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+---
+language: Groovy
+filename: learngroovy.groovy
+contributors:
+    - ["Roberto Pérez Alcolea", "http://github.com/rpalcolea"]
+filename: learngroovy.groovy
+---
+
+Groovy - A dynamic language for the Java platform [Read more here.](http://www.groovy-lang.org/)
+
+```groovy
+
+/*
+  Set yourself up:
+
+  1) Install GVM - http://gvmtool.net/
+  2) Install Groovy: gvm install groovy
+  3) Start the groovy console by typing: groovyConsole
+
+*/
+
+//  Single line comments start with two forward slashes
+/*
+Multi line comments look like this.
+*/
+
+// Hello World
+println "Hello world!"
+
+/*
+  Variables:
+
+  You can assign values to variables for later use
+*/
+
+def x = 1
+println x
+
+x = new java.util.Date()
+println x
+
+x = -3.1499392
+println x
+
+x = false
+println x
+
+x = "Groovy!"
+println x
+
+/*
+  Collections and maps
+*/
+
+//Creating an empty list
+def technologies = []
+
+/*** Adding a elements to the list ***/
+
+// As with Java
+technologies.add("Grails")
+
+// Left shift adds, and returns the list
+technologies << "Groovy"
+
+// Add multiple elements
+technologies.addAll(["Gradle","Griffon"])
+
+/*** Removing elements from the list ***/
+
+// As with Java
+technologies.remove("Griffon")
+
+// Subtraction works also
+technologies = technologies - 'Grails'
+
+/*** Iterating Lists ***/
+
+// Iterate over elements of a list
+technologies.each { println "Technology: $it"}
+technologies.eachWithIndex { it, i -> println "$i: $it"}
+
+/*** Checking List contents ***/
+
+//Evaluate if a list contains element(s) (boolean)
+contained = technologies.contains( 'Groovy' )
+
+// Or
+contained = 'Groovy' in technologies
+
+// Check for multiple contents
+technologies.containsAll(['Groovy','Grails'])
+
+/*** Sorting Lists ***/
+
+// Sort a list (mutates original list)
+technologies.sort()
+
+// To sort without mutating original, you can do:
+sortedTechnologies = technologies.sort( false )
+
+/*** Manipulating Lists ***/
+
+//Replace all elements in the list
+Collections.replaceAll(technologies, 'Gradle', 'gradle')
+
+//Shuffle a list
+Collections.shuffle(technologies, new Random())
+
+//Clear a list
+technologies.clear()
+
+//Creating an empty map
+def devMap = [:]
+
+//Add values
+devMap = ['name':'Roberto', 'framework':'Grails', 'language':'Groovy']
+devMap.put('lastName','Perez')
+
+//Iterate over elements of a map
+devMap.each { println "$it.key: $it.value" }
+devMap.eachWithIndex { it, i -> println "$i: $it"}
+
+//Evaluate if a map contains a key
+assert devMap.containsKey('name')
+
+//Evaluate if a map contains a value
+assert devMap.containsValue('Roberto')
+
+//Get the keys of a map
+println devMap.keySet()
+
+//Get the values of a map
+println devMap.values()
+
+/*
+  Groovy Beans
+
+  GroovyBeans are JavaBeans but using a much simpler syntax
+
+  When Groovy is compiled to bytecode, the following rules are used.
+
+    * If the name is declared with an access modifier (public, private or
+      protected) then a field is generated.
+
+    * A name declared with no access modifier generates a private field with
+      public getter and setter (i.e. a property).
+
+    * If a property is declared final the private field is created final and no
+      setter is generated.
+
+    * You can declare a property and also declare your own getter or setter.
+
+    * You can declare a property and a field of the same name, the property will
+      use that field then.
+
+    * If you want a private or protected property you have to provide your own
+      getter and setter which must be declared private or protected.
+
+    * If you access a property from within the class the property is defined in
+      at compile time with implicit or explicit this (for example this.foo, or
+      simply foo), Groovy will access the field directly instead of going though
+      the getter and setter.
+
+    * If you access a property that does not exist using the explicit or
+      implicit foo, then Groovy will access the property through the meta class,
+      which may fail at runtime.
+
+*/
+
+class Foo {
+    // read only property
+    final String name = "Roberto"
+
+    // read only property with public getter and protected setter
+    String language
+    protected void setLanguage(String language) { this.language = language }
+
+    // dynamically typed property
+    def lastName
+}
+
+/*
+  Logical Branching and Looping
+*/
+
+//Groovy supports the usual if - else syntax
+def x = 3
+
+if(x==1) {
+    println "One"
+} else if(x==2) {
+    println "Two"
+} else {
+    println "X greater than Two"
+}
+
+//Groovy also supports the ternary operator:
+def y = 10
+def x = (y > 1) ? "worked" : "failed"
+assert x == "worked"
+
+//For loop
+//Iterate over a range
+def x = 0
+for (i in 0 .. 30) {
+    x += i
+}
+
+//Iterate over a list
+x = 0
+for( i in [5,3,2,1] ) {
+    x += i
+}
+
+//Iterate over an array
+array = (0..20).toArray()
+x = 0
+for (i in array) {
+    x += i
+}
+
+//Iterate over a map
+def map = ['name':'Roberto', 'framework':'Grails', 'language':'Groovy']
+x = 0
+for ( e in map ) {
+    x += e.value
+}
+
+/*
+  Operators
+
+  Operator Overloading for a list of the common operators that Groovy supports:
+  http://www.groovy-lang.org/operators.html#Operator-Overloading
+
+  Helpful groovy operators
+*/
+//Spread operator:  invoke an action on all items of an aggregate object.
+def technologies = ['Groovy','Grails','Gradle']
+technologies*.toUpperCase() // = to technologies.collect { it?.toUpperCase() }
+
+//Safe navigation operator: used to avoid a NullPointerException.
+def user = User.get(1)
+def username = user?.username
+
+
+/*
+  Closures
+  A Groovy Closure is like a "code block" or a method pointer. It is a piece of
+  code that is defined and then executed at a later point.
+
+  More info at: http://www.groovy-lang.org/closures.html
+*/
+//Example:
+def clos = { println "Hello World!" }
+
+println "Executing the Closure:"
+clos()
+
+//Passing parameters to a closure
+def sum = { a, b -> println a+b }
+sum(2,4)
+
+//Closures may refer to variables not listed in their parameter list.
+def x = 5
+def multiplyBy = { num -> num * x }
+println multiplyBy(10)
+
+// If you have a Closure that takes a single argument, you may omit the
+// parameter definition of the Closure
+def clos = { print it }
+clos( "hi" )
+
+/*
+  Groovy can memorize closure results [1][2][3]
+*/
+def cl = {a, b ->
+    sleep(3000) // simulate some time consuming processing
+    a + b
+}
+
+mem = cl.memoize()
+
+def callClosure(a, b) {
+    def start = System.currentTimeMillis()
+    mem(a, b)
+    println "Inputs(a = $a, b = $b) - took ${System.currentTimeMillis() - start} msecs."
+}
+
+callClosure(1, 2)
+callClosure(1, 2)
+callClosure(2, 3)
+callClosure(2, 3)
+callClosure(3, 4)
+callClosure(3, 4)
+callClosure(1, 2)
+callClosure(2, 3)
+callClosure(3, 4)
+
+/*
+  Expando
+
+  The Expando class is a dynamic bean so we can add properties and we can add
+  closures as methods to an instance of this class
+
+  http://mrhaki.blogspot.mx/2009/10/groovy-goodness-expando-as-dynamic-bean.html
+*/
+  def user = new Expando(name:"Roberto")
+  assert 'Roberto' == user.name
+
+  user.lastName = 'Pérez'
+  assert 'Pérez' == user.lastName
+
+  user.showInfo = { out ->
+      out << "Name: $name"
+      out << ", Last name: $lastName"
+  }
+
+  def sw = new StringWriter()
+  println user.showInfo(sw)
+
+
+/*
+  Metaprogramming (MOP)
+*/
+
+//Using ExpandoMetaClass to add behaviour
+String.metaClass.testAdd = {
+    println "we added this"
+}
+
+String x = "test"
+x?.testAdd()
+
+//Intercepting method calls
+class Test implements GroovyInterceptable {
+    def sum(Integer x, Integer y) { x + y }
+
+    def invokeMethod(String name, args) {
+        System.out.println "Invoke method $name with args: $args"
+    }
+}
+
+def test = new Test()
+test?.sum(2,3)
+test?.multiply(2,3)
+
+//Groovy supports propertyMissing for dealing with property resolution attempts.
+class Foo {
+   def propertyMissing(String name) { name }
+}
+def f = new Foo()
+
+assertEquals "boo", f.boo
+
+/*
+  TypeChecked and CompileStatic
+  Groovy, by nature, is and will always be a dynamic language but it supports
+  typechecked and compilestatic
+
+  More info: http://www.infoq.com/articles/new-groovy-20
+*/
+//TypeChecked
+import groovy.transform.TypeChecked
+
+void testMethod() {}
+
+@TypeChecked
+void test() {
+    testMeethod()
+
+    def name = "Roberto"
+
+    println naameee
+
+}
+
+//Another example:
+import groovy.transform.TypeChecked
+
+@TypeChecked
+Integer test() {
+    Integer num = "1"
+
+    Integer[] numbers = [1,2,3,4]
+
+    Date date = numbers[1]
+
+    return "Test"
+
+}
+
+//CompileStatic example:
+import groovy.transform.CompileStatic
+
+@CompileStatic
+int sum(int x, int y) {
+    x + y
+}
+
+assert sum(2,5) == 7
+
+
+```
+
+## Further resources
+
+[Groovy documentation](http://www.groovy-lang.org/documentation.html)
+
+[Groovy web console](http://groovyconsole.appspot.com/)
+
+Join a [Groovy user group](http://www.groovy-lang.org/usergroups.html)
+
+## Books
+
+* [Groovy Goodness] (https://leanpub.com/groovy-goodness-notebook)
+
+* [Groovy in Action] (http://manning.com/koenig2/)
+
+* [Programming Groovy 2: Dynamic Productivity for the Java Developer] (http://shop.oreilly.com/product/9781937785307.do)
+
+[1] http://roshandawrani.wordpress.com/2010/10/18/groovy-new-feature-closures-can-now-memorize-their-results/
+[2] http://www.solutionsiq.com/resources/agileiq-blog/bid/72880/Programming-with-Groovy-Trampoline-and-Memoize
+[3] http://mrhaki.blogspot.mx/2011/05/groovy-goodness-cache-closure-results.html
+
+
+