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javadictionarylambdafunctional-programmingweak-references

Incompatible parameter types in lambda expression

Considering this code:

Map<C1, C2> map;
C1 key;
C2 value;

Why does this work:

map.computeIfAbsent(key, k -> value)

And this doesn't?

map.computeIfAbsent(key, () -> value)
Solution

  • Because the method computeIfAbsent has the following signature:

    default V computeIfAbsent(K key,
                          Function<? super K,? extends V> mappingFunction)

    so it expects a Function, which is a function interface that expects an argument and returns a value. Whereas () -> value, receives no arguments and produces a value (e.g., Supplier).