How do I use the computeIfAbsent method? How does it work, and when is it done?

Answer 1, authority 100%

Method computeIfAbsent two parameters are passed: the key key and the function for calculating the value for this key mappingFunction .

The logic of the method:

• Check for the presence of such a key in map. If there is a key and the value by the key is not null , then we do nothing
• Otherwise (there is no key or the key value is null ), we calculate the value by applying mappingFunction to key
• If the final value is not null , then write a key-value pair to 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 , especially if the computed value does not depend on the key at all.
For example, if Map & lt; Integer, List & lt; Integer & gt; & gt; map you just need to put a new list by key:

map.putIfAbsent (key, new ArrayList & lt; & gt; ());

If, at the same time, you want to immediately put the value in this list, then it will be more convenient here computeIfAbsent :

map.computeIfAbsent (key, k - & gt; new ArrayList & lt; & gt; ()). add (100);

instead of

map.putIfAbsent (key, new ArrayList & lt; & gt; ());
map.get (key) .add (100);

A good example of using the computeIfAbsent method to remember the result is to calculate Fibonacci numbers :

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

And call:

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

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