Groovy – My Study Notes – Part 1


  • Use Groovy for flexibility and readability. Use Java for performance
  • Runs on JVM – Groovy is nothing but a new way of creating Java classes – Java code can be called from Groovy and vice-versa
  • Every Groovy type is a subtype of java.lang.Object – Every Groovy object is an instance of a type in the normal way
  • Groovy class IS A Java class
  • Groovy supports dynamic typing
  • To compile a Groovy script – { groovyc –d classes Foo.groovy }
  • To run a compiled Groovy class in Java – { java -cp $GROOVY_HOME/embeddable/groovy-all-1.0.jar:classes Foo }
  • To run a Groovy script – {groovy Foo.groovy}
  • Behind the scenes it compiles to a Java class and executes
  • Any Groovy code can be executed this way as long as it can be run; that is, it is either a script, a class with a main method, a Runnable, or a GroovyTestCase.
  • Groovy is purely object-oriented
    • everything is an object. E.g 2*3 //though they look like primitives, they are actually java.lang.Integer objects
    • every operator is a method call. E.g. a+b //logic for the + operator is implemented in method plus() on the object
  • Groovy automatically imports following packages: groovy.lang.*, groovy.util.*, java.lang.*, java.util.*,*, and* as well as the classes java.math.BigInteger and BigDecimal.
  • Say there is a Groovy class called Foo, we can use Foo objects without explicitly compiling the Book class as long as Foo.groovy is on the classpath.
  • A Groovy script can also have class definitions inside them. 

Control Structure

Boolean Evaluation

Groovy’s == Is Equal to Java’s equals only if the class does not implement the Comparable interface. If it does, then it maps to the class’s compareTo() method.. Reference comparison is done via is() method. Custom truth conventions can be added by implementing asBoolean() method.
str = 'Hello'
if(str) println str + 'World' //Groovy checks if the object reference is null
list = [1]
if(list) println list //Groovy checks if list is not-null and not empty

Safe-navigation operator (?.) 

eliminates the mundane null check. If input is null, returns null instead NPE
def foo(str) { if (str != null) { str.reverse() } } //Before
def foo(str) { str?.reverse() } //After

Looping methods

Using Ranges: for(i in 0..5){println i}  //prints 0,1,2,3,4
Using times function: 5.times { println "$it" } //prints 0,1,2,3,4
Using upto function: 0.upto(5) { println "$it" } //prints 0,1,2,3,4
Using step function: 0.step(5, 2) { println "$it" } //prints 0,2,4

Static imports

import static Math.random as rand 
double value = rand() // alias name is used here to avoid confusion among static imports


  • All methods and classes are public by default.
  • Getters and setters are automatically created by Groovy. No setters created for final fields. To prevent non-final fields from modification, implement setter method manually and throw an error.
  • “hello” instead of “hello”.getClass().getName(). This class property has special meaning in Map and Builders so it won’t work.
  • We can use ‘this’ within static methods to refer to the Class object.


Optional Parameters

  • With Default value
def log(x, base=10) { Math.log(x) / Math.log(base) }
log(1024) //default base 10 is used
log(1024, 2)
  • Trailing array parameter as optional. Much like Java varargs.
def task(name, String[] details) { println "$name - $details" }
task 'name1'
task 'name2', 'blah..'
task 'name3', 'blah..blah..'

Named arguments in method calls

  • Class with no-argument constructor
class Robot { def type, height, width }
robot = new Robot(type: 'arm', width: 10, height: 40)
println "$robot.type, $robot.height, $robot.width"
  • Excess Parameters as Map – If the number of arguments sent is more than what the method parameters, and if the excess arguments are in name-value pair, then Groovy treats the name-value pairs as a Map.
class Robot { 
  def access(location, weight, fragile) {
    println "Received fragile? $fragile, weight: $weight, loc: $location"
new Robot().access(x: 30, y: 20, z: 10, 50, true)
//You can change the order
new Robot().access(50, true, x: 30, y: 20, z: 10, a:5)

Multiple Assignments

  • Method returning an array is assigned to multiple variables 
def splitName(fullName) { fullName.split(' ') }
def (firstName, lastName) = splitName('James Bond')
println "$lastName, $firstName $lastName"
  • Swapping two variables without a temporary variable using above technique
def (first, last) = ["James", "Bond"]
(first, last) = [last, first]
println "$first $last"

Implementing Interface

Block of code morphed as the implementation of an interface

interface Greeting { void greet(greeting) }
interface WellWisher { void wish(wish) }
void greeter(Greeting greeting){ greeting.greet()}
void wellwisher(WellWisher wellwisher){ wellwisher.wish()}

greeter(new Greeting(){ void greet(greeting){println 'Java style'}}) 
groovyStyle = {println 'Groovy style'}
greeter(groovyStyle as Greeting) 
//block of code is morphed into an implementation of the
// interface via 'as' operator
wellwisher(groovyStyle as WellWisher)
  • Groovy does not force us to implement all the methods in an interface. Very useful while mocking for unit testing.
  • Implementation of multi-method interface as a Map
interface Greeting { void greet(greeting); void wish(wish); void regard(regard); }
void callMe(Greeting greeting){ greeting.greet(); greeting.wish()}
//method name as key, implementation as value. Not all methods are implemented
greetingsMap = [ greet: {println 'Greet Hello World'}, wish: {println 'Wish Hello World'} ] 
callMe(greetingsMap as Greeting)

Operator Overloading

Each operator has a standard mapping to methods.

== equals
+ plus
- minus
++ next
.. next (for-each syntax)
-- previous
<< leftShift
<=> compareTo
  • Example 1: for (ch in ‘a’..’c’) { println ch }
  • Example 2: lst = [‘hello’]; lst << ‘there’; println lst
  • Example 3: Custom class and operator overriding
class Name{
  def name; 
  def plus(other){
    new Name(name: name + "~~" +
  String toString() { "name: " + name}
def name1 = new Name(name: "Hello")
def name2 = new Name(name: "World")
println name1 + name2


  • groovyc ignores @Override
  • @Canonical – auto-generates toString() implementation as comma-separated field values
import groovy.transform.*
@Canonical(excludes="age, password")
class Person {
  String firstName, lastName, password
  int age
def sara = new Person(firstName: "Sara", lastName: "Walker", age: 49, password: "passw0rd")
println sara
  • @Delegate
import groovy.transform.*
class Worker {
  def work() { println 'get work done' }
  def analyze() { println 'analyze...' }
  def writeReport() { println 'get report written' }
class Expert {
  def analyze() { println "expert analysis..." }
class Manager {
  //At compile time, Groovy examines the Manager class and brings 
  // in methods from the delegated classes only if those methods 
  // don’t already exist
  @Delegate Expert expert = new Expert() 
  //only work() and writeReport() methods are brought here
  @Delegate Worker worker = new Worker()
def bernie = new Manager()
bernie.analyze()      //invokes Expert.analyze()         //invokes
bernie.writeReport()  //invokes Worker.writeReport
  • @Immutable – Groovy adds the hashCode(), equals(), and toString() methods
import groovy.transform.*
class CreditCard { String cardNumber; int creditLimit }
println new CreditCard("4000-1111-2222-3333", 1000)
  • @Lazy – provides a painless way to implement the virtual proxy pattern with thread safety as a bonus
class AsNeeded {
def value
  //heavy1 and heavy2 are lazy-initialized only at the time of invocation
  @Lazy Heavy heavy1 = new Heavy()
  @Lazy Heavy heavy2 = { new Heavy(size: value) }()
  AsNeeded() { println "Created AsNeeded" }
  • @Newify – Create objects via Ruby-like and Python-like constructors without using ‘new Foo()’ style. Comes handy in DSL creation.
@Newify([CreditCard, Person]) //specify the list of types here. 
def fluentCreate() {
  println CreditCard("1234-5678-1234-5678", 2000) //Python-like constructor invocation with new keyword
  println"John", "Doe") //Ruby-like constructor invocation where new() is a method
  • @Singleton
@Singleton(lazy = true)
class TheUnique {
  private TheUnique() { println 'Instance created' }
  def hello() { println 'hello' }
new TheUnique().hello() //Caveat: since Groovy does not honor private methods, clients can still do this.
  • @InheritConstructors
class Car {
  def make, model, year
  Car(make, model){ this.make = make; this.model = model; this.year = 2000; }
  Car(make, model, year){ this.make = make; this.model = model; this.year = year; }
class Honda extends Car{
  //no need to explicitly override all the constructors here
println new Car("Honda", "Accord")


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