Trying to understand how the datatypes are determined in this reduce()

[Anil Philip]

tb864615.OCPJAVASE17PT.c06.079

I am trying to understand how the datatypes are determined in this reduce(). (It doesn't compile - book wants you to know why)
I understand the identity is Integer.
But shouldn't w,y,z be StringBuilder?

Based on the reduction operation, the data types of w, y, and z are Integer, while the data type of x is StringBuilder.

     

 var p = List.of(new StringBuilder("hello"),
                new StringBuilder("goodbye"));
        var q = p.parallelStream().reduce(0,
                (w,x) -> w.length() + x.length(),
                (y,z) -> y.length() + z.length());
        System.out.print(q);

[Stephan van Hulst]

Look carefully at the parameters of the reduce() function.

<U> U reduce(
  U                           identity, 
  BiFunction<U, ? super T, U> accumulator,
  BinaryOperator<U>           combiner
)

We passed in an Integer as the identity, so we can substitute Integer for U. We also know that the stream consists of StringBuilder, so we substitute StringBuilder for T.

Integer reduce(
  Integer                                             identity, 
  BiFunction<Integer, ? super StringBuilder, Integer> accumulator,
  BinaryOperator<Integer>                             combiner
)

So the accumulator is a function that accepts an Integer and a StringBuilder. That means:

(Integer w, Stringbuilder x) -> w.length() + x.length()

And the combiner is a function that takes two Integers:

(Integer y, Integer z) -> y.length() + z.length()

Now you can easily see why the program is wrong: w, y and z are Integer and thus lack a length() method.

[Stephan van Hulst]

We can fix the program like this:

.reduce(
  0,
  (w, x) -> w + x.length(),
  (y, z) -> y + z
);

Here's a way of visualizing a possible execution:

Stream:	["hello", "goodbye"]
After splitting:	[["hello"], ["goodbye"]]
After accumulating:	[5, 7]
After combining:	12

[Anil Philip]

Stephan van Hulst wrote:Look carefully at the parameters of the reduce() function.

So the accumulator is a function that accepts an Integer and a StringBuilder. That means:

(Integer w, Stringbuilder x) -> w.length() + x.length()

And the combiner is a function that takes two Integers:

(Integer y, Integer z) -> y.length() + z.length()

I see now that we must pay close attention not only to the generic types in the definition, but also to the order in which they appear, and attempt to map it to the actual arguments.

[Stephan van Hulst]

You got it.