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Before beginning...

Before playing with the examples you'll find below, you should first look at:

Some optional more advanced topics you may also wish to peruse:

Your First Groovy

Code Block
java
java
//hello.groovy
println "hello, world"
for (arg in this.args ) {
  println "Argument:" + arg;
}
// this is a comment
/* a block comment, commenting out an alternative to above:
this.args.each{ arg -> println "hello, ${arg}"}
*/

To run it from command line

Code Block
java
java
groovy hello.groovy MyName yourName HisName

Overview

Groovy classes compile down to Java bytecode and so there's a 1-1 mapping between a Groovy class and a Java class.
Indeed each Groovy class can be used inside normal Java code - since it is a Java class too.

Probably the easiest way to get groovy is to try working with collections. In Groovy List (java.util.List) and Map (java.util.Map) are both first class objects in the syntax. So to create a List of objects you can do the following...

Code Block
java
java
def list = [1, 2, 'hello', new java.util.Date()]
assert list.size() == 4
assert list.get(2) == 'hello'
assert list[2] == 'hello'

Notice that everything is an object (or that auto-boxing takes place when working with numbers). To create maps...

Code Block
java
java
def map = ['name':'James', 'location':'London']
assert map.size() == 2
assert map.get('name') == 'James'
assert map['name'] == 'James'

Iterating over collections is easy...

Code Block
java
java
def list = [1, 2, 3]
for (i in list) { println i }

Once you have some collections you can then use some of the new collection helper methods or try working with closures...

Working with closures

Closures are similar to Java's inner classes, except they are a single method which is invokable, with arbitrary parameters. A closure can have as many parameters as you wish...

Code Block
java
java
def closure = { param -> println("hello ${param}") }
closure.call("world!")

closure = { greeting, name -> println(greeting + name) }
closure.call("hello ", "world!")

If no parameter(s) is(are) specified before -> symbol then a default named parameter, called 'it' can be used. e.g.

Code Block
java
java
def closure = { println "hello " + it }
closure.call("world!")

Using closures allows us to process collections (arrays, maps, strings, files, SQL connections and so forth) in a clean way. e.g

Code Block
java
java
[1, 2, 3].each ({ item -> print "${item}-" })
["k1":"v1", "k2":"v2"].each {key, value -> println key + "=" + value}

Note: If a given closure is the last parameter of a method, its definition can reside outside of the parentheses. Thus the following code is valid:

Code Block
java
java
def fun(int i, Closure c) {
  c.call(i)
}

// put Closure out of ()
[1, 2, 3].each() { item -> print "${item}-" } // 1-2-3-
fun(123) { i -> println i } // 123

// omit ()
[1, 2, 3].each ({ item -> print "${item}-" }) // 1-2-3-

// omit enclosing ()
[1, 2, 3].each { item -> print "${item}-" } // 1-2-3-

// normal
[1, 2, 3].each(({ item -> print "${item}-" })) // 1-2-3-

// using the fun function to do the same thing
[1,2,3].each {fun(it,{item -> print "${item}-"})} // 1-2-3-

def closure = { i -> println i}

//[1, 2, 3].each() closure // error. closure has been previously defined

Here are a number of helper methods available on collections & strings...

each

iterate via a closure

Code Block
java
java
[1, 2, 3].each { item -> print "${item}-" }

collect

collect the return value of calling a closure on each item in a collection

Code Block
java
java
def value = [1, 2, 3].collect { it * 2 }
assert value == [2, 4, 6]

find

finds first item matching closure predicate

Code Block
java
java
def value = [1, 2, 3].find { it > 1 }
assert value == 2

findAll

finds all items matching closure predicate

Code Block
java
java
def value = [1, 2, 3].findAll { it > 1 }
assert value == [2, 3]

inject

allows you to pass a value into the first iteration and then pass the result of that iteration into the next iteration and so on. This is ideal for counting and other forms of processing

Code Block
java
java
def value = [1, 2, 3].inject('counting: ') { str, item -> str + item }
assert value == "counting: 123"

value = [1, 2, 3].inject(0) { count, item -> count + item }
assert value == 6

In addition there's 2 new methods for doing boolean logic on some collection...

every

returns true if all items match the closure predicate

Code Block
java
java
def value = [1, 2, 3].every { it < 5 }
assert value

value = [1, 2, 3].every { item -> item < 3 }
assert ! value

any

returns true if any item match the closure predicate

Code Block
java
java
def value = [1, 2, 3].any { it > 2 }
assert value

value = [1, 2, 3].any { item -> item > 3 }
assert value == false

Other helper methods include:

max / min

returns the max/min values of the collection - for Comparable objects

Code Block
java
java
value = [9, 4, 2, 10, 5].max()
assert value == 10
value = [9, 4, 2, 10, 5].min()
assert value == 2
value = ['x', 'y', 'a', 'z'].min()
assert value == 'a'

join

concatenates the values of the collection together with a string value

Code Block
java
java
def value = [1, 2, 3].join('-')
assert value == '1-2-3'