Your First Groovy
//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
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…
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…
def map = [‘name’:’James’, ‘location’:’London’]
assert map.size() == 2
assert map.get(‘name’) == ‘James’
assert map[‘name’] == ‘James’
Iterating over collections is easy…
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…
def closure = { param -> println(“hello ${param}”) }
closure.call(“world!”)
closure = { greeting, name -> println(greeting + name) }
closure.call(“hello “, “world!”)
If no parameter(s) is(are) specified before -> symbol then a default named parameter, called ‘it’ can be used. e.g.
def closure = { println “hello “ + it }
closure.call(“world!”)
Using closures allows us to process collections (arrays, maps, strings, files, SQL connections and so forth) in a clean way. e.g
[1, 2, 3].each ({ item -> print “${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:
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…
collect
collect the return value of calling a closure on each item in a collection
def value = [1, 2, 3].collect { it * 2 }
assert value == [2, 4, 6]find
finds first item matching closure predicate
def value = [1, 2, 3].find { it > 1 }
assert value == 2findAll
finds all items matching closure predicate
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
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
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
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
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’
