解决java.lang.Exception: java.lang.NullPointerException

启动程序，控制台日志打印信息，发现错误信息：空指针异常

先用degger排除错误：




运行弹出这个界面



我们需要在OrderComparator类给序列化

OrderComparator.java

package com.study.groupingcomparator;

import org.apache.hadoop.io.WritableComparable;
import org.apache.hadoop.io.WritableComparator;

public class OrderComparator  extends WritableComparator {

    protected OrderComparator(){
        super(OrderBean.class,true);
    }

    @Override
    public int compare(WritableComparable a, WritableComparable b) {
        OrderBean oa = (OrderBean) a;
        OrderBean ob = (OrderBean) b;

        return oa.getOrderId().compareTo(ob.getOrderId());
    }
}