制作登录表格。如果块表现良好，否则块不执行

Question

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

    String username=reg_username.getText();
    String password=reg_password.getText();
    int age=Integer.parseInt(reg_age.getText());
    String gender=null;
    if(reg_rdomale.isSelected()){
    gender="Male";
    }else if(reg_rdofemale.isSelected()){
    gender="Female";
    }
    String country=reg_country.getSelectedItem().toString();        
        try {
        Statement st=conn.createStatement();
        String Querytwo=("select * from registrationform where UserName='"+reg_username.getText()+"'");
        ResultSet rs=st.executeQuery(Querytwo);
        while(rs.next())
        if(rs.getString(1).equals(reg_username.getText())){
        JOptionPane.showMessageDialog(this, "UserName Already Exists.");
        }else{            
        String Query="insert into registrationform values('"+username+"','"+password+"','"+age+"','"+gender+"','"+country+"')";
        st.execute(Query);
        JOptionPane.showMessageDialog(this, "SignUp Complete\nYou are now Registered.");
        }
    } catch (SQLException ex) {
        Logger.getLogger(insert.class.getName()).log(Level.SEVERE, null, ex);            
        JOptionPane.showMessageDialog(this, ex.getMessage());
    }       

}

//如果我只使用插入块，它绝对工作。但当我添加if else来比较用户名是否已经存在。当条件变为假时，它执行if块但不阻塞