How to simulate a group-by in JSTL view layer by keeping track of previous & current values while looping over a List?

Question

First I retrieve the Note objects in the service layer:

List<Note> notes = this.getNotes();

Then I pass List of Notes to the view layer and set the same initial value for prevNote and currNote to the first element of the List:

<c:forEach items="${notes}" var="note" begin="0" end="1" step="1">
    <c:set var="prevNote">${note}</c:set>
    <c:set var="currNote">${note}</c:set>
</c:forEach>

Then I iterate over the List and set prevNote to currNote and currNote to the current note item (note) from the loop:

<c:forEach items="${notes}" var="note">

    <c:set var="prevNote">${currNote}</c:set>
    <c:set var="currNote">${note}</c:set>

    prev:<c:out value="${prevNote.userCode}"/>
    curr:<c:out value="${currNote.userCode}"/>

</c:forEach>

But when I run this, I get this error message:

    Unable to find a value for "note" in object of 
    class "java.lang.String" using operator ".

What I want to do here is check whether prevNote and currNote contain the same userCode value - if they do I want to skip the current element in the List and display the next Note that has a different userCode (sort of simulating a group-by operation).

Any hints on how to do this and why I'm getting this error message?

Answer 1

Setting a value in the body of c:set will essentially convert it to String using toString() method of the object. The exception is also hinting that you aren't working on a fullworthy Note instance, but on a String one.

Use the value attribute instead.

<c:set var="prevNote" value="${note}" />
<c:set var="currNote" value="${note}" />

---

As to your functional requirement:

What I want to do here is check whether prevNote and currNote contain the same userCode value - if they do I want to skip the current element in the List and display the next Note that has a different userCode (sort of simulating a group-by operation).

Just storing the current note at end of the loop is sufficient. E.g.

<c:forEach items="${notes}" var="note">
    <c:if test="${empty prevNote or note.userCode != prevNote.userCode}">
         <c:out value="${note.userCode}" />
    </c:if>
    <c:set var="prevNote" value="${note}" />
</c:forEach>