Groovy Script Tutorial for Beginners

What is a Groovy Script?

Apache Groovy is an Object-oriented programming language used for Java platform. This dynamic language has many features which are similar to Python, Ruby, Smalltalk, and Pero. It can be used as a scripting language for the Java platform.

It is almost like a super version of Java which offers Java's enterprise capabilities. It also offers many productivity features like DSL support, closures, and dynamic typing. Unlike some other languages, it is designed as a companion, not a replacement for Java.

Groovy source code gets compiled in Java Bytecode so it can run on any platform if JRE is installed on that operating system. Groovy also performs a lot of tasks behind the scene that makes it more agile and dynamic.

In this course, you will learn-

• What is a Groovy Script?
• Why Groovy?
• Groovy History
• Features of Groovy
• Install Groovy
• Groovy Hello World Example
• Groovy Variables
• Groovy-Operators
• Groovy-Loops
• Groovy- Decision Making
• Groovy List
• Groovy Maps
• Groovy- Closures
• Groovy Vs. Java
• Myths about Groovy
• Cons of using Groovy
• Groovy Tools

Why Groovy?

Here, are major reasons why you should use Groovy-

• Groovy is an agile and dynamic language
• Seamlessly integration with all existing Java objects and libraries
• Feels easy and natural to Java developers
• More concise and meaningful code compares to Java
• You can use it as much or as little as you like with Java apps

Groovy History

• 2003: Developed by Bob McWhirter & James Strachan
• 2004: Commissioned into JSR 241 but it was abandoned
• 2005: Brought back by Jeremy Rayner & Guillaume Laforge
• 2007: Groovy version 1.0
• 2012: Groovy version 2
• 2014: Groovy version 2.3 (official support given for JDK 8)
• 2015: Groovy became a project at the Apache Software Foundation

Features of Groovy

• List, map, range, regular expression literals
• Multimethod and metaprogramming
• Groovy classes and scripts are usually stored in .groovy files
• Scripts contain Groovy statements without any class declaration.
• Scripts can also contain method definitions outside of class definitions.
• It can be compiled and fully integrated with traditional Java application.
• Language level support for maps, lists, regular expressions
• Supports closures, dynamic typing, metaobject protocol
• Support for static and dynamic typing & operator overloading
• Literal declaration for lists (arrays), maps, ranges, and regular expressions

Install Groovy

Step 1) Ensure you have Java installed. https://www.guru99.com/install-java.html

Step 2) Go to http://groovy-lang.org/download.html and click installer.

Note: You can also install Groovy using the Zip file or as an Eclipse IDE. In this tutorial, we will stick to Windows Installer

Step 3) Launch the downloaded installer. Select language and click OK 

 

Step 4) Launch. In welcome screen, click NEXT

 

Step 5) Agree with the license terms

 

Step 6) Select components you want to install and click next

 

Step 7) Select Installation Directory and click Next

 

Step 8) Choose Start Menu Folder and Click Next

Step 9) Once install is done, let the paths default and click next

Step 10) Click Next.

Step 11) In start Menu search for Groovy Console

Groovy Hello World Example

Consider we want to print a simple string "Hello World" in Java. The code to achieve this would be

public class Demo {
    public static void main(String args[]) {
        System.out.println("Hello World");
    }
}

The above code is valid in both Java and Groovy as Groovy is a superset of Java. But the advantage with Groovy is that we can do we away with class creation, public method creation, etc and achieve the same output with a single line code as follows:

println "Hello World." 

There is no need for semicolons

There is no need for parenthesis

System.out.println is reduced to println

Groovy Variables

In Java, static binding is compulsory. Meaning the type of a variable has to be declared in advance.

public class Demo {
    public static void main(String args[]) {
        int x = 104;
        System.out.println(x);
        //x = "Guru99";
    }
}

In the above example type of variable (integer) is declared in advance using the keyword "int". If you were to declare a floating point number you use the keyword float.

If you try to assign a String value to an int (uncomment line #5), you will get the following error

Demo.java:5: error: incompatible types: String cannot be converted to int
x = "Guru99";

In contrast, Groovy supports Dynamic Typing. Variables are defined using the keyword "def," and the type of a variable does not need to be declared in advance. The compiler figures out the variable type at runtime and you can even the variable type.

Consider the following groovy example,

def x = 104
println x.getClass()
x = "Guru99"
println x.getClass()

Output

class java.lang.Integer
class java.lang.String

In Groovy, you can create multiline strings. Just ensure that you enclosed the String in triple quotes.

def x = """Groovy
at
Guru99"""
println x

Output

Groovy
at
Guru99

Note: You can still variable types like byte, short, int, long, etc with Groovy. But you cannot dynamically change the variable type as you have explicitly declared it.

Consider the following code:

int x = 104
println x
x = "Guru99"

It gives the following error

104
Caught: org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object 'Guru99' with class 'java.lang.String' to class 'int'
org.codehaus.groovy.runtime.typehandling.GroovyCastException: Cannot cast object 'Guru99' with class 'java.lang.String' to class 'int'
    at jdoodle.run(jdoodle.groovy:3)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
    at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
    at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
Command exited with non-zero status 1

Groovy-Operators

An operator is a symbol which tells the compiler to do certain mathematical or logical manipulations.

Groovy has the following five types of operators –

• Arithmetic operators: Add (+), Subtract (-), Multiplication (*), Division(/)
• Relational operators: equal to (==), Not equal to (!=), Less than (<) Less than or equal to (<=), Greater than (>), Greater than or equal to (>=)
• Logical operators: And (&&), Or(||), Not(!)
• Bitwise operators: And(&), Or(|), (^), Xor or Exclusive-or operator
• Assignment operators: Negation operator (~)

Groovy-Loops

In Java, you would define a loop as follows

public class Demo {
    public static void main(String args[]) {
        for (int x = 0; x <= 5; x++) {
            System.out.println(x);
        }
    }
}

Output

0
1
2
3
4
5

You can achieve the same output in Groovy using upto keywords

0.upto(4) {println "$it"}

You get the same output as above. $it is a closure that gives the value of the current loop.

Consider the following code

2.upto(4) {println "$it"}

It gives an output

2
3
4

You can also use the "times" keyword to get the same output

5.times{println "$it"}

Consider, you want to print 0,2,4,6 with for loop in Java

public class Demo {
    public static void main(String args[]) {
        for (int x = 0; x <= 5; x=x+2) {
            System.out.println(x);
        }
    }
}

Output:

0
2
4
6

You can use the step method for the same

0.step(7,2){println "$it"}

Groovy- Decision Making

Statements	Deception
if Statement	As in Java, the if statement is executed if the condition is true.
if/else Statement	In if/else statement at first a condition is evaluated in the if statement. If the condition is true then executes the statements after that. It stops before the else condition and exits out of the loop. However, If the condition is false then executes the statements in the else statement block. Then it exits the loop.
Nested If Statement	It is used when there is a requirement to have multiple if statements.
Switch Statement	The nested if-else statement could become unreadable when you have multiple conditions. To make code more readable switch statement is used.
Nested Switch Statement	In Groovy is also possible to use nested switch statements.

Groovy List

List structure allows you to store a collection of data Items. In a Groovy programming language, the List holds a sequence of object references. It also shows a position in the sequence. A List literal is presented as a series of objects separated by commas and enclosed in square brackets.

Example of Grrovy list:

A list of Strings- ['Angular', 'Nodejs,]

A list of object references - ['Groovy', 2,4 2.6]

A list of integer values - [16, 17, 18, 19]

An empty list- [ ]

Following are list methods available in Groovy:

Methods	Description
add()	Allows you to append the new value to the end of this List.
contains()	Returns true if this List contains a certain value.
get()	Returns the element at the definite position
isEmpty()	Returns true value if List contains no elements
minus()	This command allows you to create a new List composed of the elements of the original excluding those which are specified in the collection.
plus()	Allows you to create a new List composed of the elements of the original together along with mentioned in the collection.
pop()	Removes the last item from the List
remove()	Removes the element at the specific position
reverse()	Create a new List which reverses the elements of the original List
size()	Allow finding some elements
sort()	Returns a sorted copy

Consider the following example

def y = ["Guru99", "is", "Best", "for", "Groovy"]
println y
y.add("Learning")
println(y.contains("is"))
println(y.get(2))
println(y.pop())

Output

[Guru99, is, Best, for, Groovy]
true
Best
Learning

Groovy Maps

A Map Groovy is a collection of Key Value Pairs

Examples of Groovy maps:

• [Tutorial: 'Java, Tutorial: 'Groovy] – Collection of key-value pairs which has Tutorial as the key and their respective values
• [ : ] Represent an Empty map

Here, is a list of map methods available in Groovy.

Methods	Description
containsKey()	Check that map contains this key or not?
get()	This command looks up the key in this Map and returns the corresponding value. If you don't find any entry in this Map, then it will return null.
keySet()	Allows to find a set of the keys in this Map
put()	Associates the specified value with the given key in this Map. If the Map earlier contained a mapping for this key. Then the old value will be replaced by the specified value.
size()	Returns the number of key-value mappings.
values()	This command returns a collection view of the values.

Groovy Example:

def y = [fName:'Jen', lName:'Cruise', sex:'F']
print y.get("fName")

Output

Jen

Groovy- Closures

A groovy closure is a piece of code wrapped as an object. It acts as a method or a function.

Example of simple closure

def myClosure = {
       println "My First Closure"	
}
myClosure()

Output:

My First Closure

A closure can accept parameters. The list of identifies is comma separated with

an arrow (->) marking the end of the parameter list.

def myClosure = {
       a,b,c->
       y = a+b+c
       println y
}
myClosure(1,2,3)

Output

6

A closure can return a value.

def myClosure = {
       a,b,c->
       return (a+b+c)
}
println(myClosure(1,2,3))

Output

6

There are many built-in closures like "It", "identity", etc. Closures can take other closure as parameters.

Groovy Vs. Java

Groovy	Java
In Groovy, default access specifier is public. It means a method without any specified access modifier is public and accessible outside of class and package boundaries.	In Java, the default access modifier is a package, i.e., if you don't specify access modifier for fields, methods or class it becomes package-private,
Getters and setters are automatically generated for class members.	Java, you need to define getters and setters method for fields
Groovy allows variable substitution using double quotation marks with strings.	Java does not support variable substitution.
Typing information is optional.	Typing information is mandatory in Java.
Groovy it's not required to end with a semicolon.	In Java, every statement ends with a semicolon.
Groovy is automatically a wrapping class called Script for every program	In Java, you need the main method to make a class executable.

Myths about Groovy

Myth	Reality
We can use Groovy just for scripting.	It can be used for scripting. However, you can perform many other tasks apart from it.
Groovy is all about closures. "It's just functional programming language."	Groovy adopts from functional programming languages like Lisp or Closure.
Groovy is an ideal choice if you want do TDD	This statement is true. However, it is certainly not the only reason to use Groovy.
You can use Groovy only if you want to use Grails.	Grails is a powerful web development framework. But Groovy offers more than that.

Cons of using Groovy

• JVM and Groovy script start time is slow which limits OS-level scripting
• Groovy is not entirely accepted in other communities.
• It is not convenient to use Groovy without using IDE
• Groovy can be slower which increased the development time
• Groovy may need lots of memory
• Knowledge of Java is imperative.

Groovy Tools

We will discuss 3 important tools

1. groovysh: Executes code interactively.

2. groovyConsole: GUI for interactive code execution

3. groovy: Executes groovy scripts. You can use it like Perl, Python, etc.

Groovysh

• command-line shell
• Helps you to execute Groovy code interactively
• Allows entering statements or whole scripts

Groovy console

• Swing interface which acts as a minimal Groovy development editor.
• Allows you to interacts Groovy code
• Helps you to load and run Groovy script files

Groovy

It is the processor which executes Groovy programs and scripts. U

It can be used to test simple Groovy expressions.

Summary

• Groovy is an Object-oriented programming language used for Java platform
• It offers seamless integration with all existing Java objects and libraries
• Bob McWhirter & James Strachan developed groovy in 2003
• List, map, range, regular expression literals are important features of Groovy
• Four type of operators support by Groovy are 1. Relational 2.Logical 3. Bitwise 4. Assignment
• Groovy performed decision making using if, if/else, Nested if, switch, Netsted switch statements
• List structure allows you to store a collection of Data Items
• A Map Groovy is a collection of Key Value Pairs
• In Groovy, Getters and setters are automatically generated for class members
• In Java, you can use provide getters and setters method for fields
• The biggest myth about Groovy is that it can only use for scripting which is not correct
• Some time Groovy can be slower which increased the development time
• Three Groovy Tools are: groovysh which executes code, groovy Console which is GUI for interactive code execution and groovy which executes scripts