Question

I am using an Arraylist to hold heterogeneous data…a mix of String, int and class objects (These objects could be instances of any class I’ve defined.). I’ve initialised the ArrayList as ArrayList<Object> = new ArrayList<Object>.

But I think I’ve heard ppl saying that directly referring to the Objects is sth. bad/undesirable? I’m referring to the objects because I can’t find any suitable generics to use in my case. Is it ok to use or are there any suitable generics? (I’m relatively new to Java)

Answer 1

Use different list for each type and keep those lists in a map and you can use the class object as the key so it wont mix up and easy to access.

 Map<Class<?>,List<?>> map = new HashMap<Class<?>,List<?>>();
 map.put(String.class, new ArrayList<String>());
 map.put(Integer.class, new ArrayList<Integer>());
 map.put(Class.class, new ArrayList<Class<?>>());

EDIT:

P.S: You will still get an unchecked warning when you retrieve the list from the map if you want to add new object into the list.

public static void main(String[] args) {
        Map<Class<?>,List<?>> map = new HashMap<Class<?>,List<?>>();
        map.put(String.class, new ArrayList<String>());
        map.put(Integer.class, new ArrayList<Integer>());
        map.put(Class.class, new ArrayList<Class<?>>());

        String str = "deneme";

        @SuppressWarnings("unchecked")
        List<String> strList =  (List<String>) map.get(String.class);
        strList.add(str);
        strList.add("str2");
        strList.add("str2");

        @SuppressWarnings("unchecked")
        List<Class<?>> classList =  (List<Class<?>>) map.get(Class.class);
        classList.add(String.class);
        classList.add(Integer.class);
        classList.add(Double.class);


        for(String currentStr:strList){
            System.out.println(currentStr);
        }
    }