thejavajar

Happy New Year! So it is 2010 and you still haven’t looked at Groovy or had fun developing with Closures? It is time to give it a try. Download Groovy (currently 1.7), setup a GROOVY_HOME variable, and add GROOVY_HOME/bin to your system path. Now open up a text editor, I like Textmate on my Mac and Notepad ++ when I am on a Windows machine, and create a new file, something like ClosureFun.groovy. Now let’s get some coding done. First, let’s add a class:

class Person {
    def firstName
    def lastName
    def emailAddress
}

This is really not too difficult to understand, right? We have a class, Person, with three attributes; firstName, lastName and emailAddress. Now let’s create some instances of Person:

bob = new Person(firstName:"Bob", lastName:"Jones", emailAddress:"bob.jones@yahoo.com")
rob = new Person(firstName:"Rob", lastName:"Fake", emailAddress:"rob.fake@gmail.com")
ray = new Person(firstName:"Ray", lastName:"Real", emailAddress:"ray.real@gmail.com")
tom = new Person(firstName:"Tom", lastName:"Real", emailAddress:"tom.real@yahoo.com")
mike = new Person(firstName:"Mike", lastName:"Test", emailAddress:"mike.test@msn.com")
drew = new Person(firstName:"Drew", lastName:"Wilkins", emailAddress:"dwilkins@aol.com")

We now have six instances of Person; bob, rob, ray, tom, mike and drew. Groovy supports these Map-like constructors and they are given to us just like Java gives us a no arg constructor by default if we do not create a constructor for our Java class. We also get new Person(firstName:”value”), new Person(lastName:”value”), new Person(emailAddress:”value”), new Person(firstName:”value”, emailAddress:”value”)… and the other permutations. This feature is quite nice. Now let’s create a list of Persons:

def list = [bob, rob, ray, tom, mike, drew]

Maybe the Groovy syntax is a little different than Java, but it is pretty easy to see how, in the code above, to create a list of Persons. Now, we have this instance of list, as in java.util.ArrayList, that now contains our instances of Person. We have created a class Person, we have our six instances of Person and they all exist in a list. What now?

Well, I want to be able to print out the values of firstName and lastName of particular instances of Person that are in the list. I want to print out the Person instances in our list where the firstName value begins with “R”. Then I want to be able to print out the Person instances in our list where the emailAddress contains “yahoo”. Someone else wants to print out the Person instances where the last letter in lastName is “e” and their friend wants the Person instances where the last letter in lastName is “s”. Should I write a method for each scenario that will take a list as a parameter and loop through each value and check for our desired condition then print the firstName and lastName? Sure, why not?

def printPersonsWithFirstNameBeginingWithR(list) {
    list.each {
        if(it.firstName.startsWith("R")) {
            println "$it.firstName $it.lastName"
        }
    }
}

Our first method is done. If you are not familiar with Groovy, ‘list.each’ is like a ‘for’ loop in Java and ‘it’ is an implicit placeholder for each item in the list. In our case, ‘it’ represents a Person class. More information on syntax of Groovy can be found on the Groovy Web site.

def printPersonsWithEmailAddressContainingYahoo(list) {
    list.each {
        if(it.emailAddress.contains("yahoo")) {
            println "$it.firstName $it.lastName"
        }
    }
}

def printPersonsWithLastNameEndingWithE(list) {
    list.each {
        if(it.lastName.getAt(-1) == "e") {
            println "$it.firstName $it.lastName"
        }
    }
}

def printPersonsWithLastNameEndingWithS(list) {
    list.each {
        if(it.lastName.getAt(-1) == "s") {
            println "$it.firstName $it.lastName"
        }
    }
}

That would work, right? Yeah, we could then write some code to call each method passing in our list and we would get what we want. Cool, we’re done. Wait, we need to support another condition. We need to support a request to print out all Person instances in our list where the emailAddress contains “gmail”. Well, I guess we should write another method? Why not?

def printPersonsWithEmailAddressContainingGmail(list) {
    list.each {
        if(it.emailAddress.contains("gmail")) {
            println "$it.firstName $it.lastName"
        }
    }
}

In our case, we see a pattern emerging. The only thing that changes in each of the above methods is the name of the method and the conditional statement in the if block. Well, when we take advantage of Closures in Groovy we can ignore the 5 methods we wrote above and just write 1 method like the one below (yeah, 5 methods into 1):

def printPersons(list, clos) {
    list.each() {
        if(clos(it)) {
            println "$it.firstName $it.lastName"
        }
    }
}

The above method looks just like our other 5 methods but we have changed our input parameters. Now we are expecting a list and a Closure as arguments and we evaluate the Closure parameter in the if block as our conditional statement (Closures can do more than return true or false). So how do we call our re-factored, re-written method? We now call the method passing is our list as a parameter and because the Closure is the last argument in the printPersons method, we can create our conditional Closure within brackets as you can see here:

// firstName starts with 'R'
printPersons(list) { it.firstName.startsWith("R") } 

// emailAddress contains 'yahoo'
printPersons(list) { it.emailAddress.contains("yahoo") } 

// last letter of lastName is 'e'
printPersons(list) { it.lastName.getAt(-1) == "e" }

// last letter of lastName is 's'
printPersons(list) { it.lastName.getAt(-1) == "s" }

Without getting into all the syntax and semantics of Groovy, we use ‘it’ again because it is a special implicit variable in Groovy used quite often with Closures and other Groovy code structures. In our case, ‘it’ will represent each Person instance in the loop, ‘list.each’. Now when we need to add that last minute request, well, it is already supported! Just pass the new condition.

// emailAddress contains 'gmail'
printPersons(list) { it.emailAddress.contains("gmail") }

Very cool stuff. Now, we can combine everything in one file, like ClosureFun.groovy and run the script. There are plenty of articles and blog posts all over the Internet about Groovy and Closures. I have learned quite a bit by attending presentations by people like Venkat Subramaniam, Scott Davis, Guillaume LaForge, Graeme Rocher, Jeff Brown and a few others. At the end of the day, it is all about getting your hands on Groovy, so get out there and write some code!