Vaadin CRM Demo创建抽象表单类触发ComponentEventListener类型推断错误

问题根因 #

报错本质是泛型类型不匹配+父类成员隐藏未初始化两个问题共同导致的：

1. ContactForm中重复定义了Binder<Contact> binder，隐藏了父类AbstractForm中定义的Binder<T> binder，父类方法调用的是未初始化的父类binder，同时事件绑定过程中泛型推导失败
2. 事件内部类的泛型定义不严谨，导致addListener调用时无法匹配方法参数的事件类型

修复步骤 #

1. 调整AbstractForm的成员访问权限，修正事件泛型 #

修改AbstractForm类代码：

public abstract class AbstractForm<T> extends FormLayout {
    public T entity;
    // 将binder改为protected，允许子类初始化
    protected Binder<T> binder;
    Button save = new Button("Save");
    Button delete = new Button("Delete");
    Button close = new Button("Cancel");

    // 其余原有代码保持不变，仅修改事件类定义部分
    public abstract class EntityFormEvent extends ComponentEvent<AbstractForm<T>> {
        private final T entity;
        // 构造方法source添加泛型约束
        protected EntityFormEvent(AbstractForm<T> source, T entity) {
            super(source, false);
            this.entity = entity;
        }
        public T getEntity() {
            return entity;
        }
    }

    public class SaveEvent extends EntityFormEvent {
        SaveEvent(AbstractForm<T> source, T entity) {
            super(source, entity);
        }
    }

    public class DeleteEvent extends EntityFormEvent {
        DeleteEvent(AbstractForm<T> source, T entity) {
            super(source, entity);
        }
    }

    public class CloseEvent extends EntityFormEvent {
        CloseEvent(AbstractForm<T> source) {
            super(source, null);
        }
    }
}

2. 删除ContactForm中重复的binder定义，初始化父类binder #

修改ContactForm类代码，移除自身重复定义的binder，直接给父类binder赋值：

public class ContactForm extends AbstractForm<Contact> {
  TextField firstName = new TextField("First name");
  TextField lastName = new TextField("Last name");
  EmailField email = new EmailField("Email");
  ComboBox<Status> status = new ComboBox<>("Status");
  ComboBox<Company> company = new ComboBox<>("Company");

  public ContactForm(List<Company> companies, List<Status> statuses) {
    addClassName("contact-form");
    // 直接初始化父类的binder，不要重复声明
    binder = new BeanValidationBinder<>(Contact.class);
    binder.bindInstanceFields(this);

    company.setItems(companies);
    company.setItemLabelGenerator(Company::getName);
    status.setItems(statuses);
    status.setItemLabelGenerator(Status::getName);
    add(firstName,
        lastName,
        email,
        company,
        status,
        createButtonsLayout()); 
  }
}

3. 调整ListView中事件方法的参数类型 #

把saveContact、deleteContact的参数类型调整为对齐父类泛型的事件类型：

private void saveContact(AbstractForm<Contact>.SaveEvent event) {
    service.saveContact(event.getEntity());
    updateList();
    closeEditor();
}

private void deleteContact(AbstractForm<Contact>.DeleteEvent event) {
    service.deleteContact(event.getEntity());
    updateList();
    closeEditor();
}

如果仍然存在泛型推导问题，可在addListener调用时显式指定泛型：

form.<AbstractForm<Contact>.SaveEvent>addListener(AbstractForm.SaveEvent.class, this::saveContact);
form.<AbstractForm<Contact>.DeleteEvent>addListener(AbstractForm.DeleteEvent.class, this::deleteContact);
form.<AbstractForm<Contact>.CloseEvent>addListener(AbstractForm.CloseEvent.class, e -> closeEditor());

修改后重新构建即可解决报错，此时父类binder已正确初始化，事件类型完全对齐，ComponentEventListener的泛型可以正常推导。

内容的提问来源于stack exchange，提问作者Lewis Liu