How does combiner in Stream.collect method work in java 8?

Question

I created simple demo :

public static void main(String[] args) {
        List list2 = Arrays.asList(\"adf\", \"bcd\", \"abc\", \"hgr\", \"jyt\"         


        

Answer 1

It is more interesting result with and without parallel for Raniz sample:

    String collect = list2.stream().collect(StringBuilder::new,
            (res, elem) -> {
                System.out.printf("ACCUMULATE res=%s, elem=%s\n", res, elem);
                res.append(" ").append(elem);
        },
            (res1, res2) -> {
                System.out.printf("COMBINE res1=%s, res2=%s\n", res1, res2);
                res1.append(res2.toString());
            }).toString();

Without parallel combine never have been called:

ACCUMULATE res=, elem=adf
ACCUMULATE res= adf, elem=bcd
ACCUMULATE res= adf bcd, elem=abc
ACCUMULATE res= adf bcd abc, elem=hgr
ACCUMULATE res= adf bcd abc hgr, elem=jyt
ACCUMULATE res= adf bcd abc hgr jyt, elem=edr
ACCUMULATE res= adf bcd abc hgr jyt edr, elem=biu
collect= adf bcd abc hgr jyt edr biu

And with parallel

ACCUMULATE res=, elem=jyt
ACCUMULATE res=, elem=hgr
COMBINE res1= hgr, res2= jyt
ACCUMULATE res=, elem=biu
ACCUMULATE res=, elem=edr
COMBINE res1= edr, res2= biu
ACCUMULATE res=, elem=bcd
COMBINE res1= hgr jyt, res2= edr biu
ACCUMULATE res=, elem=abc
ACCUMULATE res=, elem=adf
COMBINE res1= bcd, res2= abc
COMBINE res1= adf, res2= bcd abc
COMBINE res1= adf bcd abc, res2= hgr jyt edr biu
collect= adf bcd abc hgr jyt edr biu