'Java, How to add values to Array List used as value in HashMap

What I have is a HashMap<String, ArrayList<String>> called examList. What I want to use it for is to save grades of each course a person is attending. So key for this HashMap is couresID, and value is a ArrayList of all grades (exam attempts) this person has made.

The problem is I know how to work with array lists and hashmaps normally, but I'm not sure how to even begin with this example. So how would I, or example, add something to ArrayList inside HashMap?

javaarraylisthashmap


Solution 1:[1]

String courseID = "Comp-101";
List<String> scores = new ArrayList<String> ();
scores.add("100");
scores.add("90");
scores.add("80");
scores.add("97");

Map<String, ArrayList<String>> myMap = new HashMap<String, ArrayList<String>>();
myMap.put(courseID, scores);

Hope this helps!

Solution 2:[2]

  • First create HashMap.

    HashMap> mapList = new HashMap>();

  • Get value from HashMap against your input key.

    ArrayList arrayList = mapList.get(key);

  • Add value to arraylist.

    arrayList.add(addvalue);

  • Then again put arraylist against that key value. mapList.put(key,arrayList);

It will work.....

Solution 3:[3]

First you retreieve the value (given a key) and then you add a new element to it

    ArrayList<String> grades = examList.get(courseId);
    grades.add(aGrade);

Solution 4:[4]

First, you have to lookup the correct ArrayList in the HashMap:

ArrayList<String> myAList = theHashMap.get(courseID)

Then, add the new grade to the ArrayList:

myAList.add(newGrade)

Solution 5:[5]

Java 8+ has computeIfAbsent

examList.computeIfAbsent(map.get(id), k -> new ArrayList<>());
map.get(id).add(value);

https://docs.oracle.com/javase/8/docs/api/java/util/Map.html#computeIfAbsent-K-java.util.function.Function-

Solution 6:[6]

Can also do this in Kotlin without using any external libraries.

var hashMap : HashMap<String, MutableList<String>> = HashMap()

if(hashMap.get(id) == null){
        hashMap.put(id, mutableListOf<String>("yourString"))
} else{
        hashMap.get(id)?.add("yourString")
}

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1 Mechkov
Solution 2 Chetan
Solution 3 Óscar López
Solution 4 Matt Fenwick
Solution 5 Diacrome
Solution 6

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