Java 从另一个类获取变量值

我有一个名为

Members

的类，还有一个名为

DBConnect

的类，它将连接到数据库，然后将数据库中的值存储到

Members

类。然后我想使用另一个名为

Controller

的类中

Members

类的值。但是，我正在我的

Controller

类中创建一个对象的新实例，如果从那里使用方法

get（）

，它将返回

null

但是，如果我在

DBConnect

类中执行所有操作，就可以了。它将返回第一个名称，而不是

null

成员

类

package Database;

public class Members {

    private String firstName;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

package Database;

import java.sql.*;

public class DBConnect {
    private Connection connection;
    private Statement statement;
    private ResultSet resultSet;

    Members members = new Members();

    public DBConnect() {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");

            connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/finance", "root", "");
            statement = connection.createStatement();
            System.out.println("Successfully connected to DATABASE.");
        } catch (Exception ex) {
            System.out.println("ERROR: Can't connect to the Database! -> " + ex);
        }
    }

    public void getData(int id) {
        try {
            String query = "select * from Members where MembersId = '" + id + "'";
            resultSet = statement.executeQuery(query);

            while (resultSet.next()) {
                members.setId(resultSet.getInt("MembersId"));
                members.setFirstName(resultSet.getString("MembersFirstName"));
                members.setLastName(resultSet.getString("MembersLastName"));
                members.setPosition(resultSet.getString("MembersPosition"));
                members.setHourlyRate(resultSet.getDouble("MembersHourlyRate"));
                members.setGraduated(resultSet.getString("MembersGraduated"));
                members.setLanguages(resultSet.getString("MembersLanguages"));
                members.setCountry(resultSet.getString("MembersCountry"));
                members.setCity(resultSet.getString("MembersCity"));
                members.setAddress(resultSet.getString("MembersAddress"));
            }
        }catch (Exception ex) {
            System.out.println("Error: " + ex);
        }
    }
}

package sample;

import Database.DBConnect;
import Database.Members;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Label;
import javafx.scene.effect.DropShadow;
import javafx.scene.image.Image;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.HBox;
import javafx.scene.paint.Color;
import javafx.scene.paint.ImagePattern;
import javafx.scene.shape.Circle;

import java.io.IOException;
import java.net.URISyntaxException;
import java.net.URL;
import java.util.ResourceBundle;


public class Controller implements Initializable {

    private DBConnect dbConnect;
    private Members members;

@FXML
    private HBox sidebarMembers;

@FXML
    private void openSidebarMembers(MouseEvent event) throws IOException {
        dbConnect = new DBConnect();
        dbConnect.getData(2);
        members = new Members();
        if (event.getSource() == sidebarMembers) {
            System.out.println(members.getFirstName());
        }
    }

DBConnect

Class

package Database;

public class Members {

    private String firstName;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

package Database;

import java.sql.*;

public class DBConnect {
    private Connection connection;
    private Statement statement;
    private ResultSet resultSet;

    Members members = new Members();

    public DBConnect() {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");

            connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/finance", "root", "");
            statement = connection.createStatement();
            System.out.println("Successfully connected to DATABASE.");
        } catch (Exception ex) {
            System.out.println("ERROR: Can't connect to the Database! -> " + ex);
        }
    }

    public void getData(int id) {
        try {
            String query = "select * from Members where MembersId = '" + id + "'";
            resultSet = statement.executeQuery(query);

            while (resultSet.next()) {
                members.setId(resultSet.getInt("MembersId"));
                members.setFirstName(resultSet.getString("MembersFirstName"));
                members.setLastName(resultSet.getString("MembersLastName"));
                members.setPosition(resultSet.getString("MembersPosition"));
                members.setHourlyRate(resultSet.getDouble("MembersHourlyRate"));
                members.setGraduated(resultSet.getString("MembersGraduated"));
                members.setLanguages(resultSet.getString("MembersLanguages"));
                members.setCountry(resultSet.getString("MembersCountry"));
                members.setCity(resultSet.getString("MembersCity"));
                members.setAddress(resultSet.getString("MembersAddress"));
            }
        }catch (Exception ex) {
            System.out.println("Error: " + ex);
        }
    }
}

package sample;

import Database.DBConnect;
import Database.Members;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Label;
import javafx.scene.effect.DropShadow;
import javafx.scene.image.Image;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.HBox;
import javafx.scene.paint.Color;
import javafx.scene.paint.ImagePattern;
import javafx.scene.shape.Circle;

import java.io.IOException;
import java.net.URISyntaxException;
import java.net.URL;
import java.util.ResourceBundle;


public class Controller implements Initializable {

    private DBConnect dbConnect;
    private Members members;

@FXML
    private HBox sidebarMembers;

@FXML
    private void openSidebarMembers(MouseEvent event) throws IOException {
        dbConnect = new DBConnect();
        dbConnect.getData(2);
        members = new Members();
        if (event.getSource() == sidebarMembers) {
            System.out.println(members.getFirstName());
        }
    }

控制器

类

package Database;

public class Members {

    private String firstName;

    public String getFirstName() {
        return firstName;
    }

    public void setFirstName(String firstName) {
        this.firstName = firstName;
    }

package Database;

import java.sql.*;

public class DBConnect {
    private Connection connection;
    private Statement statement;
    private ResultSet resultSet;

    Members members = new Members();

    public DBConnect() {
        try {
            Class.forName("com.mysql.cj.jdbc.Driver");

            connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/finance", "root", "");
            statement = connection.createStatement();
            System.out.println("Successfully connected to DATABASE.");
        } catch (Exception ex) {
            System.out.println("ERROR: Can't connect to the Database! -> " + ex);
        }
    }

    public void getData(int id) {
        try {
            String query = "select * from Members where MembersId = '" + id + "'";
            resultSet = statement.executeQuery(query);

            while (resultSet.next()) {
                members.setId(resultSet.getInt("MembersId"));
                members.setFirstName(resultSet.getString("MembersFirstName"));
                members.setLastName(resultSet.getString("MembersLastName"));
                members.setPosition(resultSet.getString("MembersPosition"));
                members.setHourlyRate(resultSet.getDouble("MembersHourlyRate"));
                members.setGraduated(resultSet.getString("MembersGraduated"));
                members.setLanguages(resultSet.getString("MembersLanguages"));
                members.setCountry(resultSet.getString("MembersCountry"));
                members.setCity(resultSet.getString("MembersCity"));
                members.setAddress(resultSet.getString("MembersAddress"));
            }
        }catch (Exception ex) {
            System.out.println("Error: " + ex);
        }
    }
}

package sample;

import Database.DBConnect;
import Database.Members;
import javafx.fxml.FXML;
import javafx.fxml.Initializable;
import javafx.scene.control.Label;
import javafx.scene.effect.DropShadow;
import javafx.scene.image.Image;
import javafx.scene.input.MouseEvent;
import javafx.scene.layout.HBox;
import javafx.scene.paint.Color;
import javafx.scene.paint.ImagePattern;
import javafx.scene.shape.Circle;

import java.io.IOException;
import java.net.URISyntaxException;
import java.net.URL;
import java.util.ResourceBundle;


public class Controller implements Initializable {

    private DBConnect dbConnect;
    private Members members;

@FXML
    private HBox sidebarMembers;

@FXML
    private void openSidebarMembers(MouseEvent event) throws IOException {
        dbConnect = new DBConnect();
        dbConnect.getData(2);
        members = new Members();
        if (event.getSource() == sidebarMembers) {
            System.out.println(members.getFirstName());
        }
    }

我的理解是，当您创建一个新对象时，它将导致将所有变量初始化回0。除了使它们保持静态之外，我还应该如何访问这些值

---

因此，我的问题是，如何从我的

控制器
类中的
DBConnect
类访问我为
成员设置的值？
在DBConnect类中将getData方法的返回类型更改为Member

public Member getData(int id)

现在，您可以从控制器访问该成员

Member member = dbConnect.getData(2);

好吧，你似乎有很多误解。我认为您需要更多地了解类和对象是如何工作的。您需要在此处使用对象实例，而不是类。可能您应该在getData方法中创建一个实例，然后返回此对象
Members=dbConnect.getData（2）？