Iterate arraylist and send email by grouping values

Question

I have an ArrayList like below

ArrayList<String> fList = new ArrayList<>();
    fList.add("100510-0001");
    fList.add("100510-0001");
    fList.add("100513-0004");
    fList.add("100510-0002");
    fList.add("100510-0001");
    fList.add("100513-0005");
    fList.add("100513-0006");
    fList.add("100518-0006");
    fList.add("100599-0001");
    fList.add("100593-0009");

I need to send an email based on first 6 characters of the List values.

Example: I have 100510 repeated 4 times in the list so I need to send all 4 records in the same email, like below hardcode one.

I have 100513 3 times, I can have n number of lists but I need to do recursion/iteration and send the email with the same records i.e 100510 in a separate email and 100513 in a separate email etc...

Any help?

ArrayList<String> subList = new ArrayList<>();
    for (int i = 0; i < fList.size(); i++) {
       String subString =  fList.get(0).split("-")[0];
        if(fList.get(i) == "100510"){
            subList.add(fList.get(i));
            createEmail(subList);
        }
        if(fList.get(i) == "100513"){
            subList.add(fList.get(i));
            createEmail(subList);
        }
    }

@ScaryWombat There are two parts to this question: (1) string comparison (2) multimap. How can you say its duplicate :( — Teddy, Nov 21 '18 at 06:12
You guys have an agenda to clean up and organize questions. But, at any cost? — Teddy, Nov 21 '18 at 06:12
Where is the `subString` in use? why two `if`s and not one with `||`? — Naman, Nov 21 '18 at 06:15
@nullpointer Then the appropriate comment should be added - and cannot be marked as "duplicate" of some half-related question. — Teddy, Nov 21 '18 at 06:16
@Teddy The OP has made no effort to show the `createEmail` method - how do we know if anything is wrong. Maybe all problems are caused by the String comparison error — Scary Wombat, Nov 21 '18 at 06:17
The question may be wrong and deserves closing. But "Closed as duplicate of XYZ" is also not great :) — Teddy, Nov 21 '18 at 06:20
http://stackoverflow.com/questions/513832/how-do-i-compare-strings-in-java — Scary Wombat, Nov 21 '18 at 06:23
[Please add a minimal, complete, and verifiable example](https://stackoverflow.com/help/mcve) — Scary Wombat, Nov 21 '18 at 06:24
OP what does `createEmail` do? Why do you think that you code does not work? — Scary Wombat, Nov 21 '18 at 06:24

score 2 · Accepted Answer · answered Nov 21 '18 at 06:45

If all you need is to send one e-mail per "prefix", then all you need to do is group:

Map<String, List<String>> distinctMap = fList.stream()
            .collect(Collectors.groupingBy(s -> s.split("-")[0]));

distinctMap.forEach((str1, list) -> {
    System.out.println("Sending e-mail for prefix " + str1);
    createEmail(list);
});

Of course you can do that using your for-loop, but you should only send the e-mail after all iterations:

Map<String, List<String>> emailsByPrefix = new HashMap<>();

for (int i = 0; i < fList.size(); i++) {

    String subString = fList.get(i).split("-")[0];

    emailsByPrefix.merge(subString, Arrays.asList(fList.get(i)), (list1, list2) -> {
        List<String> merged = new ArrayList<>();
        merged.addAll(list1);
        merged.addAll(list2);

        return merged;
    });
}

for (Entry<String, List<String>> emailEntry : emailsByPrefix.entrySet()) {
    System.out.println("Sending e-mail for prefix " + emailEntry.getKey());
    createEmail(emailEntry.getValue());
}

Thank you this what indeed I needed. I’ve used the map solution. — angular_architect, Nov 21 '18 at 18:40

score -1 · Answer 2 · answered Nov 21 '18 at 06:40

You can sort the original list, collect subList until first part changes, then send the subList each time the first part changes.

Collections.sort(fList);
String lastPart1 = null;
List<String> subList = null;
for (int i = 0; i < fList.size(); i++) {
   String strPart1 =  fList.get( i ).split("-")[0];
   String strPart2 =  fList.get( i ).split("-")[1];
    if( !strPart1.equals(lastPart1) ){
         if(subList!=null){ //Send the last collected subList
             createEmail( lastPart1, subList );
         }
         subList = new ArrayList<>();
         subList.add (strPart2);
         lastPart1 = strPart1;

    }else{
         subList.add (strPart2);
    }
}

Thank you but I don’t need to check the part2. – angular_architect Nov 21 '18 at 18:40 — angular_architect, Nov 21 '18 at 18:40

Iterate arraylist and send email by grouping values

2 Answers2