public void actionPerformed(ActionEvent e) { String username=uField.getText().trim(); String password =pField.getText().trim(); String phonenum=tField.getText().trim(); String gender=bg.isSelected(maleBtn.getModel())?maleBtn.getText():femaleBtn.getText(); String Driver = "com.mysql.cj.jdbc.Driver"; String url = "jdbc:mysql://localhost:3306/data2?characterEncoding=UTF8&useSSL=false"; Connection conn = null; try{ Class.forName(Driver); conn = DriverManager.getConnection(url, "root", "yxh9497"); Statement stmt=conn.createStatement(); ResultSet rs = stmt.executeQuery("SELECT * FROM userinformation"); while(rs.next()) { if(username==rs.getString(username)) { JOptionPane.showMessageDialog(null, "用户名已被注册"); break; } } String sql1="insert into userinformation values('"+ username+ "','"+password+"','"+phonenum+"','"+gender+"')"; int a= stmt.executeUpdate(sql1); }catch(Exception ex){ ex.printStackTrace(); } try{ //关闭数据库联接类 conn.close(); }catch(Exception ex){ ex.printStackTrace(); } JOptionPane.showMessageDialog(null, "注册成功"); jf.setVisible(false); new GUI().init();}我想在当用户名重复时，不再执行String sql1="insert into userinformation values('"+ username+ "','"+password+"','"+phonenum+"','"+gender+"')"; int a= stmt.executeUpdate(sql1); }catch(Exception ex){ ex.printStackTrace(); } try{ //关闭数据库联接类 conn.close(); }catch(Exception ex){ ex.printStackTrace(); } JOptionPane.showMessageDialog(null, "注册成功"); jf.setVisible(false); new GUI().init();}这些语句，该怎么办

你可以在 while 循环中加一个标志变量，如 isExist，初始值为 false。如果找到重复的用户名，将该变量设为 true，然后在 while 循环结束后根据 isExist 的值来判断是否执行插入语句和注册成功的提示框。修改后的代码如下：

public void actionPerformed(ActionEvent e) {
    String username = uField.getText().trim();
    String password = pField.getText().trim();
    String phonenum = tField.getText().trim();
    String gender = bg.isSelected(maleBtn.getModel()) ? maleBtn.getText() : femaleBtn.getText();
    String Driver = "com.mysql.cj.jdbc.Driver";
    String url = "jdbc:mysql://localhost:3306/data2?characterEncoding=UTF8&useSSL=false";
    Connection conn = null;
    boolean isExist = false; // 标志变量
    try {
        Class.forName(Driver);
        conn = DriverManager.getConnection(url, "root", "yxh9497");
        Statement stmt = conn.createStatement();
        ResultSet rs = stmt.executeQuery("SELECT * FROM userinformation");
        while (rs.next()) {
            if (username.equals(rs.getString("username"))) { // 修改此处
                JOptionPane.showMessageDialog(null, "用户名已被注册");
                isExist = true; // 修改标志变量的值
                break;
            }
        }
        if (!isExist) { // 如果用户名不存在，才执行以下语句
            String sql1 = "insert into userinformation values('" + username + "','" + password + "','" + phonenum + "','" + gender + "')";
            int a = stmt.executeUpdate(sql1);
            JOptionPane.showMessageDialog(null, "注册成功");
            jf.setVisible(false);
            new GUI().init();
        }
    } catch (Exception ex) {
        ex.printStackTrace();
    }
    try {
        conn.close();
    } catch (Exception ex) {
        ex.printStackTrace();
    }
}