How to increment by group - java-8

Question

There is a table and now add a new column -- sort_num int default 0
id level sort_num
1 1 0
2 1 0
3 2 0
4 2 0
5 2 0
6 3 0
7 3 0
8 3 0
9 3 0
Now I want to set sort_num values like below
id level sort_num
1 1 1
2 1 2
3 2 1
4 2 2
5 2 3
6 3 1
7 3 2
8 3 3
9 3 4
The Java code implement above requirement is
int sortNum = 0;
int currentLevel = fooList.get(0).getLevel();
for (RuleConf foo : fooList) {
if(currentLevel != foo.getLevel()){
sortNum = 0;
currentLevel = foo.getLevel();
}
foo.setSortNum(++sortNum);
}
I want to know if Java8 could simplify above code？
PS. Use mysql to implement this requirement
set #index:=0; update t set sort_num = (#index:=#index+1) where level = 1 order by id;
set #index:=0; update t set sort_num = (#index:=#index+1) where level = 2 order by id;
set #index:=0; update t set sort_num = (#index:=#index+1) where level = 3 order by id;

Answer 1

The best approach is to stick to your plain enhanced for loop. I don't think it is possible to come up with a single Stream solution, since you need to have intermediate values. Like:
Map<Integer, List<RuleConf>> levels = fooList.stream()
.collect(Collectors.groupingBy(RuleConf::getLevel));
levels.values().forEach(v ->
IntStream.range(0, v.size()).forEach(i -> v.get(i).setSortNum(i + 1))
);

Answer 2

If you keep track of the next order numbers yourself, you may do it with one stream. This solution is thread safe as well, hence should work with parallel streams:
Map<Integer, AtomicInteger> orders = new ConcurrentHashMap<>();
fooList.stream().forEachOrdered(foo -> {
orders.putIfAbsent(foo.getLevel(), new AtomicInteger());
foo.setOrder(orders.get(foo.getLevel()).incrementAndGet());
});
It should outperform the other stream-solutions, because it requires to iterate over the list only ones.