Groovy Tutorial 3: Variables, Objects and Classes: Part 1

As we saw in the last tutorial, variables are memory locations where we store values. We also said that in Groovy, everything is an object. So what exactly are objects?

Object-Oriented Programming (OOP)

There are many programming paradigms, each with its own strengths and weaknesses. OOP is well-suited to large and complex software systems due to its various properties such as information hiding and encapsulation.

In OOP, the building blocks of a software system are classes and objects. Classes are like a blueprint of an entity, whether concrete e.g. person, document, car or building, or abstract, such as the state of a game, an idea or a communication/ http session. Objects are instances of a class, or the realization of the blueprint (class).

Classes

Classes are blueprints or templates of a program’s building block. They group together data and operations relevant to that data. Classes are composed of fields and methods. Fields store the state of a class’ instantiation i.e. an object, while methods represent the behavior of a class.

For example, the class car will have fields to represent color, mileage and registration number while its methods will be getColor(), setMileage(), addMileage() and getRegNum(). As you can guess, each determines the way we can interact with fields.

This provides a way of achieving two of the core principle of object-oriented programming, encapsulation and information-hiding. Encapsulation means grouping together relevant information, ‘in a capsule’. Information-hiding is achieved through being able to access and modify fields via methods using access modifiers. Access modifiers will be discussed in greater detail in the next tutorial. We shall now get to code samples to show examples of using classes and object since there is much more to OOP than I can cover in this tutorial. You should definitely check it out in more detail.

By convention, variable, class and object names use camel-case, with class names starting with a capital letter while variable, object, field and method names start in lowercase.

When saving a groovy script/ file, make sure it is does not clash with any class names in the script.

Persons.groovy

class Person

{

    def name //defines a property

    def income

    def gender

}

def p1=new Person()

p1.name='Cabee'

p1.income=7000000

p1.gender='Male'

println p1.name

Output

Cabee

To see what happens when you give a file a name used in as a class name, change both instances of Person to Persons. The following compilation error will be thrown:

Invalid duplicate class definition…

For java developers, the thing to remember is that Groovy will create the getters and setters in the background, thus p1.name does not reference the field name but rather, calls the auto-created setName() method in the background.

To show some of the syntactic sugar Groovy adds to classes, here is a slightly longer code sample we shall look into more details in the next tutorial:

class Persona

{

    def name //defines a property

    def income

    def gender

}

def p1=new Persona()

p1.name='Cabee'

p1.income=7000000

p1.gender='Male'

println "name: $p1.name"

println "income: $p1.income"

println "gender: $p1.gender"

println()

p1=new Persona([name:'David',income:500000,gender:'Male'])

println "name: $p1.name"

println "income: $p1.income"

println "gender: $p1.gender"

Output

name: Cabee

income: 7000000

gender: Male

name: David

income: 500000

gender: Male

In the next tutorial, we shall look at constructors, access modifiers to control data access and get into classes and objects in more details.

Happy coding with Cabee Groovy Mastery Course!!!