public class UserDao implements UserDaoInterface {
	private Connection conn;
	public UserDao(Connection conn){
		this.conn = conn;
	}

public List<User> selectUserList() {
		String sql = "select * from t_user";
		PreparedStatement sta = null;
		ResultSet rs = null;
		try {	
			System.out.println(1);
			sta = conn.prepareStatement(sql);---------------------------------155
			System.out.println(32);
			rs = sta.executeQuery();
			List users = new ArrayList();
			
			while(rs.next()){
				User u = new User();
				u.setId(rs.getInt("id"));
				u.setUsername(rs.getString("username"));
				u.setPassword(rs.getString("password"));
				u.setCallname(rs.getString("callname"));
				u.setAge(rs.getInt("age"));
				u.setBankaccount(rs.getString("bankaccount"));
				u.setSalary(rs.getDouble("salary"));
				users.add(u);
				//System.out.println(users);
			}
			return users;
		} catch (SQLException e) {
			e.printStackTrace();
			return null;
		}finally{
			
			try {
				if(rs!=null)
					rs.close();
			} catch (SQLException e) {
				e.printStackTrace();
			}
			try {
				if(sta!=null)
				sta.close();
			} catch (SQLException e) {
				e.printStackTrace();
			}
		}
		
	}
	public static void main(String[] args) {
		UserDao us = new UserDao();
		System.out.println(us.selectUserList());-------------------------------195
	}
}
--------------------------------------------------------

控制台输出：
1
Exception in thread "main" java.lang.NullPointerException
at dao.UserDao.selectUserList(UserDao.java:155)
at dao.UserDao.main(UserDao.java:195)
----------------------------------------------------------------------------

数据库表
create table t_user(
id Integer not null primary key,
username varchar2(255) not null ,
password varchar2(255) not null,
callname varchar2(255) not null,
age Integer not null,
bankaccount varchar2(255) not null,
salary decimal(8,2) not null
)
INSERT INTO t_user (id,username,password,callname,age,bankaccount,salary) VALUES(4,'李四','gasg','ettet',24,12313,2423.00);
select * from t_user;
delete from t_user where id=1;