Explain to me how: Map>> data = new HashMap>>(); is useful.Firstly, in Java 7 or in Java 6 in a modern IDE, you only have to type:
Map>> data = new HashMap();
So let's break this down: if I was using another language, I could just do:
var data = {};
But consider what you've now lost:
1. If your code was just a simple associative array, not much. But if you have a complex nested data structure, you need to now put a comment somewhere so you don't forget what you're storing there. You can get into serious problems when multiple programmers are working on the same project if you don't clearly describe your data structures, and so you're not really saving on typing by writing Map>> because you'd have to document that in a comment instead. Since people are lazy when it comes to commenting code, Java therefore forces a degree of self documentation into data structures.
2. I notice that you're storing a Set of Strings (Set). As I'm sure you know, this means you can throw a bunch of strings into the set without worrying about duplicates. In Javascript, this would be messy. Since there is no 'Set' data type in Javascript you'd have to have an associative array where you store the string as the key and 'undefined' as the value.
3. HashMap isn't the only kind of map. In Java, you could use a TreeMap so that content is sorted in key order. This saves you code later by not having to take the associative array, converting it into a list, and then sorting the list.
4. In addition to TreeMap, another extremely useful kind of map is the ConcurrentHashMap, which allows you to manipulate the Map simultaneously from multiple threads (e.g. multiple simultaneous web request threads) without getting concurrent access problems or having to write your own synchronization code.
5. Tons of other really cool data Java datastructures available to you in the Java libs. E.g. WeakHashMap which won't prevent referenced objects from being garbage collected if they are dereferenced elsewhere, and LinkedHashMap which is useful for guaranteeing the order of stored objects even if they are removed and reinserted.
So although I agree that var data = {} can be more convenient in simple cases, that's not to say Java's approach has no upside.