Question

How do I concatenate strings in EL?

I want to do something like this but it doesn’t work:

${var1 == 0 ? 'hi' : 'hello ' + var2}

It throws an exception trying to cast 'hello' to a Double

Answer 1

The + operator always means numerical addition in JSP Expression Language. To do string concatenation you would have to use multiple adjacent EL expressions like ${str1}${str2}.

If I read your example correctly this could be written as:

${var1 == 0 ? 'hi' : 'hello '}${var1 == 0 ? '' : var2}

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Edit

Another possibility would be to use JSTL, which is longer but might be clearer if there is more text that depends on var1:

<c:choose>
    <c:when test="${var1 == 0}">hi</c:when>
    <c:otherwise>hello <c:out value="${var2}"/></c:otherwise>
</c:choose>

The c:out might not be needed, depending on the JSP version.