Compile time error when adding an Integer to a generic ArrayList of String

Even though type erasure means that at execution time the VM can't distinguish between an ArrayList<String> and an ArrayList<Integer> (they're both just instances of ArrayList) the compiler does know about the type argument, and knows what's safe.

So if you have:

ArrayList<Integer> integers = new ArrayList<Integer>();
integers.add("foo");

then the compile-time type of integers is ArrayList<Integer>, and any operations performed on integers will be checked with that in mind. The only add calls in ArrayList are

add(E e)
add(int index, E e)

Now there's no conversion from String to Integer, so neither of those overloads is applicable for the call integers.add("foo"), hence the compile-time error.

Basically, generics provide two things:

• Compile-time safety to avoid this sort of broken code; if you've said that a list should only contain Integer references, the compiler won't let you add a String reference to it

• Simplicity in terms of casts - when a method of a class with a type parameter T returns a value of type T and the compiler knows what that T is, it can insert an appropriate cast from the erased type (often Object) to the type argument. That's how:

  List<Integer> integers = getListFromSomewhere();
  Integer x = integers.get(0);
  

  works - the compiler adds a cast in the calling code. (That cast is still checked at execution time.)