java.lang.NullPointerException 这个有关问题如何解决

package mypack;public class People {    String name;    String hometown;                String local;                   String university;             String highschool;              String major;                 String grade;                     public void set(People A)    {        this.name=A.getName();        this.hometown=A.getHometown();        this.local=A.getLocal();        this.university=A.getUniversity();        this.highschool=A.getHighschool();        this.major=A.getMajor();        this.grade=A.getGrade();    }        public void setName(String name)    {        this.name=name;    }    public String getName()    {        return name;    }        public void setHometown(String hometown)    {        this.hometown=hometown;    }    public String getHometown()    {        return hometown;    }        public void setLocal(String local)    {        this.local=local;    }    public String getLocal()    {        return local;    }        public void setUniversity(String university)    {        this.university=university;    }    public String getUniversity()    {        return university;    }        public void setHighschool(String highschool)    {        this.highschool=highschool;    }    public String getHighschool()    {        return highschool;    }        public void setMajor(String major)    {        this.major=major;    }    public String getMajor()    {        return major;    }        public void setGrade(String grade)    {        this.grade=grade;    }    public String getGrade()    {        return grade;    }}

package mypack;import java.sql.Connection;import java.sql.PreparedStatement;import java.sql.ResultSet;import mypack.People;public class Count {        People sb;    int dist;    public Count(People A)    {        sb=new People();        sb.set(A);            }    public int countd(People B)    {        int flag,i;        int dist2=0;        int[] buff ={0,0,0,0,0,0};        int[] md={3,4,6,5,2,1};        if (sb.getHometown().equals(B.getHometown()))  //这报错            buff[0]=3;        if (sb.getLocal().equals(B.getLocal()))            buff[1]=4;        if (sb.getUniversity().equals(B.getUniversity()))            buff[2]=6;        if (sb.getHighschool().equals(B.getHighschool()))            buff[3]=5;        if (sb.getMajor().equals(B.getMajor()))            buff[4]=2;        if (sb.getGrade().equals(B.getGrade()))            buff[5]=1;                for (i=0;i<6;i++)        {            dist=dist+(buff[i]-md[i])*(buff[i]-md[i]);            dist2=dist2+buff[i]*buff[i];        }                if (dist<dist2)            flag=1;        else            flag=0;                return flag;    }        public int getDist()    {        return dist;    } }

package mypack;public class Getall {        public static void main(String[] args)    {                People sb=new People();        sb.setName("cop");        sb.setLocal("xxx");        sb.setUniversity("xxx");        sb.setHighschool("xxx");        sb.setMajor("xxx");        sb.setGrade("2011");        Count co=new Count(sb);//这报错        int k=co.countd(sb);        System.out.println(k);    }        }