Everything you’d know for Groovy interviews

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Call functions in primitive variable

One difference to Java, you can invoke function on primitive variables, e.g. call method plus on int as below

class test {
    static void main(String[] args){
        println("3+5="+(3.plus(5)))
    }
}

Accurate calculation on float number

In Java, as you know the calculation of float number is inaccurate, if you need such accuracy, you’ll have to use BigDecimal or others. However, it’s not a problem in Groovy.

def num1=1.000000000001
def num2=2.000000000002
println(num1+"+"+num2+"="+num1.plus(num2))

And output is:

BearSuperBrain:groovy todzhang$ groovy test.groovy 
start
3+5=8
1.000000000001+2.000000000002=3.000000000003

generate random number within 100

Using combination of nextInt and division of 100

println("next random number:"+(Math.abs(new Random().nextInt())%100))

Variable placeholder

def name="todd"
println("hello $name")

String literals

Use *3 single quote * for string in multiple lines

def body='''
    hello
        nice to see you
        bye
    '''
println("the message body is $body")

String computation

You can multiply and subtract on Groovy String

def greeting="hello world to you"*2
println(greeting-"hello")

String comparator <=>

This will return 0 if two strings are equals and -1 if former string is before the 2nd string, otherwise return +1

Following will get -1 in output as the str1 comes before str2

def str1="book"
def str2="boot" 
println("str1<=>str2:"+(str1<=>str2))

Get chars from String by index

Similar to charAt in Java String, you can access each characters easily in Groovy

def name="todd"
println("hello $name")
println("first 3 chars are:"+name[0..2])
println(" chars are:"+name[0,2,3])

List & Array

Array of multiple data types

In Groovy, you can assign different type of values into an array, it like a data class in Java.

def book=[12345,5.99,"How to coding in Groovy",[10,20,30]]
println("2nd number is:"+book[3][1])

To add & substract in Array

You can use various operators such as add, +, << Take below sample

book.add(55)
book<<66
book=book+[77,88]
println(book)
book=book-[55]
println(book)

Map

To add, update entries in Map

Map is another type of List which is key value pair

def states=[
    'capital':'Sydney',
    'population': 5_000_000
]
 
println('capital is:'+states['capital'])
states.put('flower','Rose')
states.put('capital','Unknown')

println(states.get('capital')+", flower is:"+states.get('flower'))

Range

It’s similar to List or Java’s stream

def rng1=1..10
def rng2='a'..'z'
println(rng1.getFrom())
println(rng2.getTo())

Additionally, Range can be used in switch-case as a case branch.

Implicit it in closure

Different to Java Stream, it’s no need to explicitly declare variable in the loop. You can use it as loop variable implicitly.

static void main(String... args){

    def sum=sumAll(1,2,3)
    println("total sum is:"+sum)
}

static def sumAll(int... nums){
    def sum=0
    nums.each{sum+=it}
    return sum
}

Closure

In groovy, closure can be declared in curly brace and implicit variable call can be used to represent this method

def closureFactorial = {num-> (num<=1?1:num*call(num-1))}
        
println("Get Factorial 4: "+closureFactorial(4))

It will output 24.

Single vs Double quote

If you need to access variable in a String, the “double quote” are necessary

def teams=['tom','jerry','david']
teams.each{println(it)}

def country = [
    'country':'Aus',
    'capital':'Sydney'
]
country.each{println("$it.key => $it.value")}

Filter to get even number

One line of closure can be used to quickly filter and print even numbers

def rand=1..6
def evenNumber=rand.each {num-> if(num%2==0)
    println(num)
}

To double each item in the list

println("double:"+rand.collect{it*2})

To pass in a closure as parameter

One closure can be pass into another method as parameter

def getEven = {num -> return (num%2==0)}
    def evenNumbers=listNumber(rand, getEven);
    println("All even numbesr: $evenNumbers")
}

def static listNumber(nums, closure){
    return nums.findAll(closure)

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