Why are these invalid in Java?

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

For the first one, because Java generics are not covariant. So you can't even do:

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

For more info, see this article: Java theory and practice: Generics gotchas.

For the second one, you cannot use primitives as the generic type.

Answer 2

1) What would happen if anyone added a Long to the list that is referenced by myList?

2) You can't have a primitive there.

Answer 3

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?

Answer 4

1. The first is impossible. You can use a wildcard:

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

   But now, your list is completely unusable. Adding, removing, etc doesn't compile anymore:

   myList.add(new Object()); // Error
   myList.add("error");      // Error
   

2. You can't use primitives (int, double, ....) with generics. You have to use the wrapper class:

   HashMap<Integer> myHap = new HashMap<Integer>();
   

Answer 5

for the second example Map has 2 generic parameters a Key and Value but the following still wont work

Map<Integer,Object> myMap = new HashMap<int,Object>();

that because as said before primitive types don't work with java generics (besides generic types always derive from Object and type erasure translates all generic variables to type Object)

Answer 6

For all things Java-Generics, refer to Angelika Langer. In this case, Is List<Object> a supertype of List<String>?

Even though Java does support auto-boxing, it doesn't automatically convert <int> to <Integer>. IMO this is partly to remind users that you can get nulls.