Operators

Operators

In general all operators supported in Java are identical in Groovy. Groovy goes a step further by allowing you to customize behavior of operators on Groovy types.

Arithmetic and Conditional Operators

See Operator Overloading for a list of the common operators that Groovy supports.

In addition, Groovy supports the ! (not) operator as follows:

def expression = false
assert !expression

For more details about how expressions are corced to a boolean value, see: Groovy Truth.

Collection-based Operators

Spread Operator (*.)

The Spread Operator is used to invoke an action on all items of an aggregate object. It is equivalent to calling the collect method like so:

parent*.action                             //equivalent to:
parent.collect{ child -> child?.action }

The action may either be a method call or property access, and returns a list of the items returned from each child call. As an example:

assert ['cat', 'elephant']*.size() == [3, 8]

The Spread operator will work as expected with most of the aggregate-like classes within Groovy. You can also customize your own classes to use it by defining your own iterator() method as this example shows:

class Person { String name }
class Twin {
  Person one, two
  def iterator() {
    return [one, two].iterator()
  }
}

def tw = new Twin(one: new Person(name:'Tom'),
                  two: new Person(name:'Tim'))
assert tw*.name == ['Tom', 'Tim']
// expanded equivalent of above:
assert tw.collect{ it.name } == ['Tom', 'Tim']

Object-Related Operators

• invokeMethod and get/setProperty (.)
• Java field (.@)
• The spread java field (*.@)
• Method Reference (.&)
• 'as' - "manual coercion" - asType(t) method
• Groovy == ( equals() ) behavior.
  • "is" for identity
• The instanceof operator (as in Java)

Java field (.@)

Groovy dynamically creates getter method for all your fields that can be referenced as properties:

class X
{
    def field
}

x = new X()
x.field = 1
println x.field    // 1

You can override these getters with your own implementations if you like:

class X
{
    def field

    def getField()
    {
        field += 1
    }
}

x = new X()
x.field = 1
println x.field   // 2

The @ operator allows you to override this behavior and access the field directly, so to extend the previous sample:

println x.@field   // 1

It should be mentioned that, while interesting, this is probably not a good thing to do unless you really need to. Overriding a public interface to access the internal state of an object probably means you are about to break something. Not even recommended for use in tests since it increases coupling unnecessarily.

Other Operators

• getAt() and setAt() for the subscript operator (e.g. foo[1])
• Range Operator (..) - see Collections#Collections-Ranges
• Membership Operator (in)

Elvis Operator (?: )

The "Elvis operator" is a shortening of Java's ternary operator. One instance of where this is handy is for returning a 'sensible default' value if an expression resolves to false or null. A simple example might look like this:

def gender = user.male ? "male" : "female"  //traditional ternary operator usage

def displayName = user.name ?: "Anonymous"  //more compact Elvis operator

Safe Navigation Operator (?.)

The Safe Navigation operator is used to avoid a NullPointerException. Typically when you have a reference to an object you might need to verify that it is not null before accessing methods or properties of the object. To avoid this, the safe navigation operator will simply return null instead of throwing an exception, like so:

def user = User.find( "admin" )           //this might be null if 'admin' does not exist
def streetName = user?.address?.street    //streetName will be null if user or user.address is null - no NPE thrown

Regular Expression Operators

• find (=~)
• match (==~)

For more details, see: Regular Expressions