想给这段加个剩余票数为零时点购买会提示购票失败余票为0的效果，请问该怎么做？

public class Purchase {
private Connection conn;

public Purchase(String a,String user){
    try{
        boolean j = Judge(a);
        if(j == false){
            JOptionPane.showMessageDialog(null, "购票失败！不存在此车次号！");
        }
        else{
            Class.forName("com.microsoft.sqlserver.jdbc.SQLServerDriver");
            conn = DriverManager.getConnection("jdbc:sqlserver://127.0.0.1:1433;DatabaseName=火车售票系统","sa","123456");
            String sql = "INSERT INTO "+user+"订购信息 SELECT 车次号,始发地,目的地,出发时间  FROM 车次信息 WHERE 车次号='"+a+"' GROUP BY 车次号,始发地,目的地,出发时间";    
            String sql2 = "UPDATE 车次信息 SET 剩余票数=剩余票数-1 WHERE 车次号='"+a+"'";  //更新车次信息
            java.sql.Statement stmt = conn.createStatement();
            stmt.executeUpdate(sql);
            stmt.executeUpdate(sql2);
            conn.close();
            JOptionPane.showMessageDialog(null, "购票成功！");
        }
    }catch (ClassNotFoundException e) {
        JOptionPane.showMessageDialog(null, "购票失败！连接数据库失败！");
        e.printStackTrace();
    }catch (SQLException e) {
        JOptionPane.showMessageDialog(null, "购票失败！您已购买此车次！");
        e.printStackTrace();
     } catch (Exception e) {
         JOptionPane.showMessageDialog(null, "购票失败！连接数据库失败！");
        e.printStackTrace();
    }          
}

如果a小于或等于0，if判断一下，提示票数不够