Hidden features of Groovy?

Answer 1

Remove null values from list

def list = [obj1, obj2, null, obj4, null, obj6]
list -= null
assert list == [obj1, obj2, obj4, obj6]

Answer 2

The features provided by the transformations inside the GDK's groovy.transform package, such as:

• @Immutable: The @Immutable annotation instructs the compiler to execute an AST transformation which adds the necessary getters, constructors, equals, hashCode and other helper methods that are typically written when creating immutable classes with the defined properties.
• @CompileStatic: This will let the Groovy compiler use compile time checks in the style of Java then perform static compilation, thus bypassing the Groovy meta object protocol.
• @Canonical: The @Canonical annotation instructs the compiler to execute an AST transformation which adds positional constructors, equals, hashCode and a pretty print toString to your class.

Others:

• @Slf4j This local transform adds a logging ability to your program using LogBack logging. Every method call on a unbound variable named log will be mapped to a call to the logger.
• Groovy's XML Slurper: easy parsing of XML. Killer feature!

Answer 3

I know I'am a bit late but I think there are some nice features missing here:

Collection plus/minus operators

def l = [1, 2, 3] + [4, 5, 6] - [2, 5] - 3 + (7..9)
assert l == [1, 4, 6, 7, 8, 9]

def m = [a: 1, b: 2] + [c: 3] - [a: 1]
assert m == [b: 2, c: 3]

Switch statement

switch (42) {
  case 0: .. break
  case 1..9: .. break
  case Float: .. break
  case { it % 4 == 0 }: .. break
  case ~/\d+/: .. break
}

Ranges and indexing

assert (1..10).step(2) == [1, 3, 5, 7, 9]
assert (1..10)[1, 4..8] == [2, 5, 6, 7, 8, 9]
assert ('a'..'g')[-4..-2] == ['d', 'e', 'f']

Unicode variable names

def α = 123
def β = 456
def Ω = α * β
assert Ω == 56088

Answer 4

Using the spread-dot operator

def animals = ['ant', 'buffalo', 'canary', 'dog']
assert animals.size() == 4
assert animals*.size() == [3, 7, 6, 3]

This is a shortcut for animals.collect { it.size() }.

Answer 5

Using the spread operator in method parameters

This is a great help when converting code to data:

def exec(operand1,operand2,Closure op) {
    op.call(operand1,operand2)
}

def addition = {a,b->a+b}
def multiplication = {a,b->a*b}

def instructions = [
     [1,2,addition],
     [2,2,multiplication]
]

instructions.each{instr->
    println exec(*instr)
}

Also helpful is this usage:

String locale="en_GB"

//this invokes new Locale('en','GB')
def enGB=new Locale(*locale.split('_'))

Answer 6

Coercion operator

The coercion operator (as) is a variant of casting. Coercion converts object from one type to another without them being compatible for assignment. Let’s take an example:

Integer x = 123
String s = (String) x
Integer is not assignable to a String, so it will produce a ClassCastException at runtime This can be fixed by using coercion instead:

Integer x = 123 String s = x as String
Integer is not assignable to a String, but use of as will coerce it to a String