add elements in arraylist inside hashmap

Question

I am trying to build a dynamic hashmap of type String and arraylist dynamically. I have some json data coming from server and instead of declaring many arraylist I want to save them in hashmap with String as key and arraylist as value.

Here is what I am doing now

ArrayList<classproperty> allStu;
ArrayList<classproperty> allEmp;
HashMap<String, ArrayList<classproperty>> hash;
if (type.equals("Student")) {
    prop = new classproperty("Student", info.getJSONObject(i).getJSONObject("student").getJSONArray("class").getJSONObject(s).getJSONObject("type").getString("name"));
    allStu.add(prop);       
}
if (type.equals("Emp")) {
    prop = new esSignalProperty("Emp", info.getJSONObject(m).getJSONObject("emp").getJSONObject(s).getJSONObject("dept").getString("name"));
    allemp.add(prop);        
}

hash.put("Student", allStu);
hash.put("Emp", allemp);

So it is ugly way to do it...and I would like to do it by directly putting into hashmap without declaring so many arraylist. please ignore json string extraction as it is just dummy.

Answer 1

You just need to initialize the arraylist in the beginning and then just add value based on key. if you know the keys that I guess you know you can do like this

public HashMap<String, ArrayList<classproperty>> hash
hash.put("Student", new ArrayList<classproperty>());
hash.put("Emp", new ArrayList<classproperty>());

after just as @steffen mention but with minor change

  hash.get("Student").add(prop);
  hash.get("Emp").add(prop);

It is nothing very different from what other purposed but may be still can help.

Answer 2

hash.get("Student").put(prop)

could be a solution, as you know the key inside the map.

Using this way you can leave out the 'allStu' and 'allEmp' Lists, as you can get them directly from the map.

Answer 3

I would suggest using MultiMap from Guava library that already supports this. If you don't plan to import this library, then you can roll your own manually as a wrapper of a Map<K, List<V>> :

//basic skeleton of the multimap
//as a wrapper of a map
//you can define more methods as you want/need
public class MyMultiMap<K,V> {
    Map<K, List<V>> map;
    public MyMultiMap() {
        map = new HashMap<K, List<V>>();
    }

    //in case client needs to use another kind of Map for implementation
    //e.g. ConcurrentHashMap
    public MyMultiMap(Map<K, List<V>> map) {
        this.map = map;
    }

    public void put(K key, V value) {
        List<V> values = map.get(key);
        if (values == null) {
            //ensure that there will always be a List
            //for any key/value to be inserted
            values = new ArrayList<V>();
            map.put(key, values);
        }
        values.add(value);
    }

    public List<V> get(K key) {
        return map.get(key);
    }

    @Override
    public String toString() {
        //naive toString implementation
        return map.toString();
    }
}

Then just use your multimap:

MyMultiMap myMultiMap = new MyMultiMap<String, ClassProperty>();
myMultiMap.put("student", new ClassProperty(...));
myMultiMap.put("student", new ClassProperty(...));
System.out.println(myMultiMap);