Programming Grails - Cdn.oreilly.com

You’ve already seen how property access and the compact syntax for collections and
maps can help condense your code, but there are a lot more ways.
Constructors
It’s rare to see constructors in Groovy classes. This is because the Groovy compiler adds
a constructor that takes a Map and sets field values to map values where the key corre‐
sponds to a field name. This gives you named parameters for this constructor syntax,
so it’s both more convenient and clearer which values are which. For example, a simple
POGO like this:
class Person {
String firstName
String initial
String lastName
Integer age
}
can be constructed by setting some or all of the field values:
def author = new Person(firstName: 'Hunter', initial: 's', lastName: 'Thompson')
def illustrator = new Person(firstName: 'Ralph', lastName: 'Steadman', age: 76)
def someoneElse = new Person()
In the examples, I’m taking advantage of Groovy letting me omit the [ and ] map
characters, because it makes the invocations cleaner.
This is especially useful for classes with many fields; in Java, you have to either define
multiple constructors with various signatures or pass lots of nulls where you don’t have
a value.
However, note that the Map constructor relies on the default constructor that’s added to
all classes that don’t define any explicit constructors. It calls that constructor, then sets
properties from the provided Map (this is defined in MetaClassImpl.invokeConstruc
tor(), if you’re curious). But if you declare one or more parameterized constructors,
the compiler doesn’t generate an empty one for you, and the Map constructor will fail.
Also, because it’s not a real constructor that’s added to the bytecode, you can use this
with Java classes that have a default constructor, too. So you could replace this code:
MutablePropertyValues propertyValues = ...
def beanDef = new GenericBeanDefinition()
beanDef.setBeanClassName('com.foo.bar.ClassName')
beanDef.setAutowireMode(AbstractBeanDefinition.AUTOWIRE_BY_TYPE)
beanDef.setPropertyValues(propertyValues)
with this:
MutablePropertyValues propertyValues = ...
def beanDef = new GenericBeanDefinition(
beanClassName: 'com.foo.bar.ClassName',
14 | Chapter 1: Introduction to Groovy