Data types in Groovy

Groovy offers a wide variety of built-in data types. Following is a list of data types which are defined in Groovy

Booleans

true and false are the only two primitive boolean values.

Define a boolean object:

boolean isActive = true
isActive = false

Groovy has special rules to coerce non-boolean objects to a boolean value, referred as Truthy and Falsy. Groovy decides an expression as Truthy when;

• If a boolean value is true
• Non-empty Collections and arrays are true
• True if the Matcher has at least one match
• Iterators and Enumerations with further elements are coerced to true
• Non-empty Maps are evaluated to true
• Non-empty Strings, GStrings and CharSequences are coerced to true
• Non-zero numbers are true
• Non-null object references are coerced to true

Any other value is considered Falsy. Eg: An empty string.

Collections

This section lists some of the collections in Groovy. See here for detailed information.

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.

Initialising a list:

You can initialise a list delimiting the values by commas and surrounded by square brackets. You can also create lists containing values of heterogeneous types:

def numbers = [1, 2, 3] 
def hetero = [1, "a", true]

Accessing a list:

• You can access the first element of the list on zero-based counting: hetero[0]
• You can access the last element of the list with a negative index: hetero[-1]

Set

A collection that contains no duplicate elements. Set has its implementation in various classes like HashSet, TreeSet, LinkedHashSet. All these permit null values too.

HashSet:

A HashSet class is used to create a collection that uses a hash table for storage. It inherits the AbstractSet class and implements Set interface. HashSet stores the elements by using a mechanism called hashing. HashSet contains unique elements only.

LinkedHashSet:

LinkedHashSet class is a Hash table and Linked list implementation of the set interface. It inherits HashSet class and implements Set interface. 

Initialising a Set/HashSet:

def HashSet names = [] //Initialising a HashSet

set.add("Oliver")
set.add("Casper")
set.add("elvis")
set.add("Elvis")
set.add("John")
set.add("Carter")
set.add("Oliver")
	
names == [Oliver, Elvis, John, Casper, elvis, Carter]

Maps

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.

Initialising a Map:

You need pass quoted strings if your key string isn’t a valid identifier Eg: "block-no". You must surround the variable or expression with parentheses, when you need to pass variable values as keys in your map definitions. Eg:

def colors = [red: "#FF0000", green:"#00FF00", blue: "#0000FF"] 
def doctor = [name: "Oliver","block-no":33,speciality:"Cardiology"]
place = [(address):"Oliver"]

Accessing the elements of a Map:

• Check the content associated with the red key: colors["red"]
• Add a pair to the Map: colors["pink"] = "#FF00FF" or colors.pink = "#FF00FF"
• Map contains a specific key: colors.blue == true

Arrays

Groovy reuses the list notation for arrays, but to make such literals arrays, you need to explicitly define the type of the array through coercion or type declaration. You can also create multi-dimensional arrays

Initialising an Array :

String[] arrStr = ["Apple", "Banana", "Kiwi"]  

def matrix3 = new Integer[3][3]  //Array with bounds
def matrix = new String[][]      //Array without bounds

Accessing the elements of an array:

• Accessing the second element of the array: arrStr[1]
• Adding a new value to the array: arrStr[3] = "Mango"
• Accessing an element of the multi-dimensional array: matrix3[1][1]

Operators

Groovy supports all the typical operators, such as arithmetic operators, assignment operators, logical operators, relational operators and conditional operator. See below examples using some of the operators.

1 + 2 == 3
3 / 2 == 1.5
10 % 3 == 1

def a = 5
b = a += 3
b == 8

def c = 4
def d = c++
d == 5

def c = 5
c *= 3
c == 15

a > b
b - 3 == a
b + 3 != a

x and y
x || y
!x

!true == false
!'' == true

def text = "Test"
if (text!=null && text.length()>0) {
    result = text
} else {
    result = 'Empty'
}

def text = "Test"
text ? text : 'Empty'

def text = "Test"
text ?: 'Empty'

There a few other notable operators that are specific only to the Groovy language; one of them is the Safe Navigation Operator.

Safe navigation operator

The Safe Navigation operator is used to avoid a NullPointerException. When you have a reference to an object you might need to verify that it is not null before accessing methods or properties of the object. Using this operator, you can avoid this and directly return a null.

Normally you would have to:

if(employee){
  employee.getSalary()
}

Instead, you can write it as:

employee?.getSalary()

Groovy Control structures

Groovy supports the usual if-else, "nested" if then else if, while loop, exception handling syntax. 

//Simple if
if(..){
  (...)
}
//Simple if else
if(..){
  (...)
}
else{
  (...)
}
//Nested if
if (...) {
    ...
} else if (...) {
    ...
} else {
    ...
}

Switch statement - The switch statement executes one statement from multiple conditions. It is like if-else-if ladder statement. The switch statement in Groovy can handle any kind of switch value and different kinds of matching can be performed.

def x = 1.23
def result = ""

switch ( x ) {
    case "foo":
        result = "found foo"
        // lets fall through

    case "bar":
        result += "bar"

    case [4, 5, 6, 'inList']:
        result = "list"
        break
	default:
        result = "default"
}

In Groovy it is backward compatible with Java code, one difference though is that the Groovy switch statement can handle any kind of switch value and different kinds of matching can be performed.

for loop - for loop is used to iterate a part of the program several times. If the number of iteration is fixed, it is recommended to use for loop. In Groovy the for loop is much simpler and works with any kind of array, collection, Map, etc.

// iterate over a range
def x = 0
for ( i in 0..9 ) {
    x += i
}
x == 45

// iterate over an array
def array = (0..4).toArray()
x = 0
for ( i in array ) {
    x += i
}
x == 10

Optional omitting of syntax 

Groovy allows you to leave out some elements of syntax like package prefix, parentheses, and semicolon which brings the code to the bare minimum.