How to effectively process HashMap in order described by list?

EasternDude :

This is my problem: I have a list of event names in chronological order, example:

ArrayList<String> eventsOrder = new ArrayList<>(Arrays.asList("event1", "event2", "event3", "event4", "event5"));

I have HashMap of data about events, where key is events name, example:

HashMap<String, String> eventsData = new HashMap<>();
    eventsData.put("event2", "data2");
    eventsData.put("event1", "data1");
    eventsData.put("event3", "data3");

I also have a method that merges Data from 2 events into 1:

public String merge(String previousEventsData, String nextEventsData);

I need to merge all of events' data into one object in chronological order. So in this case it would be something like this:

merge(merge("data1", "data2"),"data3");

This how I solve it now:

Firstly, this HashMap may or may not contain data about all of the events so i filter names of the ones that are available:

ArrayList<String> eventsToProcess = eventsOrder.stream()
                .filter(eventName -> eventsData.containsKey(eventName))
                .collect(Collectors.toCollection(ArrayList::new));

Then I use this ugly algorithm to merge events' data in chronological order:

    String finalData = "";

    int eventsCount = namesToProcess.size();

    finalData = eventsData.get(namesToProcess.get(0));
    if(eventsCount > 1) {
        for (int i = 1; i < eventsCount; i++){
            finalData = merge(finalData, eventsData.get(namesToProcess.get(i)));
        }

    }

Not very elegant and readable, right?

Question: How would I do same processing with Java streams or a recursive function?

kaya3 :

Something like this should be equivalent, using Stream.reduce:

eventsOrder.stream()          // "event1", "event2", "event3", "event4", "event5"
    .map(eventsData::get)     // "data1", "data2", "data3", null, null
    .filter(Objects::nonNull) // "data1", "data2", "data3"
    .reduce(YourClass::merge) // merge(merge("data1", "data2"), "data3")
    .get()                    // gets the result from the Optional

The Optional.get method throws NoSuchElementException if there are no events with data; your original code's namesToProcess.get(0) will throw IndexOutOfBoundsException in that case. That should be the only difference in behaviour, assuming your eventsData map doesn't have any null values.