java连mysql抛空指针错误

package cn.pzhu.domain;
import java.util.Date;
public class User {
    private String id;
    private String username;
    private String email;
    private String password;
    private Date birthday;
    private String nickname;
    public String getId() {
        return id;
    }
    public void setId(String id) {
        this.id = id;
    }
    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username = username;
    }
    public String getEmail() {
        return email;
    }
    public void setEmail(String email) {
        this.email = email;
    }
    public String getPassword() {
        return password;
    }
    public void setPassword(String password) {
        this.password = password;
    }
    public Date getBirthday() {
        return birthday;
    }
    public void setBirthday(Date birthday) {
        this.birthday = birthday;
    }
    public String getNickname() {
        return nickname;
    }
    public void setNickname(String nickname) {
        this.nickname = nickname;
    }
}

package cn.pzhu.dao.impl;
import java.sql.*;
import cn.pzhu.dao.UserDao;
import cn.pzhu.domain.User;

public class UserDaoImpl implements UserDao {
    static Connection connection;
    static Statement statement;
    static PreparedStatement preparedStatement;
    static String sql;
    static{
        try {
            Class.forName("com.mysql.jdbc.Driver");
            connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test1?user=root&password=pzhu");
        } catch (ClassNotFoundException e1) {
            throw new RuntimeException(e1);
        }catch (SQLException e2) {
            throw new RuntimeException(e2);
        }
    }
    //数据库user表字段顺序：username password id email birthday nickname
    public void add(User user) {
        sql = "insert into user values(?,?,?,?,?,?)";
        try {
            
            preparedStatement = connection.prepareStatement(sql);
            preparedStatement.setString(1, user.getUsername());
            preparedStatement.setString(2, user.getPassword());
            preparedStatement.setString(3, user.getId());
            preparedStatement.setString(4, user.getEmail());
            preparedStatement.setDate(5,new java.sql.Date(user.getBirthday().getTime()));
            preparedStatement.setString(6, user.getNickname());
            preparedStatement.executeUpdate();
        } catch (SQLException e3) {
            throw new RuntimeException(e3);
        }
        try {
            if(preparedStatement!=null){
                preparedStatement.close();
                preparedStatement = null;
            }
            if (connection!=null) {
                connection.close();
                connection = null;
            }
        } catch (Exception e4) {
            throw new RuntimeException(e4);
        }
        
    }
    //数据库user表字段顺序：username password id email birthday nicknameusername password id email birthday nickname
    public User find(String username,String password ) {
        sql =  "select * from user where username='"+username+"' and password='"+password+"'";
        try {
            statement = connection.createStatement();
            ResultSet resultSet = statement.executeQuery(sql);
            if(resultSet.next()){
                User user = new User();
                user.setBirthday(resultSet.getDate(5));
                user.setEmail(resultSet.getString(4));
                user.setId(resultSet.getString(3));
                user.setNickname(resultSet.getString(6));
                user.setPassword(resultSet.getString(2));
                user.setUsername(resultSet.getString(1));
                
                return user;
            }else {
                return null;
            }
        } catch (SQLException e5) {
            throw new RuntimeException(e5);
        }
        finally{
            try {
                if (statement!=null) {
                    statement.close();
                    statement=null;
                }
                if (connection!=null) {
                    connection.close();
                    connection=null;
                }
            } catch (SQLException e6) {
                throw new RuntimeException(e6);
            }
        }
    }
    
    
    //用户注册时检测数据库中是否已经存在该用户名
    public boolean find(String username) {
        sql =  "select * from user where username='"+username+"'";
        try {
            statement = connection.createStatement();
            ResultSet resultSet = statement.executeQuery(sql);
            if(resultSet.next()){
                return true;
            }else {
                return false;
            }
        } catch (SQLException e5) {
            throw new RuntimeException(e5);
        }
        finally{
            try {
                if (statement!=null) {
                    statement.close();
                    statement=null;
                }
                if (connection!=null) {
                    connection.close();
                    connection=null;
                }
            } catch (SQLException e6) {
                throw new RuntimeException(e6);
            }
        }
    }
}