How do I use the new computeIfAbsent function?

java

memoization

lambda-expressions

method-references

byAnton Shumikhin·Dec 5, 2024

The computeIfAbsent method in the Map interface helps you compute and automatically add a value into the map when absent. As the name suggests, it only performs computation if necessary, making it an efficient alternative to the Map's get method. Here’s a swift introduction:

Map<String, Integer> fruitCounts = new HashMap<>();
fruitCounts.computeIfAbsent("banana", key -> key.length()); // Adds "banana"=6, because 6-letters are a bunch 🍌

This compact code checks for "banana" and only calculates and inserts the string length if it's absent. This feature not only helps in 'peeling' off excess computations but also in providing tidier and more readable code.

Utilizing computeIfAbsent

Steering beyond its simple use, computeIfAbsent becomes particularly powerful with recursive algorithms and multimaps:

Optimization via Memoization

When working with recursive methods, computeIfAbsent paired with memoization can spare you numerous repeated calculations. In the following factorial calculator, any computed factorial gets cached in a Map:

Map<Integer, Long> memo = new HashMap<>();

public long factorial(int n) {
    // When n <= 20, use computeIfAbsent's magic to cheat recursive slowdown
    return memo.computeIfAbsent(n, k -> (k == 0) ? 1 : k * factorial(k - 1));
}

Here, calling computeIfAbsent ensures factorials are calculated once and subsequent requests for the same number are served swiftly from the Map, rather than diving into the rabbit hole of never-ending recursive calls.

Avoiding Null Management

computeIfAbsent can also prevent NullPointerExceptions by ensuring proper object initialization. Hence, working with multimaps just got a tad simpler:

Map<String, List<String>> multiMap = new HashMap<>();
multiMap.computeIfAbsent("fruits", k -> new ArrayList<>()).add("apple"); // Add 🍎 into the bag without checking if it exists

With the above approach, handling null keys becomes a piece of cake. Say goodbye to verbose boilerplate code and mind-boggling null checks!

Advanced use cases

computeIfAbsent is not just about saving a few keystrokes. It enables writing cleaner and more efficient code, even in complex scenarios:

Constructing Nested Maps

Multilayer data structures like nested Map structures can be constructed with lesser code and improved readability:

Map<String, TreeMap<Integer, TreeSet<String>>> complexMap = new HashMap<>();

complexMap.computeIfAbsent("Category", k -> new TreeMap<>())
           .computeIfAbsent(10, k -> new TreeSet<>())
           .add("Item"); // This chains faster than a rusty old bike 🚲

Note how the chained computeIfAbsent calls dynamically creates the multi-level data structure on-the-fly.

Simplifying with Method References

computeIfAbsent can efficiently combine with method references to simplify lambda expressions:

Map<String, Boolean> stringIsEmpty = new HashMap<>();
stringIsEmpty.computeIfAbsent("status", String::isEmpty); // Nobody likes an empty status, right?

Here, String::isEmpty is a more concise alternative to key -> key.isEmpty() offering integrated simplicity into the computeIfAbsent workflow.

Learning from Java gurus

Treasure troves of wisdom from experts like Stuart Marks and Brian Goetz can deepen your understanding of computeIfAbsent and turn you from a Java fledgling to a Java Jedi.

Tips and Tricks

Tackling some often overlooked details can make your computeIfAbsent journey much smoother.

Mind the Self-Referencing Lambda

One should note the self-referencing lambda—a lambda expression in computeIfAbsent can refer back to the map it’s modifying. Although helpful in some cases, if not used wisely, The Force could be disturbed!