p:dataTable javax.el.ELException: selection="#{testBean.selectedBook} cannot convert from Object To Book

Question

I have a single selection table, when i select a row, this exception happens:

com.sun.faces.context.PartialViewContextImpl.processPartial javax.el.ELException: /Test.xhtml @18,79 selection="#{testBean.selectedBook}": Cannot convert [Ljava.lang.Object;@5b9b0639 of type class [Ljava.lang.Object; to class com.obs.model.Book
javax.el.ELException: /Test.xhtml @18,79 selection="#{testBean.selectedBook}": Cannot convert [Ljava.lang.Object;@5b9b0639 of type class [Ljava.lang.Object; to class com.obs.model.Book

Caused by: javax.el.ELException: Cannot convert [Ljava.lang.Object;@5b9b0639 of type class [Ljava.lang.Object; to class com.obs.model.Book

Here is my Books table:

<h:form>
    <p:dataTable id="singleDT" var="item" value="#{testBean.booksList}" selectionMode="single"
                 selection="#{testBean.selectedBook}" rowKey="#{item[0]}">

        <p:ajax event="rowSelect" listener="#{testBean.onRowSelect}"/>
        <p:ajax event="rowUnselect" listener="#{testBean.onRowUnselect}"/>

        <p:column headerText="Id">
            <h:outputText value="#{item[0]}"/>
        </p:column>
        <p:column headerText="Title">
            <h:outputText value="#{item[1]}"/>
        </p:column>
        <f:facet name="footer">
            <p:commandButton process="singleDT" icon="ui-icon-search" value="View" action="#{testBean.showPlease}"/>
        </f:facet>
    </p:dataTable>
</h:form>

And this is testBean :

private List<Book> booksList;
private Book selectedBook;

public Book getSelectedBook() {
    return selectedBook;
}

public void setSelectedBook(Book selectedBook) {
    this.selectedBook = selectedBook;
}

public void onRowSelect(SelectEvent event) {
    System.out.println("row selected, " + event.getObject()); // displays [Ljava.lang.Object;@62699e1d
}

public void onRowUnselect(UnselectEvent event) {
    System.out.println("row Unselected");
}

@Transactional
public List<Book> allBooks() {
// fill the list with hibernate
    return booksList;
}

//getter/setters

I assigned the selected row to selectedBook

Error is cannot convert from type object to Book type.

selectedBook variable type is Book, but which variable is object which causes this error?

Answer 1

You're victim of generic type erasure.

Your List<Book> bookList is actually being filled with List<Object[]>, not List<Book>. This is confirmed by the fact that #{item[0]}, #{item[1]}, etc in your JSF page didn't throw an EL exception. You knew that because of your previous question https://stackoverflow.com/questions/29975635/java-lang-numberformatexception-for-input-string-while-iterating-over-a-pdatat and the answer in the linked duplicate.

You need to fix your Hibernate query in such way that it returns a real List<Book>, not a List<Object[]>. For example, you need to perform a SELECT b FROM Book b instead of SELECT b.id, b.title FROM Book b and set Book.class as return class when creating the query. You should not have need to add a cast on (List<Book>). A bit sane IDE would show an unchecked cast warning on that. That should already have signaled that something is possibly wrong here.

After you've fixed your Hibernate query, you need to make sure that the below test case runs successfully without ClassCastException:

List<Book> books = callYourHibernateQueryHere();

for (Book book : books) {
    System.out.println(book.getTitle());
}

Only then you can use rowKey="#{item.id}", #{item.id}, #{item.title}, etc.

Important note that this problem is technically unrelated to JSF. You would have had exactly the same problem when presenting Hibernate results in a different way, such as using System.out.println() as demonstrated above.

If fixing the Hibernate query is not an option for some reason, then you need to replace Book by Object[] over all place in your JSF managed bean. E.g. List<Object[]> bookList, Object[] selectedBook, etc. Needless to say that this is a terrible idea.

Answer 2

If you look at the FULL stacktrace, you'll most likely see that the error is in YOUR code. You can and should investigate a classcast exception easily yourself

As a sidenode:

• value="#{testBean.booksList}" refers to a method in your bean that is NOT shown in the bean code you posted (but very, very relevant, since an individual item in that list is what is in selection)
• Do you have any clue how the p:dataTable works? I think not. Look at the PF showcase and check all the differences functionalities of it and check the source code and compare with yours
• You should not only learn some basic jsf/jpa but basic developer skills