使用PreparedStatement

Question

我是Java新手。我一直在尝试使用pgadmin4从intelliJ连接到PostgreSQL

我试图创建一个表并创建了它，但是当我尝试使用准备好的语句插入数据时，它给我一个关于nullpointerexception的错误。我了解这是因为某些内容为null，但我似乎无法弄清楚什么为null。错误似乎在准备好的语句上

代码如下：

 package Datatypes1;
 import java.io.IOException;
 import java.sql.*;
 import java.sql.SQLException;
 import java.util.Scanner;
   public class Database2 {
    Scanner input = new Scanner(System.in);
     public static void main(String args[]) throws IOException {
      char ans;
      System.out.println("Welcome tp the coffee shop management system");
      Database2 database2 = new Database2();
      database2.connect();
      database2.createTable();
      do{
        database2.obtaindata();
        System.out.println("Do you want to enter more data:y/n?");
        ans =(char) System.in.read();
      }while( ans == 'y');

      database2.disconnect();
   }

//create the connection
String url="jdbc:postgresql://localhost:5432/dreamhome1";
String user = "postgres";
String pass = "postgres";


//database variables
Connection con = null;
ResultSet rs = null;
Statement stmt = null;



//connect with the database
public void connect(){
    try{
        //open connection
        con=DriverManager.getConnection(url,user,pass);

        //show that is it connected
        System.out.println("Connection successful");

    }catch(SQLException e){
        e.printStackTrace();
    }
}

//create table in the database
public void createTable(){
    try{
        stmt=con.createStatement();
        String sql="CREATE TABLE IF NOT EXISTS product (product_id SERIAL PRIMARY KEY NOT NULL,"+
                "product_name VARCHAR(100) NOT NULL,"+
                "product_price REAL)";
        stmt.executeUpdate(sql);
        System.out.println("table created successfully!");
        stmt.close();
    }catch(SQLException e){
        e.printStackTrace();

    }
}

public void obtaindata(){
    System.out.println("enter the data you want to insert");
    System.out.print("Product Name");
    String productname=input.nextLine();
    System.out.println("Product Price");
    Double productprice=input.nextDouble();
    Database2 database2=new Database2();
    database2.insertRecord(productname,productprice);

}


public void insertRecord(String prodname,Double prodprice){
    try{

        PreparedStatement st=con.prepareStatement("INSERT INTO public.product(product_name,product_price) VALUES (nextval('productid_sequence'),?,?)");
        st.setString(1,prodname);
        st.setDouble(2,prodprice);
        int i =st.executeUpdate();
        st.close();
    }catch(SQLException e){
        e.printStackTrace();
    }
}

//disconnect with the database
public void disconnect(){
    try{
        if(con != null){
            con.close();
        }

        if(stmt != null){
            stmt.close();
        }

        if(rs != null){
            rs.close();
        }

    }catch(SQLException e){
        e.printStackTrace();
    }

}

}

这是错误：

Welcome tp the coffee shop management system
Connection successful
table created successfully!
enter the data you want to insert
Product Name latte
Product Price
2.40
Exception in thread "main" java.lang.NullPointerException
    at Datatypes1.Database2.insertRecord(Database2.java:80)
    at Datatypes1.Database2.obtaindata(Database2.java:72)
    at Datatypes1.Database2.main(Database2.java:15)