This section of the document covers the syntax, keywords, operators, etc in the Groovy language.

This

...

/* This is a multi-line comment in Groovy
that starts in the first line and 
ends here */

Keywords

While writing your scripts make sure you do not use the existing keywords of the Groovy language like break, package etc to name your variables or constants. See here for the list of keywords.

String Concatenation

The Groovy strings can be concatenated with a + operator.

"a" + "b"
//returns ab

You can use the backslash character \ to escape the quotes

"\"a\"" + "b"
//returns "a"b

Semicolons

Semicolons are used to separate statements, but they are optional in Groovy as long as you write one statement per line. If you use multiple statements on the same line you can either use a semicolon or a newline to separate the statements.

def A = "a" + "b"; def B = "c" + "d" // You need a semi-colon to separate the two statements on the same line

//You can instead add a new line to separate the statements in Groovy
def A = "a" + "b"
def B = "c" + "d

Program structure

This section of the document gives an intorduction on the program structure in Groovy. You can refer to the official documentation of the Groovy language or any Java tutorial for complete information.

Packages

A package is a group of similar types of classes, interfaces and sub-packages. It is used to categorize the classes and interfaces so that they can be easily maintained. See here to know more about pacakges.

Definfing a package:

// defining a package named com.yoursite
package com.yoursite
package javax.xml.parsers

To refer to some class Foo in the com.yoursite.com package you will need to use the fully qualified name com.yoursite.com.Foo, or else you can use an import statement as shown below.

Imports

In order to refer to any class you need a qualified reference to its package. Groovy follows Java’s notion of allowing import statement to resolve class references. The import allows to access classes of a package without package qualification. See here to know more about imports.

For example MarkupBuilder is inside the package groovy.xml so in order to use this class, you need to import it as shown:

// importing the class MarkupBuilder
import groovy.xml.MarkupBuilder

Default imports:

Groovy automatically imports the packages groovy.lang.*, groovy.util.* , java.lang.* , java.util.* , java.net.* , and java.io.* as well as the classes java.math.BigInteger and BigDecimal, since these are most commonly used.

Objects

In Groovy everything is an object that has methods and properties. 

// using the imported class to create an object
def xml = new MarkupBuilder()

Methods

A method is a set of code which is referred to by name and can be invoked with parentheses () that may contain arguments. It can be invoked at any point in a program simply by utilizing its name. A method is where you put the operations on data (variables) in your Groovy code. 

Accessing a method

You can access a method using the "." dot syntax as shown in the below example, where employee is the object that has accessors or methods to get Employee data

//Accessing a method of the class 
xml.getMkp()

Direct access operator

You can use the direct access operator to call the property directly removing the paranthesis () and instead of using the getter.

xml.mkp

Script class

A script is always compiled into a class. The Groovy compiler will compile the class for you, with the body of the script copied into a run method. For example:

println 'Groovy world!'

this will be compiled as if it was the following:

import org.codehaus.groovy.runtime.InvokerHelper
class Main extends Script {                     
    def run() {                                 
        println 'Groovy world!'                 
    }
    static void main(String[] args) {           
        InvokerHelper.runScript(Main, args)     
    }
}

Variables

Variables are typically used to store information which your Groovy script needs to do its job on. See here for more information on Variables and their types. 

Variable declaration

You can declare a variable either by specifiying its type or without specifying using def. The latter can be done using def. This is because when using def Groovy determines the methods to invoke at runtime, hence improving the flexibility. See official documentation for more information.

int a,b
def a = 5
def b = 3

c = a + b

Annotations

Annotation is a tag that represents the metadata i.e. attached with class, interface, methods or fields to indicate some additional information which can be used by java compiler and JVM. In Groovy, an annotation can be used to alter the semantics of the language. It is especially true of AST transformations which will generate code based on annotations.

Simple Data types

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. 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

...

The ternary operator is a shortcut expression that is equivalent to an if/else branch assigning some value to a variable.

Instead of:

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

you can simplify it to:

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

Elvis operator is shortening of the ternary operator. You need not have to repeat the value you want to assign

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 

...

tutorial details the following chapters:

References