Groovy is a dynamic language for the Java Virtual Machine that aims to provide an alternative to Java for those seeking a more concise and expressive syntax. It is compatible with Java and allows Java code to be called from Groovy and vice versa. Groovy reduces boilerplate code through features like optional typing, closures/lambdas, operator overloading, and dynamic method dispatch. It also provides concise syntax for lists, maps, ranges, and other common data structures.

1. Groovy!
There and Back Again

2. Hello!
I am Petr Giecek
Software Engineer @ Hewlett Packard Enterprise

3. 1.
What is Groovy?

4. Groovy Programming Language
Dynamic JVM language
Static typing and static compilation capabilities
Familiar and easy to learn syntax for Java
developers
Concise, readable and expressive
Smooth Java integration
Interoperates with Java and any third-party Java libraries

5. TIOBE Index for May 2016

6. Groovy distribution
Download from Bintray
https://bintray.com/groovy/
SDKMAN! (The Software Development Kit Manager)
OSX, Linux, Cygwin
Posh-GVM (POwerSHell Groovy enVironment Manager)
Windows

7. The Groovy Development Kit
A number of helper methods added to the JDK
Working with IO
Working with collections
Working with threads
Handy utilities
...

8. Tools
groovy
Groovy interpreter
groovyc
Groovy compiler
groovysh
Groovy shell
groovyConsole
Groovy Swing console

9. Dynamic language
def var = 4
assert var.class == Integer
var = 'Hello'
assert var.class == String
var = true
assert var.class == Boolean

10. Dynamically-typed language
String greeting = "Hello";
// Compile-time error
greeting / 5
Java Groovy
def greeting = 'Hello'
// Runtime error
greeting / 5

11. Optionally typed language
// Runtime error: GroovyCastException
int greeting = '123'

13. Static type checking
@groovy.transform.TypeChecked
class Calculator {
static int sum(int x, int y) {
return "Result is ${x + y}" // Compile-time error
}
}
assert Calculator.sum(1, 10) == 11

14. Type inference
def message = 'Groovy is awesome!'
assert message.toUpperCase() == 'GROOVY IS AWESOME!'
assert message.upper() == 'GROOVY IS AWESOME!' // Compile-time error

15. Flow typing
def value = 'Groovy is awesome'
value = value.toUpperCase()
assert value == 'GROOVY IS AWESOME'
value = 9d
value = Math.sqrt(value)
assert value == 3.0
value = value.toLowerCase(value) // Compile-time error

16. Static compilation
class User {
String name
int age
}
@groovy.transform.CompileStatic
def test() {
def user = new User(name: 'John Smith', age: 30)
int age = user.age // getAge() possible runtime error: GroovyCastException
}
User.metaClass.getAge = { -> "30" } // runtime modification
test()

17. Everything clear?

18. 2.
Hello Groovy!

19. Syntax
The majority of Java syntax is valid Groovy
Primitive types
Packages and imports
Control flow statements
◇ More flexible switch
◇ For-in loop
Exception handling
◇ No distinction between checked and unchecked exceptions
Classes and object instantiation

20. You are Groovy programmers!

21. Extra keywords
� as
� def
� in
� trait

22. Default imports
java.io.*
java.lang.*
java.math.BigDecimal
java.math.BigInteger

23. Everything is an object
char c = 'C'
assert c.class == Character
int num1 = 5
assert num1.class == Integer
def num2 = 9223372036854775808
assert num2.class == BigInteger
def num3 = 10.5
assert num3.class == BigDecimal
def bool = true
assert bool.class == Boolean

24. Division
assert 10 / 4 == 2.5
assert 10.intdiv(4) == 2

25. String literals
assert 'Hello World'.class == String
assert "Hello World".class == String
assert "Hello ${name}".class instanceof GString
def c1 = "C"
assert c1.class == String
char c2 = c1
assert c2.class == Character

26. Default scope
public class User {}
class User {}
Java Groovy
class User {}
@PackageScope class User {}

27. Equality/identity check
String s1 = "Hello"
String s2 = new String("Hello")
assert s1.equals(s2) // equality check
assert s1 != s2 // identity check
Java Groovy
def s1 = 'Hello'
def s2 = new String('Hello')
assert s1 == s2 // equality check
assert !s1.is(s2) // identity check

28. Lambdas
Runnable run =
() -> System.out.println("Run");
Java Groovy
Runnable run = { println 'Run' }

29. Runtime dispatch (multimethods)
String whoIsAwesome(String arg) {
return "Groovy is awesome!";
}
String whoIsAwesome(Object arg) {
return "Java is awesome!";
}
Object o = "Object";
String result = whoIsAwesome(o);
assertEquals("Java is awesome", result);
Java
assertEquals("Groovy is awesome",
result);
Groovy

30. Other differences with Java
Interfaces do not support default implementation (Java 8)
Automatic resource management blocks not supported (Java 7)
No difference between checked and uncheck exceptions
Assertion in Groovy is always enabled
power asserts

32. 3.
Goodbye Java

33. “
Boilerplate refers to code that have to
be included in many places with little or
no alteration.

34. Hello...
public class Main {
public static void main(String... args) {
System.out.println("Goodbye Java!")
}
}
Java Groovy
println 'Hello Groovy!'

35. Optionality
Optional parentheses
Optional semicolons
Optional return keyword
Optional public keyword

36. The Groovy Truth
assert [1, 2, 3]
assert ![] // Empty collections and arrays
assert [1, 2, 3].iterator()
assert ![].iterator() // Iterators and Enumerations without further elements
assert [name: 'John']
assert ![:] // Empty maps
assert 'Hello Groovy'
assert !'' // Empty Strings, GStrings and CharSequences
assert 100
assert !0 // Zero number
assert new Object()
assert !null // Null object references

37. Example
if (users == null || users.isEmpty()) {
throw new IllegalArgumentException("No users found")
}
Java
if (!users) {
throw new IllegalArgumentException('No users found')
}
Groovy

38. Lists
def numbers = [1, 2, 3, 4]
assert numbers.class == ArrayList
def everything = [1, 2.5, true, 'Hello']
assert everything.class == ArrayList

39. Lists (cont’d)
def numbers1 = [1, 2, 3, 4] as LinkedList
assert numbers1.class == LinkedList
LinkedList numbers2 = [1, 2, 3, 4]
assert numbers2.class == LinkedList

40. Lists (cont’d)
def letters = ['a', 'b', 'c', 'd']
assert letters[0] == 'a'
assert letters[1] == 'b'
assert letters[-1] == 'd'
assert letters[-2] == 'c'
letters[2] = 'C'
assert letters[2] == 'C'
letters << 'e'
assert letters[-1] == 'e'
assert letters[1, 3] == ['b', 'd']
assert letters[2..4] == ['C', 'd', 'e']

41. Arrays
def languages = ['Java', 'Groovy', 'Scala'] as String[]
assert languages instanceof String[]
int[] numbers = [1, 2, 3]
assert numbers instanceof int[]

42. Maps
def colors = [red: '#FF0000', green: '#00FF00', blue: '#0000FF']
assert colors['red'] == '#FF0000'
assert colors.green == '#00FF00'
colors['pink'] = '#FF00FF'
colors.yellow = '#FFFF00'
assert colors.pink == '#FF00FF'
assert colors['yellow'] == '#FFFF00'
assert colors.getClass() == LinkedHashMap

43. Ranges
assert (0..5) == [0, 1, 2, 3, 4, 5]
assert ('a'..<'f') == ['a', 'b', 'c', 'd', 'e']
assert (2..-2) == [2, 1, 0, -1, -2]
assert (0..10).step(2) == [0, 2, 4, 6, 8, 10]
assert (0..5) instanceof Range
assert (0..5) instanceof List

44. Range in loops
for (i = 0; i < 10; i++) {
println i
}
for (i in 0..<10) {
println i
}

45. Elvis operator (?:)
def displayName = user.name ? user.name : 'Unknown'
def displayName = user.name ?: 'Unknown'

46. Safe navigation operator (?.)
if (headers == null || headers.getContentType() == null || headers.getContentType().getCharSet() == null) {
return DEFAULT_CHARSET;
}
return headers.getContentType().getCharSet();
Java
def charset = headers?.contentType?.charset
if (!charset) {
return DEFAULT_CHARSET;
}
charset
Groovy

48. Spaceship operator (<=>)
assert (1 <=> 1) == 0
assert (1 <=> 100) == -1
assert (100 <=> 1) == 1
assert ('a' <=> 'z') == -1
assert ('Groovy is awesome' <=> 'Java is not so bad') == -1

49. Spread operator (*.)
class User {
String name
int age
}
def users = [
new User(name: 'John', age: 33),
new User(name: 'George', age: 35),
null]
assert users*.name == ['John', 'George', null]

50. Spreading method arguments
int add(int a, int b, int c, int d) {
a + b + c + d
}
def args = [1, 2, 7, 10]
assert add(*args) == 20
args = [1, 2, 7]
assert add(*args, 10) == 20

51. Operator overloading
+ a.plus(b)
- a.minus(b)
* a.multiply(b)
/ a.div(b)
% a.mod(b)
** a.power(b)
| a.or(b)
& a.and(b)
^ a.xor(b)
as a.asType(b)
a() a.call()
a[b] a.getAt(b)
a[b] = c a.putAt(b, c)
in a.isCase(b)
<< a.leftShift(b)
>> a.rightShift(b)
++ a.next()
-- a.previous(b)

52. Coercion
BigDecimal pi1 = (BigDecimal) '3.14159265359' // GroovyCastException
BigDecimal pi2 = '3.14159265359' as BigDecimal

53. List to type coercion
class User {
String name
int age
User(String name, int age) {
this.name = name
this.age = age
}
}
def user1 = ['John', 30] as User
User user2 = ['John', 30]

54. Map to type coercion
def map
map = [
i: 10,
hasNext: { map.i > 0 },
next: { map.i-- },
]
def iter = map as Iterator
assert iter.collect { it } == [10, 9, 8, 7, 6, 5, 4, 3, 2, 1]

55. Runtime interface implementation
interface Greeter {
def sayHello(String name)
}
class DefaultGreeter {
def sayHello(String name) { println "Hello $name" }
}
def greeter = new DefaultGreeter() as Greeter
assert greeter instanceof Greeter

56. Constructors (positional arguments)
// Constructor invocation with new keyword
def user1 = new User('John', 30)
// Using coercion with as keyword
def user2 = ['John', 30] as User
// Using coercion in assignment
User user3 = ['John', 30]

57. Constructors (named arguments)
def user1 = new User()
def user2 = new User(name: 'John')
def user3 = new User(age: 30)
def user4 = new User(name: 'John', age: 30)

58. Methods
def sayHello() { 'Hello Groovy!' }
String sayHello2(def name) { 'Hello $name!' }
def sayHello3(name) { "Hello $name!" }
static String sayHello4(String name) { "Hello $name!" }

59. Named arguments
def createUser(Map args) { new User(args.name, args['age']) }
def user = createUser(name: 'John', age: 30)
assert user.name == 'John'
assert user.age == 30

60. Default arguments
def createUser(String name, int age = 30) { new User(name, age)
}
def user = createUser('John')
assert user.name == 'John'
assert user.age == 30

61. Properties
class User {
final String name = 'Jesus'
int age
}
class User {
private final String name = 'Jesus'
private int age
String getName() { return name }
void setAge(int age) { this.age = age }
int getAge() { return age }
}

62. Property access
def user = new User()
user.age = 33
user.name = 'Moses' // ReadOnlyPropertyException
assert user.name == 'Jesus'
assert user.age == 33

63. Direct field access
class User {
public String name
String getName() {
"User's name is $name"
}
}
def user = new User(name: 'John')
assert user.name == "User's name is John"
class User {
public String name
String getName() {
"User's name is $name"
}
}
def user = new User(name: 'John')
assert user.@name == 'John'

64. 4.
Closures

65. Closures syntax
{ [parameters -> ] statements }

66. Closure examples
{ 1 + 5 }
{ -> 1 + 5 }
{ println it }
{ num -> println num }
{ int x, int y = 10 -> "Sum of $x and $y is ${x + y}"}

67. def closure = { Writer w ->
w.writeLine 'Groovy creates a new BufferedWriter for the file and passes it to the closure.'
w.writeLine 'It also ensures the stream is flushed and closed after the closure returns.'
w.writeLine 'Even if an exception is thrown in the closure.'
}
assert closure instanceof Closure
def file = new File('sample.txt')
file.withWriter(closure)
Closure is an object

68. Calling a closure
def sayHello1 = { 'Hello Groovy!' }
assert sayHello1() == 'Hello Groovy!'
assert sayHello1.call() == 'Hello Groovy!'
def sayHello2 = { "Hello $it!" }
assert sayHello2('Groovy') == 'Hello Groovy!'
assert sayHello2.call('Groovy') == 'Hello Groovy!'
def sayHello3 = { name -> "Hello $name!" }
assert sayHello3('Groovy') == 'Hello Groovy!'
assert sayHello3.call('Groovy') == 'Hello Groovy!'

69. Method pointer
def sayHello(String name) { "Hello $name" }
def sayHello(int nTimes) { 'Hello' * nTimes }
def pointer = this.&sayHello
assert pointer instanceof Closure
assert pointer('John') == 'Hello John'
assert pointer(3) == 'HelloHelloHello'

70. Closure coercion
interface Predicate<T> {
boolean accept(T obj)
}
def filter2 = { it.startsWith 'J' } as Predicate
assert filter2.accept('Java')
Predicate filter1 = { it.startsWith 'G' }
assert filter1.accept('Groovy')

71. Methods accepting a SAM type
static <T> List<T> filter(List<T> source, Predicate<T> predicate) {
source.findAll { predicate.accept(it) }
}
assert filter(['Java', 'Groovy'], { it.contains 'G' }) == ['Groovy']
assert filter(['Java', 'Groovy']) { it.contains 'G' } == ['Groovy']

72. � this
The enclosing class of a closure definition
� owner
The enclosing object of a closure definition (a class or a closure)
� delegate
An object where methods calls or properties are resolved if no
receiver
of the message is defined
Delegation

73. Example
class User {
def name
}
def user = new User(name: 'John')
def getUserName = { "The name is $name" }
assert getUserName() == 'The name is John' // MissingPropertyException
getUserName.delegate = user
assert getUserName() == 'The name is John'

74. Example (Gradle)
apply plugin: 'java'
apply plugin: 'eclipse'
sourceCompatibility = 1.5
version = '1.0'
repositories {
mavenCentral()
}
dependencies {
compile group: 'commons-collections', name: 'commons-collections', version: '3.2.2'
testCompile group: 'junit', name: 'junit', version: '4.+'
}

75. Closures
First class citizens of type groovy.lang.Closure
Can access any variables of the surrounding context
Support delegation concept
Method reference via GroovyClass.&groovyMethod
Lambda expressions
Instances of SAM type they represent
Can access only effectively final variables of the surrounding context
Method reference via JavaClass::javaMethod
Closures vs Java 8 lambda expressions

76. 5.
Metaprogramming

77. “
Metaprogramming is the writing of
computer programs with the ability to
treat programs as their data.

78. Flavors
Runtime
Compile-time

79. Runtime metaprogramming
POJO
Regular Java object
Class written in Java or other JVM language
POGO
Class written in Groovy
Extends java.lang.Object and implements groovy.lang.GroovyObject
GroovyInterceptable
All methods intercepted through invokeMethod

80. POGO method handling

81. GroovyObject interface
public interface GroovyObject {
Object invokeMethod(String name, Object args);
Object getProperty(String propertyName);
void setProperty(String propertyName, Object newValue);
MetaClass getMetaClass();
void setMetaClass(MetaClass metaClass);
}

82. invokeMethod
class InvokeMethodExample implements GroovyInterceptable {
def existingMethod() { 'existingMethod()' }
def invokeMethod(String name, Object args) {
def m = metaClass.getMetaMethod(name, *args)
if (!m) return "Handle unknown method: $name($args)"
return "Intercepted: ${m.invoke(this, *args)}"
}
}
def obj = new InvokeMethodExample()
assert obj.existingMethod() == 'Intercepted: existingMethod()'
assert obj.newMethod(true) == 'Handle unknown method: newMethod([true])'

83. methodMissing
class User {
def methodMissing(String name, def args) {
if (name.startsWith("findBy")) {
def field = name[6].toLowerCase() + name[7..-1]
return "Looking for a user by $field=${args.first()}"
} else
throw new MissingMethodException(name, this.class, args)
}
}
def user = new User()
assert user.findByFirstName('John') == 'Looking for a user by firstName=John'
assert user.findByAge(30) == 'Looking for a user by age=30'

84. Example
def writer = new StringWriter()
def xml = new MarkupBuilder(writer)
xml.movies() {
movie(year: 1977) {
name('Star Wars: Episode IV')
director('George Lucas')
}
movie(year: 1989) {
name('Indiana Jines and the Last Crusade')
director('Steven Spielberg')
}
}

85. Example (cont’d)
<movies>
<movie year='1977'>
<name>Star Wars: Episode IV</name>
<director>George Lucas</director>
</movie>
<movie year='1989'>
<name>Indiana Jines and the Last Crusade</name>
<director>Steven Spielberg</director>
</movie>
</movies>

86. MetaClass
Defines the behaviour of any Groovy or Java class
Client API defined via MetaObjectProtocol
Contract with Groovy runtime system
Cached in MetaClassRegistry
Available via metaClass property

87. ExpandoMetaClass
Allows for dynamically adding/changing
Methods
Constructors
Properties

88. Adding/changing methods
class User {
String name
}
User.metaClass.nameInUpperCase = { -> name.toUpperCase() }
User.metaClass.nameInLowerCase << { -> name.toLowerCase() } // Has to be a new method
def user = new User(name: 'John Smith')
assert user.nameInUpperCase() == 'JOHN SMITH'
assert user.nameInLowerCase() == 'john smith'

89. Adding/changing constructors
class User {
String name
}
User.metaClass.constructor << { String name -> new User(name: name) }
def user = new User('John Smith')
assert user.name == 'John Smith'

90. Adding/changing properties
class User {
String name
}
User.metaClass.age = 30
def user = new User(name: 'John Smith')
assert user.age == 30
user.age = 35
assert user.age == 35

91. Adding/changing static methods
class User {
String name
}
User.metaClass.static.newInstance << { name -> new User(name: name) }
def user = User.newInstance('John Smith')
assert user.name == 'John Smith'

92. Compile-time metaprogramming
Allows code generation at compile-time
Alters Abstract Syntax Tree (AST) of a program
The changes are visible in bytecode
Callable from other JVM languages
Usually better performance

93. AST transformations
Global AST transformation
Applied as soons as they are found on compile classpath
No out-of-the-box transformations available
Local AST transformation
Applied by annotating the source code with markers
Supports parameters

94. Available AST transformations
Reducing boilerplate (code generation)
Implementing design patterns
Logging
Declarative concurrency
Cloning
Testing
...

95. Examples
� @groovy.transform.ToString
� @groovy.transform.EqualsAndHashCode
� @groovy.transform.TupleConstructor
� @groovy.transform.Canonical
@groovy.transform.InheritConstructors

96. Available AST transformations (cont’d)
� @groovy.lang.Lazy
� @groovy.transform.Sortable
� @groovy.transform.builder.Builder
� @groovy.lang.Delegate
� @groovy.transform.Immutable

98. 6.
From prototype...

99. “
It is about how fast you can write the code.

100. “
If you can, go with a language and tools you
already know.

101. Strengths
Familiar, expressive and succinct syntax
Smooth Java integration
Vibrant and rich ecosystem
Rich and easy-to-use API
Embedded dependency manager

102. 7.
… to production

103. “
It is about how fast you can understand
the code.

104. What does this code do?
def process(def arg1, def arg2, def arg3) {
[arg1, arg2, arg3].inject('') { x, y ->
if (y <= 0) return "${x}00"
if (y > 255) return "${x}ff"
def s = ''
for (int n = y; n > 0; n = n.intdiv(16)) {
int r = n % 16
s = r < 10 ? r + s : "${(char) (((int) 'A') - 10 + r)}$s"
}
s.length() == 1 ? "${x}0$s" : "$x$s"
}
}

105. And now?
String rgbToHex(int r, int g, int b) {
[r, g, b].inject('') { acc, val ->
if (val <= 0) return "${acc}00"
if (val > 255) return "${acc}ff"
def hex = ''
for (int i = val; i > 0; i = i.intdiv(16)) {
int j = i % 16
hex = j < 10 ? j + hex : "${(char) (((int) 'A') - 10 + j)}$hex"
}
hex.length() == 1 ? "${acc}0$hex" : "$acc$hex"
}
}

106. Best practices
Use explicit types for public APIs
Interfaces
Document client contract
Use descriptive names (for everything)
Avoid using the same variable name for different things
Use optional elements explicitly if it helps code readability
parentheses, return keyword

107. Best practices (cont’d)
Restrict access to members that are not part of API
private, protected, @PackageScope
Use @TypeChecked or @CompileStatic capabilities if possible
Keep your code segments small and isolated
Especially if making use of duck typing
Cover code with as many test suites as reasonable
Refactoring

108. “
Groovy is not a replacement for Java, it
is addition to Java.

109. Java?
This is Groovy!

110. References
Groovy official site
http://www.groovy-lang.org
Groovy Goodness
http://mrhaki.blogspot.cz

111. Thanks!
Any questions?
You can find me at petr.giecek.82@gmail.com
https://www.linkedin.com/in/petrgiecek
https://github.com/pgiecek

112. Credits
Special thanks to all the people who made and
released these awesome resources for free:
Presentation template by SlidesCarnival
Photographs by Unsplash