Question

I am new on Java and Android. I want to understand the following line:

List<Map<String, String>> listMap = new ArrayList<Map<String, String>>();

My doubt is: it is declared an object called listMap, which is of the type List of Map, right? But it is instantiated as an ArrayList. Sorry for the simple question, but Why is this possible?

ps1: I know List is an interface, so I can’t instantiate an object of an interface type.

ps2: I can declare listMap as being of the ArrayList type. What is the advantage of declare it as a List.

Answer 1

ps1: I know List is and interface, so I can’t instantiate and object of its type.

List<Map<String, String>> listMap = new ArrayList<Map<String, String>>();

You are right, List is an Interface hence you will not be able to instantiate an Interface. You need to use some implementing class of the interface to instantiate. ArrayList is an implementation class of interface List and being used in the able statement.

ps2: I can declare listMap as being of the type ArrayList. What is the advantage of declare it as a List.

You can always, use ArrayList in the left side as well i.e.

  ArrayList<Map<String, String>> listMap = new ArrayList<Map<String, String>>();

Advantage of using Interface: If you want to use your variable object(listMap) across the classes then better to use the Interface as type, but if your variable object (lsitMap) locally within your program locally, then I don’t think there is any added advantage.

Now you will think about Map, which is again an Interface. Please note that, you are not instantiating the Map object through above statement. You are just mentioning that your listMap list will contain the element of Map type.

Again when you will try to instantiate elements to add in the listMap, you will need an implementation class of Map interface e.g HashMap as below:

   List<Map<String, String>> listMap = new ArrayList<Map<String, String>>();

   Map<String, String> lsitMapElement1 = new HashMap<String, String>();
   lsitMapElement1.put("key1", "value1");
   listMap.add(lsitMapElement1 );

   Map<String, String> lsitMapElement2 = new HashMap<String, String>();
   lsitMapElement2.put("key2", "value2");
   listMap.add(lsitMapElement2);

Hope this makes things clear.