变量作为 null 传递给 Netbeans 中的另一个 Java 框架

Question

我正在尝试从 MSSQL 中获取一个值到 class。在我的登录 GUI 框架中，变量传递完全正确，但是当我将那个 class 对象传递给另一个框架时，名称 (String ograd) 变量 returns 为空。

这是 teachermanagement.java 文件：

package Model;

public class teachermanagement {

private int ogrID;
String ograd, ogrsoyad;

public teachermanagement(String ograd, String ogrsoyad) {
    this.ograd = ograd;
    this.ogrsoyad = ogrsoyad;
}

public int getogrID() {
    return ogrID;
}

public void setogrID(int ogrID) {
    this.ogrID = ogrID;
}

public String getograd() {
    return ograd;
}

public void setograd(String ograd) {
    this.ograd = ograd;
}

public String getogrsoyad() {
    return ogrsoyad;
}

public void setogrsoyad(String ogrsoyad) {
    this.ogrsoyad = ogrsoyad;
}

}

loginGUI.java 文件，我通过单击按钮从我的 MSSQL 数据库中获取变量，工作正常。

package View;

import Helper.dbconnection;
import Model.teachermanagement;
import java.sql.Connection;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import javax.swing.JOptionPane;


public class logingui extends javax.swing.JFrame {

    private dbconnection conn = new dbconnection();
    public logingui() {
        initComponents();
    }

    @SuppressWarnings("unchecked")
    Generated Code                     

    private void ogretmenadActionPerformed(java.awt.event.ActionEvent evt) {                                           
        // TODO add your handling code here:
    }                                          

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
               if(ogretmenad.getText().length() == 0 | ogretmenpw.getText().length() == 0)
            JOptionPane.showMessageDialog(null, "Lütfen tüm alanları doldurunuz.", "Uyarı", JOptionPane.INFORMATION_MESSAGE);
        else {  
            try {
                Connection con = conn.DBConn();
                Statement st = con.createStatement();
                ResultSet rs = st.executeQuery("SELECT * from ogretmen");
                while(rs.next()) {
                    if(ogretmenad.getText().equals(rs.getString("ograd")) && ogretmenpw.getText().equals(rs.getString("ogrsifre"))) {
                        teachermanagement tm = new teachermanagement();
                        tm.setogrID(rs.getInt("ogrID"));
                        tm.setograd(rs.getString("ograd"));
                        tm.setogrsoyad(rs.getString("ogrsoyad"));
                        //THIS IS WHERE I TRY TO PASS NAME VARIABLE TO ANOTHER FRAME                    
                        ogretmengui tgui = new ogretmengui(tm);   
                        tgui.setVisible(true);
                        dispose();
                    }
                }
            } catch (SQLException ex) {
                System.out.println(ex);
            }
        }
    }                                        

    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html 
         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(logingui.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(logingui.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(logingui.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(logingui.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                
                new logingui().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JLabel jLabel3;
    private javax.swing.JPanel jPanel1;
    private javax.swing.JPanel jPanel2;
    private javax.swing.JTabbedPane jTabbedPane1;
    private javax.swing.JTextField ogretmenad;
    private javax.swing.JPasswordField ogretmenpw;
    // End of variables declaration                   
}

dbconnection.java 在 Helper 包中：

package Helper;

import java.sql.*;
import java.util.logging.Level;
import java.util.logging.Logger;

public class dbconnection {
    Connection c = null;
    
    public dbconnection() {}
    
    public Connection DBConn() {
        try {
            this.c = DriverManager.getConnection("jdbc:sqlserver://localhost:1433; databaseName=schoolms", "sa", "1234");
            return c;
        } catch (SQLException ex) {
            Logger.getLogger(dbconnection.class.getName()).log(Level.SEVERE, null, ex);
        }
        
        return c;
    }
}

这是我试图将变量传递给的 teacherGUI 框架：

package View;

import Model.teachermanagement;

public class ogretmengui extends javax.swing.JFrame {
    
    static teachermanagement ogr = new teachermanagement();
    public ogretmengui(teachermanagement ogr) {
        initComponents();
    }


    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        jLabel1 = new javax.swing.JLabel();
        jButton1 = new javax.swing.JButton();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("OKUL YÖNETİMİ SİSTEMİ");
        setResizable(false);

        jLabel1.setFont(new java.awt.Font("Dialog", 1, 14)); // NOI18N
        jLabel1.setText("Hoşgeldiniz, " + ogr.getograd());

        jButton1.setText("Çıkış Yap");
        jButton1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton1ActionPerformed(evt);
            }
        });

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jLabel1, javax.swing.GroupLayout.DEFAULT_SIZE, 288, Short.MAX_VALUE)
                .addGap(18, 18, 18)
                .addComponent(jButton1)
                .addContainerGap())
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING, false)
                    .addComponent(jLabel1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE)
                    .addComponent(jButton1, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
                .addContainerGap(262, Short.MAX_VALUE))
        );

        pack();
        setLocationRelativeTo(null);
    }// </editor-fold>                        

    private void jButton1ActionPerformed(java.awt.event.ActionEvent evt) {                                         
        dispose();
    }                                        

    public static void main(String args[]) {
        /* Set the Nimbus look and feel */
        //<editor-fold defaultstate="collapsed" desc=" Look and feel setting code (optional) ">
        /* If Nimbus (introduced in Java SE 6) is not available, stay with the default look and feel.
         * For details see http://download.oracle.com/javase/tutorial/uiswing/lookandfeel/plaf.html

         */
        try {
            for (javax.swing.UIManager.LookAndFeelInfo info : javax.swing.UIManager.getInstalledLookAndFeels()) {
                if ("Nimbus".equals(info.getName())) {
                    javax.swing.UIManager.setLookAndFeel(info.getClassName());
                    break;
                }
            }
        } catch (ClassNotFoundException ex) {
            java.util.logging.Logger.getLogger(ogretmengui.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(ogretmengui.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(ogretmengui.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(ogretmengui.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        }
        //</editor-fold>

        /* Create and display the form */
        java.awt.EventQueue.invokeLater(new Runnable() {
            public void run() {
                try {
                    ogretmengui frame = new ogretmengui(ogr);
                    frame.setVisible(true);
                } 
                catch (Exception ex) {
                    System.out.println(ex);
                }
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JLabel jLabel1;
    // End of variables declaration                    }

如您在 teacherGUI.java 中所见，我正在尝试使用从 loginGUI 框架中获取的变量更改标签文本，但它 returns 为 NULL。当我尝试在 loginGUI 中打印变量时，它 returns 是正确的值，但在 teacherGUI 框架中情况并非如此。任何帮助将不胜感激。

PS: 我隐藏了 loginGUI innit 组件以缩短代码，因为该文件没有任何问题。

Answer 1

您创建了过多的教师管理对象并忽略了传递给构造函数的参数：

代码 1：

// create a new teachermanagement object here
teachermanagement tm = new teachermanagement();

// .....

// and pass it into a constructor here:
ogretmengui tgui = new ogretmengui(tm);   
tgui.setVisible(true);
dispose();

但是你在构造函数中忽略了 tm 对象:

public class ogretmengui extends javax.swing.JFrame {
    
    static teachermanagement ogr = new teachermanagement(); // object 1
    
    // ogr passed into the constructor is object 2, and is ignored
    public ogretmengui(teachermanagement ogr) {
        initComponents();
    }

相反，您不应该在 ogretmengui class 中创建一个新的教师管理，而是使用参数来设置一个字段：

public class ogretmengui extends javax.swing.JFrame {
    
    // should not be static
    // static teachermanagement ogr = new teachermanagement(); // object 1
    private teachermanagement ogr; // should not be static and should not be initialized
    
    // ogr passed into the constructor and used to set the variable:
    public ogretmengui(teachermanagement ogr) {
        this.ogr = ogr; // ***** here *****
        initComponents();
    }

您可能还会运行遇到问题，因为 JFrames 不是 modal 并且不会阻止程序流（例如 JOptionPane 或模态 JDialog 会）。请看一下 The Use of Multiple JFrames: Good or Bad Practice?.

此外，虽然与您的问题无关，但请考虑查看 this Java Naming Conventions 文档：Class 名称应以大写字母开头，变量以小写字母开头。同样，这与您的实际问题或问题无关，但通过遵循这些约定，您将使其他人（我们）更容易理解您的代码，这通常是一件好事。

变量作为 null 传递给 Netbeans 中的另一个 Java 框架

Variables passes as null to another Java Frame in Netbeans

java

null

swing

jframe