Using computeIfAbsent from Map

The computeIfAbsent method is passed two parameters: the key key and the function for calculating the value of this key mappingFunction .

The logic of the method:

• We check the presence of such a key in the map. If the key is and the value by the key is not null , then we do nothing
• Otherwise (no key or key value is null ), consider the value by applying mappingFunction to key
• If the final value is not null , then write the key-value pair to the map

In the form of code, this logic is described in the documentation as follows:

 if (map.get(key) == null) { V newValue = mappingFunction.apply(key); if (newValue != null) map.put(key, newValue); } 

In some situations, the putIfAbsent method will suffice , especially if the calculated value does not depend on the key at all.
For example, if in Map<Integer, List<Integer>> map you just need to put a new list by key:

 map.putIfAbsent(key, new ArrayList<>()); 

Now, if you want to immediately put the value in this list, then computeIfAbsent more convenient:

 map.computeIfAbsent(key, k -> new ArrayList<>()).add(100); 

instead

 map.putIfAbsent(key, new ArrayList<>()); map.get(key).add(100); 

A good example of using the computeIfAbsent method with memorizing the result is the calculation of Fibonacci numbers :

 private static Map<Integer, Long> map = new HashMap<>(); static { map.put(0, 0L); map.put(1, 1L); } public static long fibonacci(int x) { return map.computeIfAbsent(x, n -> fibonacci(n - 2) + fibonacci(n - 1)); } 

And the challenge:

 System.out.println(fibonacci(10)); 

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