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The Groovy Development Kit

1. Working with IO

Groovy provides a number of helper methods for working with I/O. While you could use standard Java code in Groovy to deal with those, Groovy provides much more convenient ways to handle files, streams, readers, …​

In particular, you should take a look at methods added to:

The following section focuses on sample idiomatic constructs using helper methods available above but is not meant to be a complete description of all available methods. For that, please read the GDK API.

1.1. Reading files

As a first example, let’s see how you would print all lines of a text file in Groovy:

new File(baseDir, 'haiku.txt').eachLine { line ->
    println line
}

The eachLine method is a method added to the File class automatically by Groovy and has many variants, for example if you need to know the line number, you can use this variant:

new File(baseDir, 'haiku.txt').eachLine { line, nb ->
    println "Line $nb: $line"
}

If for whatever reason the an exception is thrown in the eachLine body, the method makes sure that the resource is properly closed. This is true for all I/O resource methods that Groovy adds.

For example in some cases you will prefer to use a Reader, but still benefit from the automatic resource management from Groovy. In the next example, the reader will be closed even if the exception occurs:

def count = 0, MAXSIZE = 3
new File(baseDir,"haiku.txt").withReader { reader ->
    while (reader.readLine()) {
        if (++count > MAXSIZE) {
            throw new RuntimeException('Haiku should only have 3 verses')
        }
    }
}

Should you need to collect the lines of a text file into a list, you can do:

def list = new File(baseDir, 'haiku.txt').collect {it}

Or you can even leverage the as operator to get the contents of the file into an array of lines:

def array = new File(baseDir, 'haiku.txt') as String[]

How many times did you have to get the contents of a file into a byte[] and how much code does it require? Groovy makes it very easy actually:

byte[] contents = file.bytes

Working with I/O is not limited to dealing with files. In fact, a lot of operations rely on input/output streams, hence why Groovy adds a lot of support methods to those, as you can see in the documentation.

As an example, you can obtain an InputStream from a File very easily:

def is = new File(baseDir,'haiku.txt').newInputStream()
// do something ...
is.close()

However you can see that it requires you to deal with closing the inputstream. In Groovy it is in general a better idea to use the withInputStream idiom that will take care of that for you:

new File(baseDir,'haiku.txt').withInputStream { stream ->
    // do something ...
}

1.2. Writing files

Of course in some cases you won’t want to read but write a file. One of the options is to use a Writer:

new File(baseDir,'haiku.txt').withWriter('utf-8') { writer ->
    writer.writeLine 'Into the ancient pond'
    writer.writeLine 'A frog jumps'
    writer.writeLine 'Water’s sound!'
}

But for such a simple example, using the << operator would have been enough:

new File(baseDir,'haiku.txt') << '''Into the ancient pond
A frog jumps
Water’s sound!'''

Of course we do not always deal with text contents, so you could use the Writer or directly write bytes as in this example:

file.bytes = [66,22,11]

Of course you can also directly deal with output streams. For example, here is how you would create an output stream to write into a file:

def os = new File(baseDir,'data.bin').newOutputStream()
// do something ...
os.close()

However you can see that it requires you to deal with closing the output stream. Again it is in general a better idea to use the withOutputStream idiom that will handle the exceptions and close the stream in any case:

new File(baseDir,'data.bin').withOutputStream { stream ->
    // do something ...
}

1.3. Traversing file trees

In scripting contexts it is a common task to traverse a file tree in order to find some specific files and do something with them. Groovy provides multiple methods to do this. For example you can perform something on all files of a directory:

dir.eachFile { file ->                      (1)
    println file.name
}
dir.eachFileMatch(~/.*\.txt/) { file ->     (2)
    println file.name
}
1 executes the closure code on each file found in the directory
2 executes the closure code on files in the directory matching the specified pattern

Often you will have to deal with a deeper hierarchy of files, in which case you can use eachFileRecurse:

dir.eachFileRecurse { file ->                      (1)
    println file.name
}

dir.eachFileRecurse(FileType.FILES) { file ->      (2)
    println file.name
}
1 executes the closure code on each file or directory found in the directory, recursively
2 executes the closure code only on files, but recursively

For more complex traversal techniques you can use the traverse method, which requires you to set a special flag indicating what to do with the traversal:

dir.traverse { file ->
    if (file.directory && file.name=='bin') {
        FileVisitResult.TERMINATE                   (1)
    } else {
        println file.name
        FileVisitResult.CONTINUE                    (2)
    }

}
1 if the current file is a directory and its name is bin, stop the traversal
2 otherwise print the file name and continue

1.4. Data and objects

In Java it is not uncommon to serialize and deserialize data using the java.io.DataOutputStream and java.io.DataInputStream classes respectively. Groovy will make it even easier to deal with them. For example, you could serialize data into a file and deserialize it using this code:

boolean b = true
String message = 'Hello from Groovy'
// Serialize data into a file
file.withDataOutputStream { out ->
    out.writeBoolean(b)
    out.writeUTF(message)
}
// ...
// Then read it back
file.withDataInputStream { input ->
    assert input.readBoolean() == b
    assert input.readUTF() == message
}

And similarily, if the data you want to serialize implements the Serializable interface, you can proceed with an object output stream, as illustrated here:

Person p = new Person(name:'Bob', age:76)
// Serialize data into a file
file.withObjectOutputStream { out ->
    out.writeObject(p)
}
// ...
// Then read it back
file.withObjectInputStream { input ->
    def p2 = input.readObject()
    assert p2.name == p.name
    assert p2.age == p.age
}

1.5. Executing External Processes

The previous section described how easy it was to deal with files, readers or streams in Groovy. However in domains like system administration or devops it is often required to communicate with external processes.

Groovy provides a simple way to execute command line processes. Simply write the command line as a string and call the execute() method. E.g., on a *nix machine (or a windows machine with appropriate *nix commands installed), you can execute this:

def process = "ls -l".execute()             (1)
println "Found text ${process.text}"        (2)
1 executes the ls command in an external process
2 consume the output of the command and retrieve the text

The execute() method returns a java.lang.Process instance which will subsequently allow the in/out/err streams to be processed and the exit value from the process to be inspected etc.

e.g. here is the same command as above but we will now process the resulting stream a line at a time:

def process = "ls -l".execute()             (1)
process.in.eachLine { line ->               (2)
    println line                            (3)
}
1 executes the ls command in an external process
2 for each line of the input stream of the process
3 print the line

It is worth noting that in corresponds to an input stream to the standard output of the command. out will refer to a stream where you can send data to the process (its standard input).

Remember that many commands are shell built-ins and need special handling. So if you want a listing of files in a directory on a Windows machine and write:

def process = "dir".execute()
println "${process.text}"

you will receive an IOException saying  Cannot run program "dir": CreateProcess error=2, The system cannot find the file specified.

This is because dir is built-in to the Windows shell (cmd.exe) and can’t be run as a simple executable. Instead, you will need to write:

def process = "cmd /c dir".execute()
println "${process.text}"

Also, because this functionality currently makes use of java.lang.Process undercover, the deficiencies of that class must be taken into consideration. In particular, the javadoc for this class says:

Because some native platforms only provide limited buffer size for standard input and output streams, failure to promptly write the input stream or read the output stream of the subprocess may cause the subprocess to block, and even deadlock

Because of this, Groovy provides some additional helper methods which make stream handling for processes easier.

Here is how to gobble all of the output (including the error stream output) from your process:

def p = "rm -f foo.tmp".execute([], tmpDir)
p.consumeProcessOutput()
p.waitFor()

There are also variations of consumeProcessOutput that make use of StringBuffer, InputStream, OutputStream etc…​ For a complete list, please read the GDK API for java.lang.Process

In addition, these is a pipeTo command (mapped to | to allow overloading) which lets the output stream of one process be fed into the input stream of another process.

Here are some examples of use:

Pipes in action
proc1 = 'ls'.execute()
proc2 = 'tr -d o'.execute()
proc3 = 'tr -d e'.execute()
proc4 = 'tr -d i'.execute()
proc1 | proc2 | proc3 | proc4
proc4.waitFor()
if (proc4.exitValue()) {
    println proc4.err.text
} else {
    println proc4.text
}
Consuming errors
def sout = new StringBuilder()
def serr = new StringBuilder()
proc2 = 'tr -d o'.execute()
proc3 = 'tr -d e'.execute()
proc4 = 'tr -d i'.execute()
proc4.consumeProcessOutput(sout, serr)
proc2 | proc3 | proc4
[proc2, proc3].each { it.consumeProcessErrorStream(serr) }
proc2.withWriter { writer ->
    writer << 'testfile.groovy'
}
proc4.waitForOrKill(1000)
println "Standard output: $sout"
println "Standard error: $serr"

2. Working with collections

Groovy provides native support for various collection types, including lists, maps or ranges. Most of those are based on the Java collection types and decorated with additional methods found in the Groovy development kit.

2.1. Lists

2.1.1. List literals

You can create lists as follows. Notice that [] is the empty list expression.

def list = [5, 6, 7, 8]
assert list.get(2) == 7
assert list[2] == 7
assert list instanceof java.util.List

def emptyList = []
assert emptyList.size() == 0
emptyList.add(5)
assert emptyList.size() == 1

Each list expression creates an implementation of java.util.List.

Of course lists can be used as a source to construct another list:

def list1 = ['a', 'b', 'c']
//construct a new list, seeded with the same items as in list1
def list2 = new ArrayList<String>(list1)

assert list2 == list1 // == checks that each corresponding element is the same

// clone() can also be called
def list3 = list1.clone()
assert list3 == list1

A list is an ordered collection of objects:

def list = [5, 6, 7, 8]
assert list.size() == 4
assert list.getClass() == ArrayList     // the specific kind of list being used

assert list[2] == 7                     // indexing starts at 0
assert list.getAt(2) == 7               // equivalent method to subscript operator []
assert list.get(2) == 7                 // alternative method

list[2] = 9
assert list == [5, 6, 9, 8,]           // trailing comma OK

list.putAt(2, 10)                       // equivalent method to [] when value being changed
assert list == [5, 6, 10, 8]
assert list.set(2, 11) == 10            // alternative method that returns old value
assert list == [5, 6, 11, 8]

assert ['a', 1, 'a', 'a', 2.5, 2.5f, 2.5d, 'hello', 7g, null, 9 as byte]
//objects can be of different types; duplicates allowed

assert [1, 2, 3, 4, 5][-1] == 5             // use negative indices to count from the end
assert [1, 2, 3, 4, 5][-2] == 4
assert [1, 2, 3, 4, 5].getAt(-2) == 4       // getAt() available with negative index...
try {
    [1, 2, 3, 4, 5].get(-2)                 // but negative index not allowed with get()
    assert false
} catch (e) {
    assert e instanceof ArrayIndexOutOfBoundsException
}

2.1.2. List as a boolean expression

Lists can be evaluated as a boolean value:

assert ![]             // an empty list evaluates as false

//all other lists, irrespective of contents, evaluate as true
assert [1] && ['a'] && [0] && [0.0] && [false] && [null]

2.1.3. Iterating on a list

Iterating on elements of a list is usually done calling the each and eachWithIndex methods, which execute code on each item of a list:

[1, 2, 3].each {
    println "Item: $it" // `it` is an implicit parameter corresponding to the current element
}
['a', 'b', 'c'].eachWithIndex { it, i -> // `it` is the current element, while `i` is the index
    println "$i: $it"
}

In addition to iterating, it is often useful to create a new list by transforming each of its elements into something else. This operation, often called mapping, is done in Groovy thanks to the collect method:

assert [1, 2, 3].collect { it * 2 } == [2, 4, 6]

// shortcut syntax instead of collect
assert [1, 2, 3]*.multiply(2) == [1, 2, 3].collect { it.multiply(2) }

def list = [0]
// it is possible to give `collect` the list which collects the elements
assert [1, 2, 3].collect(list) { it * 2 } == [0, 2, 4, 6]
assert list == [0, 2, 4, 6]

2.1.4. Manipulating lists

Filtering and searching

The Groovy development kit contains a lot of methods on collections that enhance the standard collections with pragmatic methods, some of which are illustrated here:

assert [1, 2, 3].find { it > 1 } == 2           // find 1st element matching criteria
assert [1, 2, 3].findAll { it > 1 } == [2, 3]   // find all elements matching critieria
assert ['a', 'b', 'c', 'd', 'e'].findIndexOf {      // find index of 1st element matching criteria
    it in ['c', 'e', 'g']
} == 2

assert ['a', 'b', 'c', 'd', 'c'].indexOf('c') == 2  // index returned
assert ['a', 'b', 'c', 'd', 'c'].indexOf('z') == -1 // index -1 means value not in list
assert ['a', 'b', 'c', 'd', 'c'].lastIndexOf('c') == 4

assert [1, 2, 3].every { it < 5 }               // returns true if all elements match the predicate
assert ![1, 2, 3].every { it < 3 }
assert [1, 2, 3].any { it > 2 }                 // returns true if any element matches the predicate
assert ![1, 2, 3].any { it > 3 }

assert [1, 2, 3, 4, 5, 6].sum() == 21                // sum anything with a plus() method
assert ['a', 'b', 'c', 'd', 'e'].sum {
    it == 'a' ? 1 : it == 'b' ? 2 : it == 'c' ? 3 : it == 'd' ? 4 : it == 'e' ? 5 : 0
    // custom value to use in sum
} == 15
assert ['a', 'b', 'c', 'd', 'e'].sum { ((char) it) - ((char) 'a') } == 10
assert ['a', 'b', 'c', 'd', 'e'].sum() == 'abcde'
assert [['a', 'b'], ['c', 'd']].sum() == ['a', 'b', 'c', 'd']

// an initial value can be provided
assert [].sum(1000) == 1000
assert [1, 2, 3].sum(1000) == 1006

assert [1, 2, 3].join('-') == '1-2-3'           // String joining
assert [1, 2, 3].inject('counting: ') {
    str, item -> str + item                     // reduce operation
} == 'counting: 123'
assert [1, 2, 3].inject(0) { count, item ->
    count + item
} == 6

And here is idiomatic Groovy code for finding the maximum and minimum in a collection:

def list = [9, 4, 2, 10, 5]
assert list.max() == 10
assert list.min() == 2

// we can also compare single characters, as anything comparable
assert ['x', 'y', 'a', 'z'].min() == 'a'

// we can use a closure to specify the sorting behaviour
def list2 = ['abc', 'z', 'xyzuvw', 'Hello', '321']
assert list2.max { it.size() } == 'xyzuvw'
assert list2.min { it.size() } == 'z'

In addition to closures, you can use a Comparator to define the comparison criteria:

Comparator mc = { a, b -> a == b ? 0 : (a < b ? -1 : 1) }

def list = [7, 4, 9, -6, -1, 11, 2, 3, -9, 5, -13]
assert list.max(mc) == 11
assert list.min(mc) == -13

Comparator mc2 = { a, b -> a == b ? 0 : (Math.abs(a) < Math.abs(b)) ? -1 : 1 }


assert list.max(mc2) == -13
assert list.min(mc2) == -1

assert list.max { a, b -> a.equals(b) ? 0 : Math.abs(a) < Math.abs(b) ? -1 : 1 } == -13
assert list.min { a, b -> a.equals(b) ? 0 : Math.abs(a) < Math.abs(b) ? -1 : 1 } == -1
Adding or removing elements

We can use [] to assign a new empty list and << to append items to it:

def list = []
assert list.empty

list << 5
assert list.size() == 1

list << 7 << 'i' << 11
assert list == [5, 7, 'i', 11]

list << ['m', 'o']
assert list == [5, 7, 'i', 11, ['m', 'o']]

//first item in chain of << is target list
assert ([1, 2] << 3 << [4, 5] << 6) == [1, 2, 3, [4, 5], 6]

//using leftShift is equivalent to using <<
assert ([1, 2, 3] << 4) == ([1, 2, 3].leftShift(4))

We can add to a list in many ways:

assert [1, 2] + 3 + [4, 5] + 6 == [1, 2, 3, 4, 5, 6]
// equivalent to calling the `plus` method
assert [1, 2].plus(3).plus([4, 5]).plus(6) == [1, 2, 3, 4, 5, 6]

def a = [1, 2, 3]
a += 4      // creates a new list and assigns it to `a`
a += [5, 6]
assert a == [1, 2, 3, 4, 5, 6]

assert [1, *[222, 333], 456] == [1, 222, 333, 456]
assert [*[1, 2, 3]] == [1, 2, 3]
assert [1, [2, 3, [4, 5], 6], 7, [8, 9]].flatten() == [1, 2, 3, 4, 5, 6, 7, 8, 9]

def list = [1, 2]
list.add(3)
list.addAll([5, 4])
assert list == [1, 2, 3, 5, 4]

list = [1, 2]
list.add(1, 3) // add 3 just before index 1
assert list == [1, 3, 2]

list.addAll(2, [5, 4]) //add [5,4] just before index 2
assert list == [1, 3, 5, 4, 2]

list = ['a', 'b', 'z', 'e', 'u', 'v', 'g']
list[8] = 'x' // the [] operator is growing the list as needed
// nulls inserted if required
assert list == ['a', 'b', 'z', 'e', 'u', 'v', 'g', null, 'x']

It is however important that the + operator on a list is not mutating. Compared to <<, it will create a new list, which is often not what you want and can lead to performance issues.

The Groovy development kit also contains methods allowing you to easily remove elements from a list by value:

assert ['a','b','c','b','b'] - 'c' == ['a','b','b','b']
assert ['a','b','c','b','b'] - 'b' == ['a','c']
assert ['a','b','c','b','b'] - ['b','c'] == ['a']

def list = [1,2,3,4,3,2,1]
list -= 3           // creates a new list by removing `3` from the original one
assert list == [1,2,4,2,1]
assert ( list -= [2,4] ) == [1,1]

It is also possible to remove an element by referring to its index, in which case the list is mutated:

def list = [1,2,3,4,5,6,2,2,1]
assert list.remove(2) == 3          // remove the third element, and return it
assert list == [1,2,4,5,6,2,2,1]

In case you only want to remove the first element having the same value in a list, instead of removing all elements, you call call the remove method:

def list= ['a','b','c','b','b']
assert list.remove('c')             // remove 'c', and return true because element removed
assert list.remove('b')             // remove first 'b', and return true because element removed

assert ! list.remove('z')           // return false because no elements removed
assert list == ['a','b','b']

And removing all the elements in a list can be done by calling the clear method:

def list= ['a',2,'c',4]
list.clear()
assert list == []
Set operations

The Groovy development kit also includes methods making it easy to reason on sets:

assert 'a' in ['a','b','c']             // returns true if an element belongs to the list
assert ['a','b','c'].contains('a')      // equivalent to the `contains` method in Java
assert [1,3,4].containsAll([1,4])       // `containsAll` will check that all elements are found

assert [1,2,3,3,3,3,4,5].count(3) == 4  // count the number of elements which have some value
assert [1,2,3,3,3,3,4,5].count {
    it%2==0                             // count the number of elements which match the predicate
} == 2

assert [1,2,4,6,8,10,12].intersect([1,3,6,9,12]) == [1,6,12]

assert [1,2,3].disjoint( [4,6,9] )
assert ![1,2,3].disjoint( [2,4,6] )
Sorting

Working with collections often implies sorting. Groovy offers a variety of options to sort lists, from using closures to comparators, as in the following examples:

assert [6, 3, 9, 2, 7, 1, 5].sort() == [1, 2, 3, 5, 6, 7, 9]

def list = ['abc', 'z', 'xyzuvw', 'Hello', '321']
assert list.sort {
    it.size()
} == ['z', 'abc', '321', 'Hello', 'xyzuvw']

def list2 = [7, 4, -6, -1, 11, 2, 3, -9, 5, -13]
assert list2.sort { a, b -> a == b ? 0 : Math.abs(a) < Math.abs(b) ? -1 : 1 } ==
        [-1, 2, 3, 4, 5, -6, 7, -9, 11, -13]

Comparator mc = { a, b -> a == b ? 0 : Math.abs(a) < Math.abs(b) ? -1 : 1 }

// JDK 8+ only
// list2.sort(mc)
// assert list2 == [-1, 2, 3, 4, 5, -6, 7, -9, 11, -13]

def list3 = [6, -3, 9, 2, -7, 1, 5]

Collections.sort(list3)
assert list3 == [-7, -3, 1, 2, 5, 6, 9]

Collections.sort(list3, mc)
assert list3 == [1, 2, -3, 5, 6, -7, 9]
Duplicating elements

The Groovy development kit also takes advantage of operator overloading to provide methods allowing duplication of elements of a list:

assert [1, 2, 3] * 3 == [1, 2, 3, 1, 2, 3, 1, 2, 3]
assert [1, 2, 3].multiply(2) == [1, 2, 3, 1, 2, 3]
assert Collections.nCopies(3, 'b') == ['b', 'b', 'b']

// nCopies from the JDK has different semantics than multiply for lists
assert Collections.nCopies(2, [1, 2]) == [[1, 2], [1, 2]] //not [1,2,1,2]

2.2. Maps

2.2.1. Map literals

In Groovy, maps (also known as associative arrays) can be created using the map literal syntax: [:]:

def map = [name: 'Gromit', likes: 'cheese', id: 1234]
assert map.get('name') == 'Gromit'
assert map.get('id') == 1234
assert map['name'] == 'Gromit'
assert map['id'] == 1234
assert map instanceof java.util.Map

def emptyMap = [:]
assert emptyMap.size() == 0
emptyMap.put("foo", 5)
assert emptyMap.size() == 1
assert emptyMap.get("foo") == 5

Map keys are strings by default: [a:1] is equivalent to ['a':1]. This can be confusing if you define a variable named a and that you want the value of a to be the key in your map. If this is the case, then you must escape the key by adding parenthesis, like in the following example:

def a = 'Bob'
def ages = [a: 43]
assert ages['Bob'] == null // `Bob` is not found
assert ages['a'] == 43     // because `a` is a literal!

ages = [(a): 43]            // now we escape `a` by using parenthesis
assert ages['Bob'] == 43   // and the value is found!

In addition to map literals, it is possible, to get a new copy of a map, to clone it:

def map = [
        simple : 123,
        complex: [a: 1, b: 2]
]
def map2 = map.clone()
assert map2.get('simple') == map.get('simple')
assert map2.get('complex') == map.get('complex')
map2.get('complex').put('c', 3)
assert map.get('complex').get('c') == 3

The resulting map is a shallow copy of the original one, as illustrated in the previous example.

2.2.2. Map property notation

Maps also act like beans so you can use the property notation to get/set items inside the Map as long as the keys are strings which are valid Groovy identifiers:

def map = [name: 'Gromit', likes: 'cheese', id: 1234]
assert map.name == 'Gromit'     // can be used instead of map.get('Gromit')
assert map.id == 1234

def emptyMap = [:]
assert emptyMap.size() == 0
emptyMap.foo = 5
assert emptyMap.size() == 1
assert emptyMap.foo == 5

Note: by design map.foo will always look for the key foo in the map. This means foo.class will return null on a map that doesn’t contain the class key. Should you really want to know the class, then you must use getClass():

def map = [name: 'Gromit', likes: 'cheese', id: 1234]
assert map.class == null
assert map.get('class') == null
assert map.getClass() == LinkedHashMap // this is probably what you want

map = [1      : 'a',
       (true) : 'p',
       (false): 'q',
       (null) : 'x',
       'null' : 'z']
assert map.containsKey(1) // 1 is not an identifier so used as is
assert map.true == null
assert map.false == null
assert map.get(true) == 'p'
assert map.get(false) == 'q'
assert map.null == 'z'
assert map.get(null) == 'x'

2.2.3. Iterating on maps

As usual in the Groovy development kit, idiomatic iteration on maps makes use of the each and eachWithIndex methods. It’s worth noting that maps created using the map literal notation are ordered, that is to say that if you iterate on map entries, it is guaranteed that the entries will be returned in the same order they were added in the map.

def map = [
        Bob  : 42,
        Alice: 54,
        Max  : 33
]

// `entry` is a map entry
map.each { entry ->
    println "Name: $entry.key Age: $entry.value"
}

// `entry` is a map entry, `i` the index in the map
map.eachWithIndex { entry, i ->
    println "$i - Name: $entry.key Age: $entry.value"
}

// Alternatively you can use key and value directly
map.each { key, value ->
    println "Name: $key Age: $value"
}

// Key, value and i as the index in the map
map.eachWithIndex { key, value, i ->
    println "$i - Name: $key Age: $value"
}

2.2.4. Manipulating maps

Adding or removing elements

Adding an element to a map can be done either using the put method, the subscript operator or using putAll:

def defaults = [1: 'a', 2: 'b', 3: 'c', 4: 'd']
def overrides = [2: 'z', 5: 'x', 13: 'x']

def result = new LinkedHashMap(defaults)
result.put(15, 't')
result[17] = 'u'
result.putAll(overrides)
assert result == [1: 'a', 2: 'z', 3: 'c', 4: 'd', 5: 'x', 13: 'x', 15: 't', 17: 'u']

Removing all the elements of a map can be done by calling the clear method:

def m = [1:'a', 2:'b']
assert m.get(1) == 'a'
m.clear()
assert m == [:]

Maps generated using the map literal syntax are using the object equals and hashcode methods. This means that you should never use an object which hash code is subject to change over time, or you wouldn’t be able to get the associated value back.

It is also worth noting that you should never use a GString as the key of a map, because the hash code of a GString is not the same as the hash code of an equivalent String:

def key = 'some key'
def map = [:]
def gstringKey = "${key.toUpperCase()}"
map.put(gstringKey,'value')
assert map.get('SOME KEY') == null
Keys, values and entries

We can inspect the keys, values, and entries in a view:

def map = [1:'a', 2:'b', 3:'c']

def entries = map.entrySet()
entries.each { entry ->
  assert entry.key in [1,2,3]
  assert entry.value in ['a','b','c']
}

def keys = map.keySet()
assert keys == [1,2,3] as Set

Mutating values returned by the view (be it a map entry, a key or a value) is highly discouraged because success of the operation directly depends on the type of the map being manipulated. In particular, Groovy relies on collections from the JDK that in general make no guarantee that a collection can safely be manipulated through keySet, entrySet, or values.

Filtering and searching

The Groovy development kit contains filtering, searching and collecting methods similar to those found for lists:

def people = [
    1: [name:'Bob', age: 32, gender: 'M'],
    2: [name:'Johnny', age: 36, gender: 'M'],
    3: [name:'Claire', age: 21, gender: 'F'],
    4: [name:'Amy', age: 54, gender:'F']
]

def bob = people.find { it.value.name == 'Bob' } // find a single entry
def females = people.findAll { it.value.gender == 'F' }

// both return entries, but you can use collect to retrieve the ages for example
def ageOfBob = bob.value.age
def agesOfFemales = females.collect {
    it.value.age
}

assert ageOfBob == 32
assert agesOfFemales == [21,54]

// but you could also use a key/pair value as the parameters of the closures
def agesOfMales = people.findAll { id, person ->
    person.gender == 'M'
}.collect { id, person ->
    person.age
}
assert agesOfMales == [32, 36]

// `every` returns true if all entries match the predicate
assert people.every { id, person ->
    person.age > 18
}

// `any` returns true if any entry matches the predicate

assert people.any { id, person ->
    person.age == 54
}
Grouping

We can group a list into a map using some criteria:

assert ['a', 7, 'b', [2, 3]].groupBy {
    it.class
} == [(String)   : ['a', 'b'],
      (Integer)  : [7],
      (ArrayList): [[2, 3]]
]

assert [
        [name: 'Clark', city: 'London'], [name: 'Sharma', city: 'London'],
        [name: 'Maradona', city: 'LA'], [name: 'Zhang', city: 'HK'],
        [name: 'Ali', city: 'HK'], [name: 'Liu', city: 'HK'],
].groupBy { it.city } == [
        London: [[name: 'Clark', city: 'London'],
                 [name: 'Sharma', city: 'London']],
        LA    : [[name: 'Maradona', city: 'LA']],
        HK    : [[name: 'Zhang', city: 'HK'],
                 [name: 'Ali', city: 'HK'],
                 [name: 'Liu', city: 'HK']],
]

2.3. Ranges

Ranges allow you to create a list of sequential values. These can be used as List since Range extends java.util.List.

Ranges defined with the .. notation are inclusive (that is the list contains the from and to value).

Ranges defined with the ..< notation are half-open, they include the first value but not the last value.

// an inclusive range
def range = 5..8
assert range.size() == 4
assert range.get(2) == 7
assert range[2] == 7
assert range instanceof java.util.List
assert range.contains(5)
assert range.contains(8)

// lets use a half-open range
range = 5..<8
assert range.size() == 3
assert range.get(2) == 7
assert range[2] == 7
assert range instanceof java.util.List
assert range.contains(5)
assert !range.contains(8)

//get the end points of the range without using indexes
range = 1..10
assert range.from == 1
assert range.to == 10

Note that int ranges are implemented efficiently, creating a lightweight Java object containing a from and to value.

Ranges can be used for any Java object which implements java.lang.Comparable for comparison and also have methods next() and previous() to return the next / previous item in the range. For example, you can create a range of String elements:

// an inclusive range
def range = 'a'..'d'
assert range.size() == 4
assert range.get(2) == 'c'
assert range[2] == 'c'
assert range instanceof java.util.List
assert range.contains('a')
assert range.contains('d')
assert !range.contains('e')

You can iterate on a range using a classic for loop:

for (i in 1..10) {
    println "Hello ${i}"
}

but alternatively you can achieve the same effect in a more Groovy idiomatic style, by iterating a range with each method:

(1..10).each { i ->
    println "Hello ${i}"
}

Ranges can be also used in the switch statement:

switch (years) {
    case 1..10: interestRate = 0.076; break;
    case 11..25: interestRate = 0.052; break;
    default: interestRate = 0.037;
}

2.4. Syntax enhancements for collections

2.4.1. GPath support

Thanks to the support of property notation for both lists and maps, Groovy provides syntactic sugar making it really easy to deal with nested collections, as illustrated in the following examples:

def listOfMaps = [['a': 11, 'b': 12], ['a': 21, 'b': 22]]
assert listOfMaps.a == [11, 21] //GPath notation
assert listOfMaps*.a == [11, 21] //spread dot notation

listOfMaps = [['a': 11, 'b': 12], ['a': 21, 'b': 22], null]
assert listOfMaps*.a == [11, 21, null] // caters for null values
assert listOfMaps*.a == listOfMaps.collect { it?.a } //equivalent notation
// But this will only collect non-null values
assert listOfMaps.a == [11,21]

2.4.2. Spread operator

The spread operator can be used to "inline" a collection into another. It is syntactic sugar which often avoids calls to putAll and facilitates the realization of one-liners:

assert [ 'z': 900,
         *: ['a': 100, 'b': 200], 'a': 300] == ['a': 300, 'b': 200, 'z': 900]
//spread map notation in map definition
assert [*: [3: 3, *: [5: 5]], 7: 7] == [3: 3, 5: 5, 7: 7]

def f = { [1: 'u', 2: 'v', 3: 'w'] }
assert [*: f(), 10: 'zz'] == [1: 'u', 10: 'zz', 2: 'v', 3: 'w']
//spread map notation in function arguments
f = { map -> map.c }
assert f(*: ['a': 10, 'b': 20, 'c': 30], 'e': 50) == 30

f = { m, i, j, k -> [m, i, j, k] }
//using spread map notation with mixed unnamed and named arguments
assert f('e': 100, *[4, 5], *: ['a': 10, 'b': 20, 'c': 30], 6) ==
        [["e": 100, "b": 20, "c": 30, "a": 10], 4, 5, 6]

2.4.3. The star-dot `*.' operator

The "star-dot" operator is a shortcut operator allowing you to call a method or a property on all elements of a collection:

assert [1, 3, 5] == ['a', 'few', 'words']*.size()

class Person {
    String name
    int age
}
def persons = [new Person(name:'Hugo', age:17), new Person(name:'Sandra',age:19)]
assert [17, 19] == persons*.age

2.4.4. Slicing with the subscript operator

You can index into lists, arrays, maps using the subscript expression. It is interesting that strings are considered as special kinds of collections in that context:

def text = 'nice cheese gromit!'
def x = text[2]

assert x == 'c'
assert x.class == String

def sub = text[5..10]
assert sub == 'cheese'

def list = [10, 11, 12, 13]
def answer = list[2,3]
assert answer == [12,13]

Notice that you can use ranges to extract part of a collection:

list = 100..200
sub = list[1, 3, 20..25, 33]
assert sub == [101, 103, 120, 121, 122, 123, 124, 125, 133]

The subscript operator can be used to update an existing collection (for collection type which are not immutable):

list = ['a','x','x','d']
list[1..2] = ['b','c']
assert list == ['a','b','c','d']

It is worth noting that negative indices are allowed, to extract more easily from the end of a collection:

You can use negative indices to count from the end of the List, array, String etc.

text = "nice cheese gromit!"
x = text[-1]
assert x == "!"

def name = text[-7..-2]
assert name == "gromit"

Eventually, if you use a backwards range (the starting index is greater than the end index), then the answer is reversed.

text = "nice cheese gromit!"
name = text[3..1]
assert name == "eci"

2.5. Enhanced Collection Methods

In addition to lists, maps or ranges, Groovy offers a lot of additional methods for filtering, collecting, grouping, counting, …​ which are directly available on either collections or more easily iterables.

In particular, we invite you to read the Groovy development kit API docs and specifically:

  • methods added to Iterable can be found here

  • methods added to Iterator can be found here

  • methods added to Collection can be found here

  • methods added to List can be found here

  • methods added to Map can be found here

3. Handy utilities

3.1. ConfigSlurper

ConfigSlurper is a utility class for reading configuration files defined in the form of Groovy scripts. Like it is the case with Java *.properties files, ConfigSlurper allows a dot notation. But in addition, it allows for Closure scoped configuration values and arbitrary object types.

def config = new ConfigSlurper().parse('''
    app.date = new Date()  (1)
    app.age  = 42
    app {                  (2)
        name = "Test${42}"
    }
''')

assert config.app.date instanceof Date
assert config.app.age == 42
assert config.app.name == 'Test42'
1 Usage of the dot notation
2 Usage of Closure scopes as an alternative to the dot notation

As can be seen in the above example, the parse method can be used to retrieve groovy.util.ConfigObject instances. The ConfigObject is a specialized java.util.Map implementation that either returns the configured value or a new ConfigObject instance but never null.

def config = new ConfigSlurper().parse('''
    app.date = new Date()
    app.age  = 42
    app.name = "Test${42}"
''')

assert config.test != null   (1)
1 config.test has not been specified yet it returns a ConfigObject when being called.

In the case of a dot being part of a configuration variable name, it can be escaped by using single or double quotes.

def config = new ConfigSlurper().parse('''
    app."person.age"  = 42
''')

assert config.app."person.age" == 42

In addition, ConfigSlurper comes with support for environments. The environments method can be used to hand over a Closure instance that itself may consist of a several sections. Let’s say we wanted to create a particular configuration value for the development environment. When creating the ConfigSlurper instance we can use the ConfigSlurper(String) constructor to specify the target environment.

def config = new ConfigSlurper('development').parse('''
  environments {
       development {
           app.port = 8080
       }

       test {
           app.port = 8082
       }

       production {
           app.port = 80
       }
  }
''')

assert config.app.port == 8080
The ConfigSlurper environments aren’t restricted to any particular environment names. It solely depends on the ConfigSlurper client code what value are supported and interpreted accordingly.

The environments method is built-in but the registerConditionalBlock method can be used to register other method names in addition to the environments name.

def slurper = new ConfigSlurper()
slurper.registerConditionalBlock('myProject', 'developers')   (1)

def config = slurper.parse('''
  sendMail = true

  myProject {
       developers {
           sendMail = false
       }
  }
''')

assert !config.sendMail
1 Once the new block is registered ConfigSlurper can parse it.

For Java integration purposes the toProperties method can be used to convert the ConfigObject to a java.util.Properties object that might be stored to a *.properties text file. Be aware though that the configuration values are converted to String instances during adding them to the newly created Properties instance.

def config = new ConfigSlurper().parse('''
    app.date = new Date()
    app.age  = 42
    app {
        name = "Test${42}"
    }
''')

def properties = config.toProperties()

assert properties."app.date" instanceof String
assert properties."app.age" == '42'
assert properties."app.name" == 'Test42'

3.2. Expando

The Expando class can be used to create a dynamically expandable object. Despite its name it does not use the ExpandoMetaClass underneath. Each Expando object represents a standalone, dynamically-crafted instance that can be extended with properties (or methods) at runtime.

def expando = new Expando()
expando.name = 'John'

assert expando.name == 'John'

A special case occurs when a dynamic property registers a Closure code block. Once being registered it can be invoked as it would be done with a method call.

def expando = new Expando()
expando.toString = { -> 'John' }
expando.say = { String s -> "John says: ${s}" }

assert expando as String == 'John'
assert expando.say('Hi') == 'John says: Hi'

3.3. Observable list, map and set

Groovy comes with observable lists, maps and sets. Each of these collections trigger java.beans.PropertyChangeEvent events when elements are added, removed or changed. Note that a PropertyChangeEvent is not only signalling that a certain event has occurred, moreover, it holds information on the property name and the old/new value a certain property has been changed to.

Depending on the type of change that has happened, observable collections might fire more specialized PropertyChangeEvent types. For example, adding an element to an observable list fires an ObservableList.ElementAddedEvent event.

def event                                       (1)
def listener = {
    if (it instanceof ObservableList.ElementEvent)  {  (2)
        event = it
    }
} as PropertyChangeListener


def observable = [1, 2, 3] as ObservableList    (3)
observable.addPropertyChangeListener(listener)  (4)

observable.add 42                               (5)

assert event instanceof ObservableList.ElementAddedEvent

def elementAddedEvent = event as ObservableList.ElementAddedEvent
assert elementAddedEvent.changeType == ObservableList.ChangeType.ADDED
assert elementAddedEvent.index == 3
assert elementAddedEvent.oldValue == null
assert elementAddedEvent.newValue == 42
1 Declares a PropertyChangeEventListener that is capturing the fired events
2 ObservableList.ElementEvent and its descendant types are relevant for this listener
3 Registers the listener
4 Creates an ObservableList from the given list
5 Triggers an ObservableList.ElementAddedEvent event
Be aware that adding an element in fact causes two events to be triggered. The first is of type ObservableList.ElementAddedEvent, the second is a plain PropertyChangeEvent that informs listeners about the change of property size.

The ObservableList.ElementClearedEvent event type is another interesting one. Whenever multiple elements are removed, for example when calling clear(), it holds the elements being removed from the list.

def event
def listener = {
    if (it instanceof ObservableList.ElementEvent)  {
        event = it
    }
} as PropertyChangeListener


def observable = [1, 2, 3] as ObservableList
observable.addPropertyChangeListener(listener)

observable.clear()

assert event instanceof ObservableList.ElementClearedEvent

def elementClearedEvent = event as ObservableList.ElementClearedEvent
assert elementClearedEvent.values == [1, 2, 3]
assert observable.size() == 0

To get an overview of all the supported event types the reader is encouraged to have a look at the JavaDoc documentation or the source code of the observable collection in use.

ObservableMap and ObservableSet come with the same concepts as we have seen for ObservableList in this section.