How to concatenate a String in EL?

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If you’re already on EL 3.0 (Java EE 7; WildFly, Tomcat 8, GlassFish 4, etc), then you could use the new += operator for this:

<p>${not empty promoPrice ? 'ONLY $' += promoPrice : 'FREE'}</p>

If you’re however not on EL 3.0 yet, then use EL 2.2 (Java EE 7; JBoss AS 6/7, Tomcat 7, GlassFish 3, etc) capability of invoking direct methods with arguments, which you then apply on String#concat():

<p>${not empty promoPrice ? 'ONLY $'.concat(promoPrice) : 'FREE'}</p>

Or if you’re even not on EL 2.2 yet, then use JSTL <c:set> to create a new EL variable with the concatenated values just inlined in value:

<c:set var="promoPriceString" value="ONLY $${promoPrice}" />
<p>${not empty promoPrice ? promoPriceString : 'FREE'}</p>

In your particular case, another way is to split the expression in two parts:

<p>${not empty promoPrice ? 'ONLY $' : 'FREE'}${promoPrice}</p>

If ${promoPrice} is null or empty, it won’t be printed anyway.

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