经过不断百度，程序修改为：

import java.sql.DriverManager;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class DBDemo {
Connection con;
Statement stm;
String sql;
ResultSet rs;

DBDemo() { //构造方法进行初始化
try {
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");

String url = "jdbc:odbc:driver={Microsoft Access Driver (*.mdb)};DBQ=D:\\实例.MDB";
Class.forName("sun.jdbc.odbc.JdbcOdbcDriver");
Connection conn = DriverManager.getConnection(url);
System.out.println(conn);
} catch(SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
} catch(ClassNotFoundException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

public void insert() { //插入数据
sql = "insert into 学生表(学号,学生姓名,身份证号,性别,年级,专业)values('20030004','王五','37021545678','男','2016','专业')";
try {
int i = stm.executeUpdate(sql);
// System.out.println("i="+i);;
if(i > 0) { //i的值表示语句操作的记录数量，i=0表示未进行数据库操作
System.out.println("添加成功！");
}
} catch(SQLException e) {
// TODO Auto-generated catch block
System.out.println("添加失败！");
// e.printStackTrace();
}
}




public static void main(String[] args) {
DBDemo d = new DBDemo();
d.insert();



try {
d.rs.close();
d.stm.close();
d.con.close();
} catch(SQLException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}
}

但依旧报错：

Exception in thread "main" java.lang.NullPointerException
at sun.jdbc.odbc.JdbcOdbcDriver.initialize(JdbcOdbcDriver.java:436)
at sun.jdbc.odbc.JdbcOdbcDriver.connect(JdbcOdbcDriver.java:153)
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
at DBDemo.<init>(DBDemo.java:24)
at DBDemo.main(DBDemo.java:54)