How to achieve this Map<String, List<>> structure [closed]

Question

I have data like below:

 Key       value
-----      ------
car         toyota
car         bmw
car         honda

fruit       apple
fruit       banana

computer    acer
computer    asus
computer    ibm 
...

(Each row of above data is an object with fields "key" and "value", all in one List List<DataObject>)

I would like to construct the data to a Map<String, List<String>> like following:

 "car"      : ["toyota", "bmw", "honda"]
 "fruit"    : ["apple","banana"]
 "computer" : ["acer","asus","ibm"]

How to achieve above Map structure from the data objects?

******Besides******

I am more interested in using pure JDK provided classes or interfaces to achieve the result instead of using external library. Any help?

Answer 1

    Map<String, List<String>> myMaps = new HashMap<String, List<String>>();
    for (DataObject item : myList) {
        if (!myMaps.containsKey(item.getKey())) {
            myMaps.put(item.getKey(), new ArrayList<String>());
        }
        myMaps.get(item.getKey()).add(item.getValue());
    }

Answer 2

I would use the guavas Multimap implementation. But it is easy doable with the standard JDK aswell.

Example standard JDK:

public static void main(String[] args) {
    Scanner s = new Scanner(
            "car         toyota\n" +
            "car         bmw\n" +
            "car         honda\n" +
            "fruit       apple\n" +
            "fruit       banana\n" +
            "computer    acer\n" +
            "computer    asus\n" +
            "computer    ibm");

    Map<String, List<String>> map = new LinkedHashMap<String, List<String>>();

    while (s.hasNext()) {

        String key = s.next();
        if (!map.containsKey(key))
            map.put(key, new LinkedList<String>());

        map.get(key).add(s.next());
    }

    System.out.println(map);
}

Example guava:

public static void main(String[] args) {
    Scanner s = new Scanner(
            "car         toyota\n" +
            "car         bmw\n" +
            "car         honda\n" +
            "fruit       apple\n" +
            "fruit       banana\n" +
            "computer    acer\n" +
            "computer    asus\n" +
            "computer    ibm");

    Multimap<String, String> map = LinkedListMultimap.create();

    while (s.hasNext()) 
        map.put(s.next(), s.next());

    System.out.println(map);
}

---

Output (both implementations):

{car=[toyota, bmw, honda], fruit=[apple, banana], computer=[acer, asus, ibm]}