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I would like to concatenate a string within a ternary operator in EL(Expression Language).

Suppose there is a variable named value. If it's empty, I want to use some default text. Otherwise, I need to append it with some static text.

${(empty value)? "none" : value + " enabled"}

This will not compile however. What would be a correct way to write this? Or is this even possible?

Aniket Kulkarni
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Tom Tucker
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7 Answers7

108

With EL 2 you can do the following:

#{'this'.concat(' is').concat(' a').concat(' test!')}
Joel
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    `${'this'.concat(' is').concat(' a').concat(' test!')}` would be the normal form. – Per Lindberg Jan 29 '14 at 11:46
  • Does this work with non-string types, e.g. `${'I am'.concat(' number ').concat(1).concat('!')}`? – Martin Jul 29 '15 at 22:30
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    @MartinCarney concat() does only support string arguments – Joel Jul 30 '15 at 07:23
  • You need EL 2.2 to call non-getter method (see [How can I check what version of EL is server using](http://stackoverflow.com/a/7237216/4944847)) – agabrys Sep 24 '15 at 14:34
  • @Joel doesn't Java coerce the (non-String) argument of `String.concat()` to String automatically calling its `toString()` method? – Mindwin Dec 11 '15 at 12:19
  • @Mindwin I haven't used EL for a while, but with normal Java definitely not. If it's a non-null object, you could call toString() on it though. – Joel Dec 11 '15 at 16:54
  • @joel EL coerces the argument before submitting to backing Java code. – Mindwin Dec 11 '15 at 17:00
48

This answer is obsolete. Technology has moved on. Unless you're working with legacy systems see Joel's answer.

There is no string concatenation operator in EL. If you don't need the concatenated string to pass into some other operation, just put these expressions next to each other:

${value}${(empty value)? 'none' : ' enabled'}
Community
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McDowell
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45

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:

<c:out value="${empty value ? 'none' : value += ' enabled'}" />

If you're however not on EL 3.0 yet, and the value is a genuine java.lang.String instance (and thus not e.g. java.lang.Long), 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():

<c:out value="${empty value ? 'none' : value.concat(' enabled')}" />

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="enabled" value="${value} enabled" />
<c:out value="${empty value ? 'none' : enabled}" />
BalusC
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14

Since Expression Language 3.0, it is valid to use += operator for string concatenation.

${(empty value)? "none" : value += " enabled"}  // valid as of EL 3.0

Quoting EL 3.0 Specification.

String Concatenation Operator

To evaluate 

A += B
  • Coerce A and B to String.
  • Return the concatenated string of A and B.
Vsevolod Golovanov
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Aniket Kulkarni
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1

Mc Dowell's answer is right. I just want to add an improvement if in case you may need to return the variable's value as:

${ empty variable ? '<variable is empty>' : variable }
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Mohammad Faisal
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0

it also can be a great idea using concat for EL + MAP + JSON problem like in this example :

#{myMap[''.concat(myid)].content}

Benjamin Fuentes
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0

1.The +(operator) has not effect to that in using EL. 2.so this is the way,to use that 

<c:set var="enabled" value="${value} enabled" />


<c:out value="${empty value ? 'none' : enabled}" />

is this helpful to You ?

usr-local-ΕΨΗΕΛΩΝ
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Adalarasan Sachithanantham
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