Purpose of third argument to 'reduce' function in Java 8 functional programming

Question

Under what circumstances is the third argument to \'reduce\' called in Java 8 streams?

The code below attempts to traverse a list of strings and add up the code poin

Answer 1

Simple test code to confirm the usage of combiner:

String[] strArray = {"abc", "mno", "xyz"};
List strList = Arrays.asList(strArray);

System.out.println("stream test");
int streamResult = strList.stream().reduce(
        0, 
        (total,s) -> { System.out.println("accumulator: total[" + total + "] s[" + s + "] s.codePointAt(0)[" + s.codePointAt(0) + "]"); return total + s.codePointAt(0); }, 
        (a,b) -> { System.out.println("combiner: a[" + a + "] b[" + b + "]"); return 1000000;}
    );
System.out.println("streamResult: " + streamResult);

System.out.println("parallelStream test");
int parallelStreamResult = strList.parallelStream().reduce(
        0, 
        (total,s) -> { System.out.println("accumulator: total[" + total + "] s[" + s + "] s.codePointAt(0)[" + s.codePointAt(0) + "]"); return total + s.codePointAt(0); }, 
        (a,b) -> { System.out.println("combiner: a[" + a + "] b[" + b + "]"); return 1000000;}
    );
System.out.println("parallelStreamResult: " + parallelStreamResult);

System.out.println("parallelStream test2");
int parallelStreamResult2 = strList.parallelStream().reduce(
        0, 
        (total,s) -> { System.out.println("accumulator: total[" + total + "] s[" + s + "] s.codePointAt(0)[" + s.codePointAt(0) + "]"); return total + s.codePointAt(0); }, 
        (a,b) -> { System.out.println("combiner: a[" + a + "] b[" + b + "] a+b[" + (a+b) + "]"); return a+b;}
    );
System.out.println("parallelStreamResult2: " + parallelStreamResult2);

Output:

stream test
accumulator: total[0] s[abc] s.codePointAt(0)[97]
accumulator: total[97] s[mno] s.codePointAt(0)[109]
accumulator: total[206] s[xyz] s.codePointAt(0)[120]
streamResult: 326
parallelStream test
accumulator: total[0] s[mno] s.codePointAt(0)[109]
accumulator: total[0] s[abc] s.codePointAt(0)[97]
accumulator: total[0] s[xyz] s.codePointAt(0)[120]
combiner: a[109] b[120]
combiner: a[97] b[1000000]
parallelStreamResult: 1000000
parallelStream test2
accumulator: total[0] s[mno] s.codePointAt(0)[109]
accumulator: total[0] s[xyz] s.codePointAt(0)[120]
accumulator: total[0] s[abc] s.codePointAt(0)[97]
combiner: a[109] b[120] a+b[229]
combiner: a[97] b[229] a+b[326]
parallelStreamResult2: 326