Question

List<Object> myList = new ArrayList<String>(); //(hint: no)
Map<Integer> myMap = new HashMap<int>(); // (hint: also no)

Why are the statements wrong in the above declarations?

Answer 1

Let’s look at the first example. Think of the operations you should be able to do on a List<Object>: Add, Remove, Retrieve any Object.

You’re trying to fill those requirements with a collection that you can Add, Remove, and Retrieve only strings.

List<Object> myList = new ArrayList<String>();

// This should be valid based on the List<Object> interface but obviously
// MyClass isn't String...so what would the statement do?
myList.add(new MyClass());

The second is simply because Generics in Java don’t support primitive types. For more information, you could check out:

java – Why don’t Generics support primitive types?