Problem with statelessness

[Vimugdh Lall]

I'm having trouble understanding statelessness and associativity of accumulators in the reduce() method for Streams. I'd appreciate some insight into it or even a link to some article which explains it properly. Thanks!

[Paul Clapham]

I looked at the API documentation for the Stream.reduce() method; it mentioned that the operation must be associative. And the word "associative" was a link to an explanation of why that is necessary and what the ramifications are. Did you read that? If not, do you still have questions after reading it?

Likewise with "stateless"; the word in the documentation is also a link to a section which explains further.

[Vimugdh Lall]

Thanks Paul for pointing those out. I just went through them. Got the associativity part nailed down. I've understood the statelessness too I guess. I'll go through some problems relating to those to solidify my understanding!
Cheers!  

[Paul Clapham]

Great!

[Stephan van Hulst]

Here's an example that violates statelessness:

var sentence = words.stream().reduce(new StringBuilder(), StringBuilder::append, StringBuilder::append);

Each time StringBuilder.append() is called, it modifies the state of the StringBuilder instance. This instance is also used when other elements of the stream are iterated over, potentially leading to issues. To be stateless, the operations you pass to the reduce() method must not call any mutators, and should always return the reduced result as a new immutable value.

The example above is associative though: (("a" + "b") + "c") yields the same result as ("a" + ("b" + "c")).

Here's an example that violates associativity:

var finalCredit = expenses.stream().reduce(startingCredit, (currentCredit, expense) -> currentCredit - expense);

It is stateless, but it's not associative because ((100 - 20) - 15) yields a different result than (100 - (20 - 15)).