Java 如何将JFrame居中？ 嘿，伙计们，我做了一个连接到我的DB的小登录应用程序，但是我有一个小问题，我不知道该如何处理这个框架，我的意思是当我启动应用程序使它正好在屏幕中间时。 我试过这样做： Dimension screenSize = Toolkit.getDefaultToolkit().getScreenSize()

但我不知道该把它放在哪里，而且它是不完整的。 提前谢谢

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package mysqltryouts;

import java.awt.event.WindowAdapter;
import java.awt.event.WindowEvent;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.Statement;
import java.util.Scanner;
import javax.swing.JFrame;
import javax.swing.JOptionPane;

/**
 *
 * @author ExtremeSwat
 */





public class login_panel extends javax.swing.JFrame {


  private Connection connect = null;
  private Statement statement = null;
  private PreparedStatement preparedStatement = null;
  private ResultSet resultSet = null;




  public void readDataBase() throws Exception
  {

       String username = jTextField1.getText();

      String password = jPasswordField1.getText();

//    System.out.println(username);
//    System.out.println(password);
//    System.out.println("test");

      //---------------------

//      String databaseUsername = "";
//      String databasePassword = "";

    try {
      // this will load the MySQL driver, each DB has its own driver

      Class.forName("com.mysql.jdbc.Driver");

      // setup the connection with the DB.

      connect = DriverManager
          .getConnection("jdbc:mysql://localhost/cards?"
              + "user=root&password=password");





     //String sqlQuery = "select count(*) > 0 as match_found FROM username WHERE username = ? and password = MD5(?)";
      String sqlQuery = "SELECT * FROM username WHERE username = ? and password = MD5(?)";
       // String sqlQuery = "select count(*) > 0 as cnt FROM username WHERE username = ? and password = MD5(?)";

      PreparedStatement pst = connect.prepareStatement( sqlQuery );
pst.setString( 1, username );
pst.setString( 2, password );

ResultSet rs = pst.executeQuery();

if( rs.next() ) {


    System.out.println("You did it!");
}
else
    System.out.println("Bad PW");


    } catch (Exception e) {
      throw e;
    } finally {
      close();
    }

  }


            private void close() {
    close(resultSet);
    close(statement);
    close(connect);
  }

  private void close(AutoCloseable c) throws UnsupportedOperationException {
    try {
      if (c != null) {
        c.close();
      }
    } catch (Exception e) {
    // don't throw now as it might leave following closables in undefined state
    }

}



    /**
     * Creates new form login_panel
     */
    public login_panel() {
        initComponents();
    }



    /**
     * This method is called from within the constructor to initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is always
     * regenerated by the Form Editor.
     */
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                          
    private void initComponents() {

        jTextField1 = new javax.swing.JTextField();
        jPasswordField1 = new javax.swing.JPasswordField();
        jLabel1 = new javax.swing.JLabel();
        jLabel2 = new javax.swing.JLabel();
        jButton1 = new javax.swing.JButton();
        jButton2 = new javax.swing.JButton();
        jLabel3 = new javax.swing.JLabel();

        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);

        jTextField1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jTextField1ActionPerformed(evt);
            }
        });

        jPasswordField1.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jPasswordField1ActionPerformed(evt);
            }
        });

        jLabel1.setText("Username");

        jLabel2.setText("Password");

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

        jButton2.setText("eXIT");
        jButton2.addActionListener(new java.awt.event.ActionListener() {
            public void actionPerformed(java.awt.event.ActionEvent evt) {
                jButton2ActionPerformed(evt);
            }
        });

        jLabel3.setFont(new java.awt.Font("Old English Text MT", 1, 36)); // NOI18N
        jLabel3.setText("Login B0$$");

        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
        getContentPane().setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                .addContainerGap(66, Short.MAX_VALUE)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                        .addComponent(jButton2)
                        .addContainerGap())
                    .addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
                        .addComponent(jLabel3)
                        .addGap(53, 53, 53))))
            .addGroup(layout.createSequentialGroup()
                .addGap(20, 20, 20)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addComponent(jLabel1)
                    .addComponent(jLabel2))
                .addGap(18, 18, 18)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
                    .addGroup(layout.createSequentialGroup()
                        .addComponent(jTextField1, javax.swing.GroupLayout.PREFERRED_SIZE, 92, javax.swing.GroupLayout.PREFERRED_SIZE)
                        .addGap(18, 18, 18)
                        .addComponent(jButton1))
                    .addComponent(jPasswordField1, javax.swing.GroupLayout.PREFERRED_SIZE, 92, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addContainerGap(javax.swing.GroupLayout.DEFAULT_SIZE, Short.MAX_VALUE))
        );
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGroup(layout.createSequentialGroup()
                .addContainerGap()
                .addComponent(jLabel3)
                .addGap(38, 38, 38)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel1)
                    .addComponent(jTextField1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE)
                    .addComponent(jButton1))
                .addGap(4, 4, 4)
                .addGroup(layout.createParallelGroup(javax.swing.GroupLayout.Alignment.BASELINE)
                    .addComponent(jLabel2)
                    .addComponent(jPasswordField1, javax.swing.GroupLayout.PREFERRED_SIZE, javax.swing.GroupLayout.DEFAULT_SIZE, javax.swing.GroupLayout.PREFERRED_SIZE))
                .addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED, 63, Short.MAX_VALUE)
                .addComponent(jButton2))
        );

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

    private void jTextField1ActionPerformed(java.awt.event.ActionEvent evt) {                                            
        // TODO add your handling code here:
    }                                           

    private void jPasswordField1ActionPerformed(java.awt.event.ActionEvent evt) {                                                
        // TODO add your handling code here:
    }                                               

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








  try{ 


//   login_panel dao = new login_panel();
//   
//   
//   dao.readDataBase();

      this.readDataBase();

     }
  catch(Exception e)
  {
      System.out.println(e.getMessage());
  }


    }                                        

    private void jButton2ActionPerformed(java.awt.event.ActionEvent evt) {                                         


   int dialogButton = JOptionPane.YES_NO_OPTION;

    int dialogResult = JOptionPane.showConfirmDialog(null, "Exit? ", "Information",dialogButton);

        if(dialogResult == JOptionPane.YES_OPTION)
        {
            JOptionPane.showMessageDialog(null, "Thanks for the stay","Confirm",JOptionPane.INFORMATION_MESSAGE);
            System.exit(0);

        }else
        {
            JOptionPane.showMessageDialog(null, "Remaining...","Remaining....",JOptionPane.INFORMATION_MESSAGE);

        }

    }                                        

    /**
     * @param args the command line arguments
     */
    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(login_panel.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (InstantiationException ex) {
            java.util.logging.Logger.getLogger(login_panel.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (IllegalAccessException ex) {
            java.util.logging.Logger.getLogger(login_panel.class.getName()).log(java.util.logging.Level.SEVERE, null, ex);
        } catch (javax.swing.UnsupportedLookAndFeelException ex) {
            java.util.logging.Logger.getLogger(login_panel.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 login_panel().setVisible(true);
            }
        });
    }

    // Variables declaration - do not modify                     
    private javax.swing.JButton jButton1;
    private javax.swing.JButton jButton2;
    private javax.swing.JLabel jLabel1;
    private javax.swing.JLabel jLabel2;
    private javax.swing.JLabel jLabel3;
    private javax.swing.JPasswordField jPasswordField1;
    private javax.swing.JTextField jTextField1;
    // End of variables declaration                   
}

这样做

imension dim = Toolkit.getDefaultToolkit().getScreenSize();
frame.setLocation(dim.width/2-this.getSize().width/2, dim.height/2-this.getSize().height/2);

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如果您使用的是Java 1.4或更高版本，则可以使用对话框、框架或窗口上的简单方法setLocationRelativeTonull将其居中

//new login_panel().setVisible(true);  <-- get rid of this
login_panel frame = new login_panel();
frame.setLocationRelativeTo(null);
frame.setVisible(true);

在你的跑步中

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旁注：遵循Java命名约定。类名使用大写字母，使用驼峰式大小写。登录面板→ LoginPanel

frame.setLocationRelativeToNull我应该将其放置在哪里？对于帧定位，不能使用setLocationByPlatformtrue。请参见演示。