How to get a value from a map with a default

Short answer

getOrDefault() returns a fallback value if the key is missing:

void main() {
    var prices = Map.of("tea", 3, "cake", 12);
 
    IO.println(prices.getOrDefault("tea", 0));
    IO.println(prices.getOrDefault("coffee", 0));
}

Output:

3
0

Add if missing

putIfAbsent() writes a value only if the key is not present yet:

void main() {
    var settings = new HashMap<>(Map.of("theme", "dark"));
    settings.putIfAbsent("theme", "light");
    settings.putIfAbsent("lang", "en");
 
    IO.println(new TreeMap<>(settings));
}

Output:

{lang=en, theme=dark}

Create on first access

computeIfAbsent() creates the value only when needed. Handy for grouping into lists:

void main() {
    var byLetter = new TreeMap<Character, List<String>>();
    for (String name : List.of("Anna", "Oleg", "Alex")) {
        byLetter.computeIfAbsent(name.charAt(0), key -> new ArrayList<>()).add(name);
    }
    IO.println(byLetter);
}

Output:

{A=[Anna, Alex], O=[Oleg]}

Common mistake

Using get() without a check. It returns null for a missing key, and unboxing into an int throws:

void main() {
    Map<String, Integer> stock = Map.of("tea", 5);
    int coffee = stock.get("coffee");
}

Output:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "java.lang.Integer.intValue()" because the return value of "java.util.Map.get(Object)" is null

Thanks for reading!

Short answer

Add if missing

Create on first access

Common mistake

Functions used in this recipe

Thanks for reading!