What are Groovy Closures?

One of the more difficult concepts that I had to get my head around as a Java developer was the concept of Groovy closures.

Closures:

• are classes (defined by Closure.class)
• like defining new Closure() {}
  
• are similar to anonymous classes
• are defined with braces { and }
• provide one default iterator parameter (if none specified) called "it" for iterator
• (1..3).each {println "number $it" }
  

• can accept parameters
• (1..3).each {someNum -> println "Some number $someNum" }
  

• can be predefined with a variable name and invoked (they are classes after all)
• def myClosure = {x, y -> println "you sent $x and $y" }
• myClosure 'Feet', 'Hans'
• prints: you sent Feet and Hans

• do not require an input parameter if its not used, do not provide one
• def myClosure = { -> "returning this dude" }
• println myClosure