.ClassCastException: kx.c$Dict cannot be cast to kx.c$Flip
    at Aggregate.main(Aggregate.java:40)
Exception in thread "main" java.lang.NullPointerException
    at Aggregate$KxTableModel.getRowCount(Aggregate.java:17)
    at Aggregate.main(Aggregate.java:47)

Answer 1

您的查询返回一个键控表(因为 by city)您现在可以处理 c.Dict 或在选择之前插入 0! 以取消对表的键控。

0!从选项卡中按城市选择总金额