Getting Started

Setting up your Java environment

Groovy requires Java, so you need to have a version available (while groovy 1.6 supported JDK 1.4 or greater, for groovy 1.7 onwards, minimum JDK 1.5 is needed). Here are the steps if you don't already have Java installed:

Setting up your Groovy environment

Download the Groovy installer or binaries from the downloads page and follow the installation instructions.  (There is currently an issue where you cannot have spaces in the path where Groovy is installed under windows.  So, instead of accepting the default installation path of "c:\Program Files\Groovy" you will want to change the path to something like "c:\Groovy")

OR

Setting up optional jar files

You may wish to obtain optional jar files, either corresponding to Groovy modules (see module documentation for details) or corresponding to other Java classes you wish to make use of from Groovy. Some possibilities are listed below:

Name

From

Description

jtds-version.jar

http://jtds.sourceforge.net

Database driver for SQL Server and/or Sybase

hsqldb-version.jar

http://www.hsqldb.org/

Database driver for HSQLDB, a 100% Java database

The recommended way for making Groovy be aware of your additional jar files is to place them in a predefined location. Your Groovy install should include a file called groovy-starter.conf. Within that file, make sure a line such as

load ${user.home}/.groovy/lib/*


is not commented out. The user.home system property is set by your operating system. (Mine is C:\Document and Settings\paul. Now simply place your jar files into the .groovy/lib directory.
(Note: as an alternative, you can set up a CLASSPATH variable and make sure it mentions all of your additional jar files, otherwise Groovy works fine with an empty or no CLASSPATH variable.)

Hello, World

In the top part of the window of the groovyConsole, type the following

println "Hello, World!"


And then type <CTRL-R>.
Notice that the text gets printed out in the OS console window (the black one behind the groovyConsole window) and the bottom part of the groovyConsole says:

groovy> println "Hello, World!"
null


The line starting with "groovy>" is just the text of what the console processed. The "null" is what the expression "evaluated to". Turns out the expression to print out a message doesn't have any "value" so the groovyConsole printed "null".
Next try something with an actual value. Replace the text in the console with:

123+45*67


or your favorite arithmetic expression, and then type <CTRL-R> (I'm going to stop telling you to hit <CTRL-R>, I think you get the idea). Now the "value" printed at the bottom of the groovyConsole has more meaning.

Variables

You can assign values to variables for later use. Try the following:

x = 1
println x

x = new java.util.Date()
println x

x = -3.1499392
println x

x = false
println x

x = "Hi"
println x

 

Lists and Maps

The Groovy language has built-in support for two important data types, lists and maps (Lists can be operated as arrays in Java language). Lists are used to store ordered collections of data. For example an integer list of your favorite integers might look like this:

myList = [1776, -1, 33, 99, 0, 928734928763]


You can access a given item in the list with square bracket notation (indexes start at 0):

println myList[0]


Should result in this output:

1776


You can get the length of the list with the "size" method:

println myList.size()


Should print out:

6


But generally you shouldn't need the length, because unlike Java, the preferred method to loop over all the elements in an list is to use the "each" method, which is described below in the "Code as Data" section.
Another native data structure is called a map. A map is used to store "associative arrays" or "dictionaries". That is unordered collections of heterogeneous, named data. For example, let's say we wanted to store names with IQ scores we might have:

scores = [ "Brett":100, "Pete":"Did not finish", "Andrew":86.87934 ]


Note that each of the values stored in the map is of a different type. Brett's is an integer, Pete's is a string, and Andrew's is a floating point number. We can access the values in a map in two main ways:

println scores["Pete"]
println scores.Pete


Should produce the output:

Did not finish
Did not finish


To add data to a map, the syntax is similar to adding values to an list. For example, if Pete re-took the IQ test and got a 3, we might:

scores["Pete"] = 3


Then later when we get the value back out, it will be 3.

println scores["Pete"]


should print out 3.
Also as an aside, you can create an empty map or an empty list with the following:

emptyMap = [:]
emptyList = []


To make sure the lists are empty, you can run the following lines:

println emptyMap.size()
println emptyList.size()


Should print a size of 0 for the List and the Map.

Conditional Execution

One of the most important features of any programming language is the ability to execute different code under different conditions. The simplest way to do this is to use the '''if''' construct. For example:

amPM = Calendar.getInstance().get(Calendar.AM_PM)
if (amPM == Calendar.AM)
{
	println("Good morning")
} else {
	println("Good evening")
}


Don't worry too much about the first line, it's just some code to determine whether it is currently before noon or after. The rest of the code executes as follows: first it evaluates the expression in the parentheses, then depending on whether the result is '''true''' or '''false''' it executes the first or the second code block. See the section below on boolean expressions.
Note that the "else" block is not required, but the "then" block is:

amPM = Calendar.getInstance().get(Calendar.AM_PM)
if (amPM == Calendar.AM)
{
	println("Have another cup of coffee.")
}

 

Boolean Expressions

There is a special data type in most programming languages that is used to represent truth values, '''true''' and '''false'''. The simplest boolean expression are simply those words. Boolean values can be stored in variables, just like any other data type:

myBooleanVariable = true


A more complex boolean expression uses one of the boolean operators:

* ==
* !=
* >
* >=
* <
* <=


Most of those are probably pretty intuitive. The equality operator is '''==''' to distinguish from the assignment operator '''='''. The opposite of equality is the '''!=''' operator, that is "not equal"
So some examples:

titanicBoxOffice = 1234600000
titanicDirector = "James Cameron"

trueLiesBoxOffice = 219000000
trueLiesDirector = "James Cameron"

returnOfTheKingBoxOffice = 752200000
returnOfTheKingDirector = "Peter Jackson"

theTwoTowersBoxOffice = 581200000
theTwoTowersDirector = "PeterJackson"

titanicBoxOffice > returnOfTheKingBoxOffice  // evaluates to true
titanicBoxOffice >= returnOfTheKingBoxOffice // evaluates to true
titanicBoxOffice >= titanicBoxOffice         // evaluates to true
titanicBoxOffice > titanicBoxOffice          // evaluates to false
titanicBoxOffice + trueLiesBoxOffice < returnOfTheKingBoxOffice + theTwoTowersBoxOffice  // evaluates to false

titanicDirector > returnOfTheKingDirector    // evaluates to false, because "J" is before "P"
titanicDirector < returnOfTheKingDirector    // evaluates to true
titanicDirector >= "James Cameron"           // evaluates to true
titanicDirector == "James Cameron"           // evaluates to true


Boolean expressions are especially useful when used in conjunction with the '''if''' construct. For example:

if (titanicBoxOffice + trueLiesBoxOffice > returnOfTheKingBoxOffice + theTwoTowersBoxOffice)
{
	println(titanicDirector + " is a better director than " + returnOfTheKingDirector)
}


An especially useful test is to test whether a variable or expression is null (has no value). For example let's say we want to see whether a given key is in a map:

suvMap = ["Acura MDX":"\$36,700", "Ford Explorer":"\$26,845"]
if (suvMap["Hummer H3"] != null)
{
 	println("A Hummer H3 will set you back "+suvMap["Hummer H3"]);
}


Generally null is used to indicate the lack of a value in some location.

Debugging and Troubleshooting Tips

If you are a Java developer