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Thanks  Nice Article.

It would be create if you could enrich it with code through github URL etc.

Good explanation.

Actually, @JosePaumard, in the "Optionals: First Patterns" section where you have...

     Optional<Person> opt = ...;    
     if (opt.isPresent()) {        
          int value = opt.get(); // there is a value    
     } else {        
          // decide what to do    
     }

... the line with the comment "// there is a value" would fail compilation with a "incompatible types: Person cannot be converted to int" error.

You wrote that

The Optional class could have been returned by the map.get() method

could be used to distinguish between having a  'null' value stored in the map and not having the given key. This does not work since Optional does not allow 'null'  as a value (at least under Java 8) - 'null' is used to flag the empty Optional (no value present)!

@JosePaumard In the code in the "Calculating the Inverse of a Square Root, Second Version" section, the inverse operation is done first, and then the result of that inverse operation is passed to the sqare root operation. So that code is actually calculating the square root of the inverse...

Function<Double, Stream<Double>> invSqrt =

    d ‐> OptionalMath.inv(d).flatMap(OptionalMath::sqrt)

                       .map(Stream::of)

                       .orElseGet(Stream::empty);

Java- it is my like programme