how to sort the array list in HashMap<Integer, ArrayList<String>>

Question

I'm having issues sorting out the ArrayList found inside of the hashmap, or treemap. the goal is to have the program print out the key, along with the sorted arraylist. I tried using Collections, but it didnt work, any help is welcome :)

public static void main(String[] args) {

    ArrayList<StudentCourse> List = new ArrayList<StudentCourse>();

    List.add(new StudentCourse(2, "MATH210"));
    List.add(new StudentCourse(2, "CS105"));
    List.add(new StudentCourse(1, "S300"));
    List.add(new StudentCourse(1, "CS200"));

    HashMap<Integer, ArrayList<String>> HMap = new HashMap<>();

    for (StudentCourse st : List) {
        if (HMap.containsKey(st.getStudentID())) {
            HMap.get(st.getStudentID()).add(st.getCourse());
        } else {
            HMap.put(st.getStudentID(), new ArrayList<>());
            HMap.get(st.getStudentID()).add(st.getCourse());
        }
    }
    Collections.sort(List);//this leads to an error.



    Map<Integer, ArrayList<String>> TrMap = new TreeMap<Integer, ArrayList<String>>(HMap);
    System.out.println(TrMap.toString());             
}

the output is this : {1=[S300, CS200], 2=[MATH210, CS105]}

while the intent is to have the the arraylist sorted, so:

1=[cS200, S300], 2=[CS105, MATH210]

Can you show the code that you tried which didn't work? Because there is really not much to it, just call Collections.sort(list); and it's sorted (create a minimal reproducible example). Please stick to naming conventions, variable names should always start with lower case characters. — Zabuzard
– Zabuzard, Commented Apr 12, 2018 at 0:40
Maybe you forgot to specify a Comparator object or to define a natural ordering on your custom elements (by letting StudentCourse implement Comparable). Otherwise nobody knows how StudentCourse objects should be sorted. — Zabuzard
– Zabuzard, Commented Apr 12, 2018 at 0:42
You can and should post your solution as a formal answer and accept it after the 48-hour delay, provided no better answers come along. — MutantOctopus
– MutantOctopus, Commented Apr 12, 2018 at 1:28
That's a comment, the answer box should be down below. Large box with a blue button labeled "Post your answer". — MutantOctopus
– MutantOctopus, Commented Apr 12, 2018 at 1:50

Zabuzard · Accepted Answer · 2018-04-12 13:03:23Z

There is really not much to it. You just iterate all value entries and call some sort algorithm on it. For example the one provided by the Collections#sort (documentation) method. The method sorts inline so you don't have to re-add the values or anything, just call the method and your value entry will be sorted:

Probably the shortest code to realize this is

map.values().forEach(Collections::sort);

The values method returns a Set of all your values in the map. The forEach applies the given method to all entries.

If you are more familiar with regular loops:

for (ArrayList<String> value : map.values()) {
    Collections.sort(value);
}

The approach you have posted in your comments works too:

for (Entry<Integer, ArrayList<String>> entry : map.entrySet()) {
    Collections.sort(entry.getValue());
}

but it is unnecessary to pull the whole Entry (with key) out of the map, you only need the values. So consider using the map#values method instead of the map#entrySet.

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