Maps

A map stores key → value pairs. [key: value] creates a java.util.LinkedHashMap, which remembers insertion order.

def user = [name: 'Ada', age: 36, city: 'London']

println user.name              // property-style access
println user['age']            // subscript access
println user.get('city')
println user.email             // missing key: null
println user.getOrDefault('email', 'n/a')
println user.size()
println user.keySet()
println user.values()

Ada
36
London
null
n/a
3
[name, age, city]
[Ada, 36, London]

Keys written like name: are strings. user.name and user['name'] both look up the key 'name'.

Changing a map

def stock = [apples: 3]
stock.pears = 5
stock['plums'] = 2
stock.apples += 10
stock.remove('pears')
println stock
println stock.containsKey('plums')

def key = 'kiwis'
def more = [(key): 4, 'with space': 1]   // (expr) uses a variable as key
println stock + more

[apples:13, plums:2]
true
[apples:13, plums:2, kiwis:4, with space:1]

To use a variable's value as a key in a literal, wrap it in parentheses: [(key): 4]. Without them, [key: 4] means the string 'key'.

Looping over a map

def prices = [coffee: 3.5, tea: 2.8, cake: 4.2]

prices.each { item, price ->
    println "$item costs $price"
}

for (entry in prices) {
    println "${entry.key.toUpperCase()}=${entry.value}"
}

coffee costs 3.5
tea costs 2.8
cake costs 4.2
COFFEE=3.5
TEA=2.8
CAKE=4.2

Map gotchas

def m = [class: 'A', size: 3]
println m.class                // looks up the key "class"!
println m.getClass().simpleName
println m.size                 // key "size"
println m.size()               // the method
println([:].isEmpty())

A
LinkedHashMap
3
2
true

Property syntax on a map always means "look up this key", so m.class returns null when there's no class key. Use getClass() when you want the class. [:] is an empty map.

Practice

Count how many times each word appears in 'a b a c b a' using a map, and print the result.

def counts = [:]
'a b a c b a'.tokenize().each { word ->
    counts[word] = (counts[word] ?: 0) + 1
}
println counts

[a:3, b:2, c:1]