显示JTable时出错

Question

我想显示数据库中存在的JTable内容问题，当我点击“Afficher”时，它是一个简单的显示但它没有用，我没有发现错误

这是结果

public class PreparerTestAutomatique extends JPanel
List<Question> questions;
JButton btnAfficher = new JButton("Afficher");
    btnAfficher.addActionListener(new ActionListener() {
        public void actionPerformed(ActionEvent arg0) {
            categorie=GestionCategorieDelegate.doFindCategorieById(PreparerTest.idCategorie);
            if(Facile.getText().length()!=0){
                questions=GestionTestDelegate.doPrepareRandomTest(Integer.parseInt(Facile.getText()), categorie,"Facile");
            } else{

            }
            if(Moyen.getText().length()!=0){
                questions=GestionTestDelegate.doPrepareRandomTest(Integer.parseInt(Moyen.getText()), categorie, "Moyen");
            }else{

            }
            if(Difficile.getText().length()!=0){
                questions=GestionTestDelegate.doPrepareRandomTest(Integer.parseInt(Difficile.getText()), categorie, "Difficile");
            }else{

            }
        }
    });

这是方法PrepareRandomTest它运行得很好我测试了它

 @Override
public List<Question> prepareRandomTest(int number_of_questions, Categorie categorie, String niveauDeDifficulte){ 

          String jpql = "SELECT q FROM Question q where q.categorie=:categorie and q.niveauDeDifficulte=:niveauDeDifficulte ORDER BY RAND()";
          Query query = entityManager.createQuery(jpql);
          query.setParameter("categorie", categorie).setParameter("niveauDeDifficulte", niveauDeDifficulte);
          query.setMaxResults(number_of_questions);
          List<Question> result = query.getResultList();
          Collections.shuffle(result);
          return result;      
}