在java中从数据库中检索值 [英] retrieving values from database in java

问题描述

我正在制作一个程序，用于从我创建的数据库中检索输入的字段数据/值。但是当我运行它时，输出始终为null。我想知道出了什么问题？这是我的代码：

I am making a program that retrieves the inputted data/values of fields from the database I created. But when I'm running it the output is always null. I wonder what's wrong? Here's my code:

import java.sql.*;

public class GuestsInfo
{
private String  firstName, lastName;
private int  age;
private Connection con;
private PreparedStatement statement;

public GuestsInfo()
{
    try {
         Class.forName("com.mysql.jdbc.Driver");
         con = DriverManager.getConnection(
                        "jdbc:mysql://localhost:3306/3moronsdb","root","");
    } catch (Exception e) {
         e.printStackTrace();
    }
}
public GuestsInfo(String firstName, String lastName, int age)
{
    this();
    try
    {   
        statement = con.prepareStatement("Insert into guest(firstName,lastName,age)values(?,?,?)");

        statement.setString(1, firstName);
        statement.setString(2, lastName);
        statement.setInt(3, age);

        statement.executeUpdate();
        con.close();
    }
    catch(Exception e)
    {
        e.printStackTrace();
        return;
    }   
}


public void setFirstName(String firstName)
{

    try{
        statement= con.prepareStatement("SELECT * FROM guest WHERE firstName = ?");
        statement.setString(1, firstName);
        ResultSet rs = statement.executeQuery();
        while(rs.next()){
            firstName = rs.getString(1);
        }
    }catch(Exception e){
        System.out.print(e);
    }
}

public String getFirstName(){
    return firstName;
}

public void setLastName(String lastName)
{

    try{
        statement= con.prepareStatement("SELECT * FROM guest WHERE lastName = ?");
        statement.setString(2, lastName);
        ResultSet rs = statement.executeQuery();
        while(rs.next()){
            lastName = rs.getString(2);
        }
    }catch(Exception e){
        System.out.print(e);
    }
}

public String getLastName(){
    return lastName;
}

public void setAge(int age){
    try{
        statement = con.prepareStatement("SELECT * FROM guest WHERE age=?");
        statement.setInt(3, age);
        ResultSet rs = statement.executeQuery();
        while(rs.next()){
            age = rs.getInt(3);
        }
    }catch(Exception e){
        System.out.print(e);
    }
}

public int getAge(){
    return age;
}

在我的主要课程中，我有以下代码：

In my main class I have this codes:

 public class TestMain {
public static void main(String [] args){

    GuestsInfo gi = new GuestsInfo();
    gi.setFirstName("Ericka");
    System.out.println(gi.getFirstName());

}
 }

我想知道它为什么总是说null当我运行它时。

I wonder why it always says null when I'm running it.

推荐答案

你实际上并没有设置你的二传手中的任何东西。

You are not actually "setting" anything in your setters.

public void setFirstName(String firstName)
{

    try{
    // Here you create a READing statement.
        statement= con.prepareStatement("SELECT * FROM guest WHERE firstName = ?");
        statement.setString(1, firstName);
        ResultSet rs = statement.executeQuery();
        while(rs.next()){
    // Here you set the >>method param<< to a non-existing String
            firstName = rs.getString(1);
        }
    }catch(Exception e){
       System.out.print(e);
    }
}
// After the method is done you have done nothing because you didn't even set your class member variable "firstName", which you would have set using "this.firstName".

除此之外：我对getter和setter的理解是这样的，你可以在你的吸气。

On top of that: My understanding of getters and setter is such that you would do the query in your getter.

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