Groovy for Domain-specific Languages - Second Edition

Chapter 7

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Expandos

An Expando is a dynamic representation of a typical Groovy bean. Expandos support
typical get and set style bean access but, in addition to this, they accept gets and
sets to arbitrary properties. If we try to access a non-existing property, the Expando
does not mind and, instead of causing an exception, it will return null. If we set a
non-existent property, the Expando will add that property and set the value. In order
to create an Expando, we instantiate an object of class groovy.util.Expando:

given:

def customer = new Expando()

expect:

customer.properties == [:]

customer.id == null

customer.properties == [:]

when:

customer.id = 1001

customer.firstName = "Fred"

customer.surname = "Flintstone"

customer.street = "1 Rock Road"

then:

customer.id == 1001

customer.properties == [

id:1001, firstName:'Fred',

surname:'Flintstone', street:'1 Rock Road']

The id field of customer is accessible on the Expando shown in the preceding
example even when it does not exist as a property of the bean. Once a property
has been set, it can be accessed by using the normal field getter, for example,
customer.id. Expandos are a useful extension to normal beans when we need
to be able to dump arbitrary properties into a bag and we don't want to write a
custom class to do so.