Groovy: Sort elements of a list

Groovy provides number of ways to sort elements of a list

a.   Using sort method

b.   Using Collections.sort method

Using sort method

Groovy provides below overloaded forms of sort method to sort the elements of list.

public List sort()

public List sort(boolean mutate)

public List sort(Closure closure)

public List sort(boolean mutate, Closure closure)

public List sort(boolean mutate, Comparator comparator)

public List sort()

Sort the elements of a list. If the Collection is a List, it is sorted in place and returned. Otherwise, the elements are first placed into a new list which is then sorted and returned - leaving the original Collection unchanged.

HelloWorld.groovy

numbers = [2, 9, 7, 3, 19, 8]

println "Before sorting. numbers: $numbers"
sortedNumbers = numbers.sort()

println "numbers: $numbers"
println "sortedNumbers: $sortedNumbers"

Output

Before sorting. numbers: [2, 9, 7, 3, 19, 8]

numbers: [2, 3, 7, 8, 9, 19]

sortedNumbers: [2, 3, 7, 8, 9, 19]

As you see the output, the elements of the original list ‘numbers’ also sorted.

public List sort(boolean mutate)

Sorts the element of list. If the Iterable is a List and mutate is true, it is sorted in place and returned. Otherwise, the elements are first placed into a new list which is then sorted and returned - leaving the original Iterable unchanged.

HelloWorld.groovy

numbers1 = [2, 9, 7, 3, 19, 8]
numbers2 = [2, 9, 7, 3, 19, 8]

println "Before sorting. numbers: $numbers1"
println "Before sorting. numbers: $numbers2"

sortedNumbers = numbers1.sort(true)
println "numbers1: $numbers1"
println "sortedNumbers: $sortedNumbers"
println "numbers1.is(sortedNumbers): ${numbers1.is(sortedNumbers)}"

sortedNumber = numbers2.sort(false)
println "\nnumbers2: $numbers2"
println "sortedNumbers: $sortedNumbers"
println "numbers2.is(sortedNumbers): ${numbers2.is(sortedNumbers)}"

Output

Before sorting. numbers: [2, 9, 7, 3, 19, 8]

Before sorting. numbers: [2, 9, 7, 3, 19, 8]

numbers1: [2, 3, 7, 8, 9, 19]

sortedNumbers: [2, 3, 7, 8, 9, 19]

numbers1.is(sortedNumbers): true

numbers2: [2, 9, 7, 3, 19, 8]

sortedNumbers: [2, 3, 7, 8, 9, 19]

numbers2.is(sortedNumbers): false

public List sort(Closure closure)

Sorts this Iterable using the given Closure to determine the correct ordering. If the Iterable is a List, it is sorted in place and returned. Otherwise, the elements are first placed into a new list which is then sorted and returned - leaving the original Iterable unchanged

HelloWorld.groovy

countries = ["India", "Indonesia", "Brazil", "America", "Japan"]

println "countries : $countries"

println "\nSorting countries"

countries.sort {it ->  it.length()}
println "\nCountries by name length : $countries"

countries.sort()
println "\nCountries by name : $countries"

Output

countries : [India, Indonesia, Brazil, America, Japan]

Sorting countries

Countries by name length : [India, Japan, Brazil, America, Indonesia]

Countries by name : [America, Brazil, India, Indonesia, Japan]

public List sort(boolean mutate, Closure closure)

Sort the iterable using given closure. If the Iterable is a List and mutate is true, it is sorted in place and returned. Otherwise, the elements are first placed into a new list which is then sorted and returned - leaving the original Iterable unchanged.

HelloWorld.groovy

countries = ["India", "Indonesia", "Brazil", "America", "Japan"]

println "countries : $countries"

println "\nSorting countries by setting mutate to false"

sortedCountries = countries.sort (false, {it ->  it.length()})
println "sortedCountries : $sortedCountries"
println "countries : $countries"

println "\nSorting countries by setting mutate to true"
sortedCountries1 = countries.sort (true, {it ->  it.length()})
println "sortedCountries : $sortedCountries"
println "countries : $countries"

Output

countries : [India, Indonesia, Brazil, America, Japan]

Sorting countries by setting mutate to false

sortedCountries : [India, Japan, Brazil, America, Indonesia]

countries : [India, Indonesia, Brazil, America, Japan]

Sorting countries by setting mutate to true

sortedCountries : [India, Japan, Brazil, America, Indonesia]

countries : [India, Japan, Brazil, America, Indonesia]

public List sort(boolean mutate, Comparator comparator)

Sort the list using comparator. If the Iterable is a List and mutate is true, it is sorted in place and returned. Otherwise, the elements are first placed into a new list which is then sorted and returned - leaving the original Iterable unchanged.

HelloWorld.groovy

countries = ["India", "Indonesia", "Brazil", "America", "Japan"]

println "countries : $countries"

println "\nSorting countries by setting mutate to false"

sortedCountries = countries.sort (false, { a, b -> a.length() <=> b.length() } as Comparator )
println "sortedCountries : $sortedCountries"
println "countries : $countries"

println "\nSorting countries by setting mutate to true"
sortedCountries1 = countries.sort (true, { a, b -> a.length() <=> b.length() } as Comparator )
println "sortedCountries : $sortedCountries"
println "countries : $countries"

Output

countries : [India, Indonesia, Brazil, America, Japan]

Sorting countries by setting mutate to false

sortedCountries : [India, Japan, Brazil, America, Indonesia]

countries : [India, Indonesia, Brazil, America, Japan]

Sorting countries by setting mutate to true

sortedCountries : [India, Japan, Brazil, America, Indonesia]

countries : [India, Japan, Brazil, America, Indonesia]

Using Collections.sort method

java.util.Collections class provides below methods to sort the elements of a list.

public static <T extends Comparable<? super T>> void sort(List<T> list)

public static <T> void sort(List<T> list, Comparator<? super T> c)

First method sorts the elements in their natural ordering. Second method sort the elements using the provided comparator.

HelloWorld.groovy

countries = ["India", "Indonesia", "Brazil", "America", "Japan"]

println "countries : $countries"
println "Sorting the countries using their name length"

Collections.sort (countries, { a, b -> a.length() <=> b.length() } as Comparator )

println "countries : $countries"

Output

countries : [India, Indonesia, Brazil, America, Japan]

Sorting the countries using their name length

countries : [India, Japan, Brazil, America, Indonesia]

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