# Syntax

version 2.5.6

This chapter covers the syntax of the Groovy programming language. The grammar of the language derives from the Java grammar, but enhances it with specific constructs for Groovy, and allows certain simplifications.

## 1. Comments

### 1.1. Single line comment

Single line comments start with `//` and can be found at any position in the line. The characters following `//`, till the end of the line, are considered part of the comment.

    // a standalone single line comment
    println "hello" // a comment till the end of the line

### 1.2. Multiline comment

A multiline comment starts with `/*` and can be found at any position in the line. The characters following `/*` will be considered part of the comment, including new line characters, up to the first `*/` closing the comment. Multiline comments can thus be put at the end of a statement, or even inside a statement.

    /* a standalone multiline comment
       spanning two lines */
    println "hello" /* a multiline comment starting
                       at the end of a statement */
    println 1 /* one */ + 2 /* two */

### 1.3. GroovyDoc comment

Similarly to multiline comments, GroovyDoc comments are multiline, but start with `/**` and end with `*/`. Lines following the first GroovyDoc comment line can optionally start with a star `*`. Those comments are associated with:

- type definitions (classes, interfaces, enums, annotations),

- fields and properties definitions

- methods definitions

Although the compiler will not complain about GroovyDoc comments not being associated with the above language elements, you should prepend those constructs with the comment right before it.

    /**
     * A Class description
     */
    class Person {
        /** the name of the person */
        String name

        /**
         * Creates a greeting method for a certain person.
         *
         * @param otherPerson the person to greet
         * @return a greeting message
         */
        String greet(String otherPerson) {
           "Hello ${otherPerson}"
        }
    }

GroovyDoc follows the same conventions as Java’s own JavaDoc. So you’ll be able to use the same tags as with JavaDoc.

### 1.4. Shebang line

Beside the single line comment, there is a special line comment, often called the shebang line understood by UNIX systems which allows scripts to be run directly from the command-line, provided you have installed the Groovy distribution and the `groovy` command is available on the `PATH`.

    #!/usr/bin/env groovy
    println "Hello from the shebang line"

| The `#` character must be the first character of the file. Any indentation would yield a compilation error.

## 2. Keywords

The following list represents all the keywords of the Groovy language:

Table 1. Keywords
as | assert | break | case
catch | class | const | continue
def | default | do | else
enum | extends | false | finally
for | goto | if | implements
import | in | instanceof | interface
new | null | package | return
super | switch | this | throw
throws | trait | true | try
while |  |  |

## 3. Identifiers

### 3.1. Normal identifiers

Identifiers start with a letter, a dollar or an underscore. They cannot start with a number.

A letter can be in the following ranges:

- 'a' to 'z' (lowercase ascii letter)

- 'A' to 'Z' (uppercase ascii letter)

- '\u00C0' to '\u00D6'

- '\u00D8' to '\u00F6'

- '\u00F8' to '\u00FF'

- '\u0100' to '\uFFFE'

Then following characters can contain letters and numbers.

Here are a few examples of valid identifiers (here, variable names):

    def name
    def item3
    def with_underscore
    def $dollarStart

But the following ones are invalid identifiers:

    def 3tier
    def a+b
    def a#b

All keywords are also valid identifiers when following a dot:

    foo.as
    foo.assert
    foo.break
    foo.case
    foo.catch

### 3.2. Quoted identifiers

Quoted identifiers appear after the dot of a dotted expression. For instance, the `name` part of the `person.name` expression can be quoted with `person."name"` or `person.'name'`. This is particularly interesting when certain identifiers contain illegal characters that are forbidden by the Java Language Specification, but which are allowed by Groovy when quoted. For example, characters like a dash, a space, an exclamation mark, etc.

    def map = [:]

    map."an identifier with a space and double quotes" = "ALLOWED"
    map.'with-dash-signs-and-single-quotes' = "ALLOWED"

    assert map."an identifier with a space and double quotes" == "ALLOWED"
    assert map.'with-dash-signs-and-single-quotes' == "ALLOWED"

As we shall see in the following section on strings, Groovy provides different string literals. All kind of strings are actually allowed after the dot:

    map.'single quote'
    map."double quote"
    map.'''triple single quote'''
    map."""triple double quote"""
    map./slashy string/
    map.$/dollar slashy string/$

There’s a difference between plain character strings and Groovy’s GStrings (interpolated strings), as in that the latter case, the interpolated values are inserted in the final string for evaluating the whole identifier:

    def firstname = "Homer"
    map."Simpson-${firstname}" = "Homer Simpson"

    assert map.'Simpson-Homer' == "Homer Simpson"

## 4. Strings

Text literals are represented in the form of chain of characters called strings. Groovy lets you instantiate `java.lang.String` objects, as well as GStrings (`groovy.lang.GString`) which are also called interpolated strings in other programming languages.

### 4.1. Single quoted string

Single quoted strings are a series of characters surrounded by single quotes:

    'a single quoted string'

| Single quoted strings are plain `java.lang.String` and don’t support interpolation.

### 4.2. String concatenation

All the Groovy strings can be concatenated with the `+` operator:

    assert 'ab' == 'a' + 'b'

### 4.3. Triple single quoted string

Triple single quoted strings are a series of characters surrounded by triplets of single quotes:

    '''a triple single quoted string'''

| Triple single quoted strings are plain `java.lang.String` and don’t support interpolation.

Triple single quoted strings are multiline. You can span the content of the string across line boundaries without the need to split the string in several pieces, without contatenation or newline escape characters:

    def aMultilineString = '''line one
    line two
    line three'''

If your code is indented, for example in the body of the method of a class, your string will contain the whitespace of the indentation. The Groovy Development Kit contains methods for stripping out the indentation with the `String#stripIndent()` method, and with the `String#stripMargin()` method that takes a delimiter character to identify the text to remove from the beginning of a string.

When creating a string as follows:

    def startingAndEndingWithANewline = '''
    line one
    line two
    line three
    '''

You will notice that the resulting string contains a newline character as first character. It is possible to strip that character by escaping the newline with a backslash:

    def strippedFirstNewline = '''\
    line one
    line two
    line three
    '''

    assert !strippedFirstNewline.startsWith('\n')

#### 4.3.1. Escaping special characters

You can escape single quotes with the the backslash character to avoid terminating the string literal:

    'an escaped single quote: \' needs a backslash'

And you can escape the escape character itself with a double backslash:

    'an escaped escape character: \\ needs a double backslash'

Some special characters also use the backslash as escape character:

Escape sequence | Character
--- | ---
'\t' | tabulation
'\b' | backspace
'\n' | newline
'\r' | carriage return
'\f' | formfeed
'\\' | backslash
'\'' | single quote (for single quoted and triple single quoted strings)
'\"' | double quote (for double quoted and triple double quoted strings)

#### 4.3.2. Unicode escape sequence

For characters that are not present on your keyboard, you can use unicode escape sequences: a backslash, followed by 'u', then 4 hexadecimal digits.

For example, the Euro currency symbol can be represented with:

    'The Euro currency symbol: \u20AC'

### 4.4. Double quoted string

Double quoted strings are a series of characters surrounded by double quotes:

    "a double quoted string"

| Double quoted strings are plain `java.lang.String` if there’s no interpolated expression, but are `groovy.lang.GString` instances if interpolation is present.

| To escape a double quote, you can use the backslash character: "A double quote: \"".

#### 4.4.1. String interpolation

Any Groovy expression can be interpolated in all string literals, apart from single and triple single quoted strings. Interpolation is the act of replacing a placeholder in the string with its value upon evaluation of the string. The placeholder expressions are surrounded by `${}` or prefixed with `$` for dotted expressions. The expression value inside the placeholder is evaluated to its string representation when the GString is passed to a method taking a String as argument by calling `toString()` on that expression.

Here, we have a string with a placeholder referencing a local variable:

    def name = 'Guillaume' // a plain string
    def greeting = "Hello ${name}"

    assert greeting.toString() == 'Hello Guillaume'

But any Groovy expression is valid, as we can see in this example with an arithmetic expression:

    def sum = "The sum of 2 and 3 equals ${2 + 3}"
    assert sum.toString() == 'The sum of 2 and 3 equals 5'

| Not only are expressions allowed in between the `${}` placeholder, but so are statements. However, a statement’s value is just `null`. So if several statements are inserted in that placeholder, the last one should somehow return a meaningful value to be inserted. For instance, "The sum of 1 and 2 is equal to ${def a = 1; def b = 2; a + b}" is supported and works as expected but a good practice is usually to stick to simple expressions inside GString placeholders.

In addition to `${}` placeholders, we can also use a lone `$` sign prefixing a dotted expression:

    def person = [name: 'Guillaume', age: 36]
    assert "$person.name is $person.age years old" == 'Guillaume is 36 years old'

But only dotted expressions of the form `a.b`, `a.b.c`, etc, are valid, but expressions that would contain parentheses like method calls, curly braces for closures, or arithmetic operators would be invalid. Given the following variable definition of a number:

    def number = 3.14

The following statement will throw a `groovy.lang.MissingPropertyException` because Groovy believes you’re trying to access the `toString` property of that number, which doesn’t exist:

    shouldFail(MissingPropertyException) {
        println "$number.toString()"
    }

| You can think of `"$number.toString()"` as being interpreted by the parser as `"${number.toString}()"`.

If you need to escape the `$` or `${}` placeholders in a GString so they appear as is without interpolation, you just need to use a `\` backslash character to escape the dollar sign:

    assert '${name}' == "\${name}"

#### 4.4.2. Special case of interpolating closure expressions

So far, we’ve seen we could interpolate arbitrary expressions inside the `${}` placeholder, but there is a special case and notation for closure expressions. When the placeholder contains an arrow, `${→}`, the expression is actually a closure expression — you can think of it as a closure with a dollar prepended in front of it:

    def sParameterLessClosure = "1 + 2 == ${-> 3}" (1)
    assert sParameterLessClosure == '1 + 2 == 3'

    def sOneParamClosure = "1 + 2 == ${ w -> w << 3}" (2)
    assert sOneParamClosure == '1 + 2 == 3'

1 | The closure is a parameterless closure which doesn’t take arguments.
2 | Here, the closure takes a single `java.io.StringWriter` argument, to which you can append content with the `<<` leftShift operator. In either case, both placeholders are embedded closures.

In appearance, it looks like a more verbose way of defining expressions to be interpolated, but closures have an interesting advantage over mere expressions: lazy evaluation.

Let’s consider the following sample:

    def number = 1 (1)
    def eagerGString = "value == ${number}"
    def lazyGString = "value == ${ -> number }"

    assert eagerGString == "value == 1" (2)
    assert lazyGString ==  "value == 1" (3)

    number = 2 (4)
    assert eagerGString == "value == 1" (5)
    assert lazyGString ==  "value == 2" (6)

1 | We define a `number` variable containing `1` that we then interpolate within two GStrings, as an expression in `eagerGString` and as a closure in `lazyGString`.
2 | We expect the resulting string to contain the same string value of 1 for `eagerGString`.
3 | Similarly for `lazyGString`
4 | Then we change the value of the variable to a new number
5 | With a plain interpolated expression, the value was actually bound at the time of creation of the GString.
6 | But with a closure expression, the closure is called upon each coercion of the GString into String, resulting in an updated string containing the new number value.

| An embedded closure expression taking more than one parameter will generate an exception at runtime. Only closures with zero or one parameters are allowed.

#### 4.4.3. Interoperability with Java

When a method (whether implemented in Java or Groovy) expects a `java.lang.String`, but we pass a `groovy.lang.GString` instance, the `toString()` method of the GString is automatically and transparently called.

    String takeString(String message) {         (4)
        assert message instanceof String        (5)
        return message
    }

    def message = "The message is ${'hello'}"   (1)
    assert message instanceof GString           (2)

    def result = takeString(message)            (3)
    assert result instanceof String
    assert result == 'The message is hello'

1 | We create a GString variable
2 | We double check it’s an instance of the GString
3 | We then pass that GString to a method taking a String as parameter
4 | The signature of the `takeString()` method explicitly says its sole parameter is a String
5 | We also verify that the parameter is indeed a String and not a GString.

#### 4.4.4. GString and String hashCodes

Although interpolated strings can be used in lieu of plain Java strings, they differ with strings in a particular way: their hashCodes are different. Plain Java strings are immutable, whereas the resulting String representation of a GString can vary, depending on its interpolated values. Even for the same resulting string, GStrings and Strings don’t have the same hashCode.

    assert "one: ${1}".hashCode() != "one: 1".hashCode()

GString and Strings having different hashCode values, using GString as Map keys should be avoided, especially if we try to retrieve an associated value with a String instead of a GString.

    def key = "a"
    def m = ["${key}": "letter ${key}"]     (1)

    assert m["a"] == null                   (2)

1 | The map is created with an initial pair whose key is a GString
2 | When we try to fetch the value with a String key, we will not find it, as Strings and GString have different hashCode values

### 4.5. Triple double quoted string

Triple double quoted strings behave like double quoted strings, with the addition that they are multiline, like the triple single quoted strings.

    def name = 'Groovy'
    def template = """
        Dear Mr ${name},

        You're the winner of the lottery!

        Yours sincerly,

        Dave
    """

    assert template.toString().contains('Groovy')

| Neither double quotes nor single quotes need be escaped in triple double quoted strings.

### 4.6. Slashy string

Beyond the usual quoted strings, Groovy offers slashy strings, which use `/` as delimiters. Slashy strings are particularly useful for defining regular expressions and patterns, as there is no need to escape backslashes.

Example of a slashy string:

    def fooPattern = /.*foo.*/
    assert fooPattern == '.*foo.*'

Only forward slashes need to be escaped with a backslash:

    def escapeSlash = /The character \/ is a forward slash/
    assert escapeSlash == 'The character / is a forward slash'

Slashy strings are multiline:

    def multilineSlashy = /one
        two
        three/

    assert multilineSlashy.contains('\n')

Slashy strings can also be interpolated (ie. a GString):

    def color = 'blue'
    def interpolatedSlashy = /a ${color} car/

    assert interpolatedSlashy == 'a blue car'

There are a few gotchas to be aware of.

An empty slashy string cannot be represented with a double forward slash, as it’s understood by the Groovy parser as a line comment. That’s why the following assert would actually not compile as it would look like a non-terminated statement:

    assert '' == //

| As slashy strings were mostly designed to make regexp easier so a few things that are errors in GStrings like `$()` will work with slashy strings.

### 4.7. Dollar slashy string

Dollar slashy strings are multiline GStrings delimited with an opening `$/` and and a closing `/$`. The escaping character is the dollar sign, and it can escape another dollar, or a forward slash. But both dollar and forward slashes don’t need to be escaped, except to escape the dollar of a string subsequence that would start like a GString placeholder sequence, or if you need to escape a sequence that would start like a closing dollar slashy string delimiter.

Here’s an example:

    def name = "Guillaume"
    def date = "April, 1st"

    def dollarSlashy = $/
        Hello $name,
        today we're ${date}.

        $ dollar sign
        $$ escaped dollar sign
        \ backslash
        / forward slash
        $/ escaped forward slash
        $$$/ escaped opening dollar slashy
        $/$$ escaped closing dollar slashy
    /$

    assert [
        'Guillaume',
        'April, 1st',
        '$ dollar sign',
        '$ escaped dollar sign',
        '\\ backslash',
        '/ forward slash',
        '/ escaped forward slash',
        '$/ escaped opening dollar slashy',
        '/$ escaped closing dollar slashy'
    ].every { dollarSlashy.contains(it) }

### 4.8. String summary table

String name | String syntax | Interpolated | Multiline | Escape character
Single quoted | `'…​'` |  |  | `\`
Triple single quoted | `'''…​'''` |  |  | `\`
Double quoted | `"…​"` |  |  | `\`
Triple double quoted | `"""…​"""` |  |  | `\`
Slashy | `/…​/` |  |  | `\`
Dollar slashy | `$/…​/$` |  |  | `$`

### 4.9. Characters

Unlike Java, Groovy doesn’t have an explicit character literal. However, you can be explicit about making a Groovy string an actual character, by three different means:

    char c1 = 'A' (1)
    assert c1 instanceof Character

    def c2 = 'B' as char (2)
    assert c2 instanceof Character

    def c3 = (char)'C' (3)
    assert c3 instanceof Character

1 | by being explicit when declaring a variable holding the character by specifying the `char` type
2 | by using type coercion with the `as` operator
3 | by using a cast to char operation

| The first option 1 is interesting when the character is held in a variable, while the other two (2 and 3) are more interesting when a char value must be passed as argument of a method call.

## 5. Numbers

Groovy supports different kinds of integral literals and decimal literals, backed by the usual `Number` types of Java.

### 5.1. Integral literals

The integral literal types are the same as in Java:

- `byte`

- `char`

- `short`

- `int`

- `long`

- `java.lang.BigInteger`

You can create integral numbers of those types with the following declarations:

    // primitive types
    byte  b = 1
    char  c = 2
    short s = 3
    int   i = 4
    long  l = 5

    // infinite precision
    BigInteger bi =  6

If you use optional typing by using the `def` keyword, the type of the integral number will vary: it’ll adapt to the capacity of the type that can hold that number.

For positive numbers:

    def a = 1
    assert a instanceof Integer

    // Integer.MAX_VALUE
    def b = 2147483647
    assert b instanceof Integer

    // Integer.MAX_VALUE + 1
    def c = 2147483648
    assert c instanceof Long

    // Long.MAX_VALUE
    def d = 9223372036854775807
    assert d instanceof Long

    // Long.MAX_VALUE + 1
    def e = 9223372036854775808
    assert e instanceof BigInteger

As well as for negative numbers:

    def na = -1
    assert na instanceof Integer

    // Integer.MIN_VALUE
    def nb = -2147483648
    assert nb instanceof Integer

    // Integer.MIN_VALUE - 1
    def nc = -2147483649
    assert nc instanceof Long

    // Long.MIN_VALUE
    def nd = -9223372036854775808
    assert nd instanceof Long

    // Long.MIN_VALUE - 1
    def ne = -9223372036854775809
    assert ne instanceof BigInteger

#### 5.1.1. Alternative non-base 10 representations

Numbers can also be represented in binary, octal, hexadecimal and decimal bases.

##### Binary literal

Binary numbers start with a `0b` prefix:

    int xInt = 0b10101111
    assert xInt == 175

    short xShort = 0b11001001
    assert xShort == 201 as short

    byte xByte = 0b11
    assert xByte == 3 as byte

    long xLong = 0b101101101101
    assert xLong == 2925l

    BigInteger xBigInteger = 0b111100100001
    assert xBigInteger == 3873g

    int xNegativeInt = -0b10101111
    assert xNegativeInt == -175

##### Octal literal

Octal numbers are specified in the typical format of `0` followed by octal digits.

    int xInt = 077
    assert xInt == 63

    short xShort = 011
    assert xShort == 9 as short

    byte xByte = 032
    assert xByte == 26 as byte

    long xLong = 0246
    assert xLong == 166l

    BigInteger xBigInteger = 01111
    assert xBigInteger == 585g

    int xNegativeInt = -077
    assert xNegativeInt == -63

##### Hexadecimal literal

Hexadecimal numbers are specified in the typical format of `0x` followed by hex digits.

    int xInt = 0x77
    assert xInt == 119

    short xShort = 0xaa
    assert xShort == 170 as short

    byte xByte = 0x3a
    assert xByte == 58 as byte

    long xLong = 0xffff
    assert xLong == 65535l

    BigInteger xBigInteger = 0xaaaa
    assert xBigInteger == 43690g

    Double xDouble = new Double('0x1.0p0')
    assert xDouble == 1.0d

    int xNegativeInt = -0x77
    assert xNegativeInt == -119

### 5.2. Decimal literals

The decimal literal types are the same as in Java:

- `float`

- `double`

- `java.lang.BigDecimal`

You can create decimal numbers of those types with the following declarations:

    // primitive types
    float  f = 1.234
    double d = 2.345

    // infinite precision
    BigDecimal bd =  3.456

Decimals can use exponents, with the `e` or `E` exponent letter, followed by an optional sign, and a integral number representing the exponent:

    assert 1e3  ==  1_000.0
    assert 2E4  == 20_000.0
    assert 3e+1 ==     30.0
    assert 4E-2 ==      0.04
    assert 5e-1 ==      0.5

Conveniently for exact decimal number calculations, Groovy choses `java.lang.BigDecimal` as its decimal number type. In addition, both `float` and `double` are supported, but require an explicit type declaration, type coercion or suffix. Even if `BigDecimal` is the default for decimal numbers, such literals are accepted in methods or closures taking `float` or `double` as parameter types.

| Decimal numbers can’t be represented using a binary, octal or hexadecimal representation.

### 5.3. Underscore in literals

When writing long literal numbers, it’s harder on the eye to figure out how some numbers are grouped together, for example with groups of thousands, of words, etc. By allowing you to place underscore in number literals, it’s easier to spot those groups:

    long creditCardNumber = 1234_5678_9012_3456L
    long socialSecurityNumbers = 999_99_9999L
    double monetaryAmount = 12_345_132.12
    long hexBytes = 0xFF_EC_DE_5E
    long hexWords = 0xFFEC_DE5E
    long maxLong = 0x7fff_ffff_ffff_ffffL
    long alsoMaxLong = 9_223_372_036_854_775_807L
    long bytes = 0b11010010_01101001_10010100_10010010

### 5.4. Number type suffixes

We can force a number (including binary, octals and hexadecimals) to have a specific type by giving a suffix (see table below), either uppercase or lowercase.

Type | Suffix
--- | ---
BigInteger | `G` or `g`
Long | `L` or `l`
Integer | `I` or `i`
BigDecimal | `G` or `g`
Double | `D` or `d`
Float | `F` or `f`

Examples:

    assert 42I == new Integer('42')
    assert 42i == new Integer('42') // lowercase i more readable
    assert 123L == new Long("123") // uppercase L more readable
    assert 2147483648 == new Long('2147483648') // Long type used, value too large for an Integer
    assert 456G == new BigInteger('456')
    assert 456g == new BigInteger('456')
    assert 123.45 == new BigDecimal('123.45') // default BigDecimal type used
    assert 1.200065D == new Double('1.200065')
    assert 1.234F == new Float('1.234')
    assert 1.23E23D == new Double('1.23E23')
    assert 0b1111L.class == Long // binary
    assert 0xFFi.class == Integer // hexadecimal
    assert 034G.class == BigInteger // octal

### 5.5. Math operations

Although operators are covered later on, it’s important to discuss the behavior of math operations and what their resulting types are.

Division and power binary operations aside (covered below),

- binary operations between `byte`, `char`, `short` and `int` result in `int`

- binary operations involving `long` with `byte`, `char`, `short` and `int` result in `long`

- binary operations involving `BigInteger` and any other integral type result in `BigInteger`

- binary operations involving `BigDecimal` with `byte`, `char`, `short`, `int` and `BigInteger` result in `BigDecimal`

- binary operations between `float`, `double` and `BigDecimal` result in `double`

- binary operations between two `BigDecimal` result in `BigDecimal`

The following table summarizes those rules:

| byte | char | short | int | long | BigInteger | float | double | BigDecimal
--- | --- | --- | --- | --- | --- | --- | --- | --- | ---
byte | int | int | int | int | long | BigInteger | double | double | BigDecimal
char |  | int | int | int | long | BigInteger | double | double | BigDecimal
short |  |  | int | int | long | BigInteger | double | double | BigDecimal
int |  |  |  | int | long | BigInteger | double | double | BigDecimal
long |  |  |  |  | long | BigInteger | double | double | BigDecimal
BigInteger |  |  |  |  |  | BigInteger | double | double | BigDecimal
float |  |  |  |  |  |  | double | double | double
double |  |  |  |  |  |  |  | double | double
BigDecimal |  |  |  |  |  |  |  |  | BigDecimal

| Thanks to Groovy’s operator overloading, the usual arithmetic operators work as well with `BigInteger` and `BigDecimal`, unlike in Java where you have to use explicit methods for operating on those numbers.

#### 5.5.1. The case of the division operator

The division operators `/` (and `/=` for division and assignment) produce a `double` result if either operand is a `float` or `double`, and a `BigDecimal` result otherwise (when both operands are any combination of an integral type `short`, `char`, `byte`, `int`, `long`, `BigInteger` or `BigDecimal`).

`BigDecimal` division is performed with the `divide()` method if the division is exact (i.e. yielding a result that can be represented within the bounds of the same precision and scale), or using a `MathContext` with a precision of the maximum of the two operands' precision plus an extra precision of 10, and a scale of the maximum of 10 and the maximum of the operands' scale.

| For integer division like in Java, you should use the `intdiv()` method, as Groovy doesn’t provide a dedicated integer division operator symbol.

#### 5.5.2. The case of the power operator

The power operation is represented by the `**` operator, with two parameters: the base and the exponent. The result of the power operation depends on its operands, and the result of the operation (in particular if the result can be represented as an integral value).

The following rules are used by Groovy’s power operation to determine the resulting type:

- If the exponent is a decimal value

  - if the result can be represented as an `Integer`, then return an `Integer`

  - else if the result can be represented as a `Long`, then return a `Long`

  - otherwise return a `Double`

- If the exponent is an integral value

  - if the exponent is strictly negative, then return an `Integer`, `Long` or `Double` if the result value fits in that type

  - if the exponent is positive or zero

    - if the base is a `BigDecimal`, then return a `BigDecimal` result value

    - if the base is a `BigInteger`, then return a `BigInteger` result value

    - if the base is an `Integer`, then return an `Integer` if the result value fits in it, otherwise a `BigInteger`

    - if the base is a `Long`, then return a `Long` if the result value fits in it, otherwise a `BigInteger`

We can illustrate those rules with a few examples:

    // base and exponent are ints and the result can be represented by an Integer
    assert    2    **   3    instanceof Integer    //  8
    assert   10    **   9    instanceof Integer    //  1_000_000_000

    // the base is a long, so fit the result in a Long
    // (although it could have fit in an Integer)
    assert    5L   **   2    instanceof Long       //  25

    // the result can't be represented as an Integer or Long, so return a BigInteger
    assert  100    **  10    instanceof BigInteger //  10e20
    assert 1234    ** 123    instanceof BigInteger //  170515806212727042875...

    // the base is a BigDecimal and the exponent a negative int
    // but the result can be represented as an Integer
    assert    0.5  **  -2    instanceof Integer    //  4

    // the base is an int, and the exponent a negative float
    // but again, the result can be represented as an Integer
    assert    1    **  -0.3f instanceof Integer    //  1

    // the base is an int, and the exponent a negative int
    // but the result will be calculated as a Double
    // (both base and exponent are actually converted to doubles)
    assert   10    **  -1    instanceof Double     //  0.1

    // the base is a BigDecimal, and the exponent is an int, so return a BigDecimal
    assert    1.2  **  10    instanceof BigDecimal //  6.1917364224

    // the base is a float or double, and the exponent is an int
    // but the result can only be represented as a Double value
    assert    3.4f **   5    instanceof Double     //  454.35430372146965
    assert    5.6d **   2    instanceof Double     //  31.359999999999996

    // the exponent is a decimal value
    // and the result can only be represented as a Double value
    assert    7.8  **   1.9  instanceof Double     //  49.542708423868476
    assert    2    **   0.1f instanceof Double     //  1.0717734636432956

## 6. Booleans

Boolean is a special data type that is used to represent truth values: `true` and `false`. Use this data type for simple flags that track true/false conditions.

Boolean values can be stored in variables, assigned into fields, just like any other data type:

    def myBooleanVariable = true
    boolean untypedBooleanVar = false
    booleanField = true

`true` and `false` are the only two primitive boolean values. But more complex boolean expressions can be represented using logical operators.

In addition, Groovy has special rules (often referred to as Groovy Truth) for coercing non-boolean objects to a boolean value.

## 7. Lists

Groovy uses a comma-separated list of values, surrounded by square brackets, to denote lists. Groovy lists are plain JDK `java.util.List`, as Groovy doesn’t define its own collection classes. The concrete list implementation used when defining list literals are `java.util.ArrayList` by default, unless you decide to specify otherwise, as we shall see later on.

    def numbers = [1, 2, 3]         (1)

    assert numbers instanceof List  (2)
    assert numbers.size() == 3      (3)

1 | We define a list numbers delimited by commas and surrounded by square brackets, and we assign that list into a variable
2 | The list is an instance of Java’s `java.util.List` interface
3 | The size of the list can be queried with the `size()` method, and shows our list contains 3 elements

In the above example, we used a homogeneous list, but you can also create lists containing values of heterogeneous types:

    def heterogeneous = [1, "a", true]  (1)

1 | Our list here contains a number, a string and a boolean value

We mentioned that by default, list literals are actually instances of `java.util.ArrayList`, but it is possible to use a different backing type for our lists, thanks to using type coercion with the `as` operator, or with explicit type declaration for your variables:

    def arrayList = [1, 2, 3]
    assert arrayList instanceof java.util.ArrayList

    def linkedList = [2, 3, 4] as LinkedList    (1)
    assert linkedList instanceof java.util.LinkedList

    LinkedList otherLinked = [3, 4, 5]          (2)
    assert otherLinked instanceof java.util.LinkedList

1 | We use coercion with the `as` operator to explicitly request a `java.util.LinkedList` implementation
2 | We can say that the variable holding the list literal is of type `java.util.LinkedList`

You can access elements of the list with the `[]` subscript operator (both for reading and setting values) with positive indices or negative indices to access elements from the end of the list, as well as with ranges, and use the `<<` leftShift operator to append elements to a list:

    def letters = ['a', 'b', 'c', 'd']

    assert letters[0] == 'a'     (1)
    assert letters[1] == 'b'

    assert letters[-1] == 'd'    (2)
    assert letters[-2] == 'c'

    letters[2] = 'C'             (3)
    assert letters[2] == 'C'

    letters << 'e'               (4)
    assert letters[ 4] == 'e'
    assert letters[-1] == 'e'

    assert letters[1, 3] == ['b', 'd']         (5)
    assert letters[2..4] == ['C', 'd', 'e']    (6)

1 | Access the first element of the list (zero-based counting)
2 | Access the last element of the list with a negative index: -1 is the first element from the end of the list
3 | Use an assignment to set a new value for the third element of the list
4 | Use the `<<` leftShift operator to append an element at the end of the list
5 | Access two elements at once, returning a new list containing those two elements
6 | Use a range to access a range of values from the list, from a start to an end element position

As lists can be heterogeneous in nature, lists can also contain other lists to create multi-dimensional lists:

    def multi = [[0, 1], [2, 3]]     (1)
    assert multi[1][0] == 2          (2)

1 | Define a list of list of numbers
2 | Access the second element of the top-most list, and the first element of the inner list

## 8. Arrays

Groovy reuses the list notation for arrays, but to make such literals arrays, you need to explicitely define the type of the array through coercion or type declaration.

    String[] arrStr = ['Ananas', 'Banana', 'Kiwi']  (1)

    assert arrStr instanceof String[]    (2)
    assert !(arrStr instanceof List)

    def numArr = [1, 2, 3] as int[]      (3)

    assert numArr instanceof int[]       (4)
    assert numArr.size() == 3

1 | Define an array of strings using explicit variable type declaration
2 | Assert that we created an array of strings
3 | Create an array of ints with the `as` operator
4 | Assert that we created an array of primitive ints

You can also create multi-dimensional arrays:

    def matrix3 = new Integer[3][3]         (1)
    assert matrix3.size() == 3

    Integer[][] matrix2                     (2)
    matrix2 = [[1, 2], [3, 4]]
    assert matrix2 instanceof Integer[][]

1 | You can define the bounds of a new array
2 | Or declare an array without specifying its bounds

Access to elements of an array follows the same notation as for lists:

    String[] names = ['Cédric', 'Guillaume', 'Jochen', 'Paul']
    assert names[0] == 'Cédric'     (1)

    names[2] = 'Blackdrag'          (2)
    assert names[2] == 'Blackdrag'

1 | Retrieve the first element of the array
2 | Set the value of the third element of the array to a new value

| Java’s array initializer notation is not supported by Groovy, as the curly braces can be misinterpreted with the notation of Groovy closures.

## 9. Maps

Sometimes called dictionaries or associative arrays in other languages, Groovy features maps. Maps associate keys to values, separating keys and values with colons, and each key/value pairs with commas, and the whole keys and values surrounded by square brackets.

    def colors = [red: '#FF0000', green: '#00FF00', blue: '#0000FF']   (1)

    assert colors['red'] == '#FF0000'    (2)
    assert colors.green  == '#00FF00'    (3)

    colors['pink'] = '#FF00FF'           (4)
    colors.yellow  = '#FFFF00'           (5)

    assert colors.pink == '#FF00FF'
    assert colors['yellow'] == '#FFFF00'

    assert colors instanceof java.util.LinkedHashMap

1 | We define a map of string color names, associated with their hexadecimal-coded html colors
2 | We use the subscript notation to check the content associated with the `red` key
3 | We can also use the property notation to assert the color green’s hexadecimal representation
4 | Similarly, we can use the subscript notation to add a new key/value pair
5 | Or the property notation, to add the `yellow` color

| When using names for the keys, we actually define string keys in the map.

| Groovy creates maps that are actually instances of `java.util.LinkedHashMap`.

If you try to access a key which is not present in the map:

    assert colors.unknown == null

You will retrieve a `null` result.

In the examples above, we used string keys, but you can also use values of other types as keys:

    def numbers = [1: 'one', 2: 'two']

    assert numbers[1] == 'one'

Here, we used numbers as keys, as numbers can unambiguously be recognized as numbers, so Groovy will not create a string key like in our previous examples. But consider the case you want to pass a variable in lieu of the key, to have the value of that variable become the key:

    def key = 'name'
    def person = [key: 'Guillaume']      (1)

    assert !person.containsKey('name')   (2)
    assert person.containsKey('key')     (3)

1 | The `key` associated with the `'Guillaume'` name will actually be the `"key"` string, not the value associated with the `key` variable
2 | The map doesn’t contain the `'name'` key
3 | Instead, the map contains a `'key'` key

| You can also pass quoted strings as well as keys: ["name": "Guillaume"]. This is mandatory if your key string isn’t a valid identifier, for example if you wanted to create a string key containing a hash like in: ["street-name": "Main street"].

When you need to pass variable values as keys in your map definitions, you must surround the variable or expression with parentheses:

    person = [(key): 'Guillaume']        (1)

    assert person.containsKey('name')    (2)
    assert !person.containsKey('key')    (3)

1 | This time, we surround the `key` variable with parentheses, to instruct the parser we are passing a variable rather than defining a string key
2 | The map does contain the `name` key
3 | But the map doesn’t contain the `key` key as before