The Fine Print

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The Fine Print

A Generic Class is Shared by All Its Invocations

What does the following code fragment print?

List l1 = new ArrayList();
List l2 = new ArrayList();
System.out.println(l1.getClass() == l2.getClass());
You might be tempted to say false, but you’d be wrong. It prints true, because all instances of a generic class have the same run-time class, regardless of their actual type parameters.

Indeed, what makes a class generic is the fact that it has the same behavior for all of its possible type parameters; the same class can be viewed as having many different types.

As consequence, the static variables and methods of a class are also shared among all the instances. That is why it is illegal to refer to the type parameters of a type declaration in a static method or initializer, or in the declaration or initializer of a static variable.

Casts and InstanceOf

Another implication of the fact that a generic class is shared among all its instances, is that it usually makes no sense to ask an instance if it is an instance of a particular invocation of a generic type:

Collection cs = new ArrayList();
// Illegal.
if (cs instanceof Collection) { … }
similarly, a cast such as

// Unchecked warning,
Collection cstr = (Collection) cs;
gives an unchecked warning, since this isn’t something the runtime system is going to check for you.

The same is true of type variables

// Unchecked warning.
T badCast(T t, Object o) {
return (T) o;
}
Type variables don’t exist at run time. This means that they entail no performance overhead in either time nor space, which is nice. Unfortunately, it also means that you can’t reliably use them in casts.

Arrays

The component type of an array object may not be a type variable or a parameterized type, unless it is an (unbounded) wildcard type.You can declare array types whose element type is a type variable or a parameterized type, but not array objects.

This is annoying, to be sure. This restriction is necessary to avoid situations like:

// Not really allowed.
List[] lsa = new List[10];
Object o = lsa;
Object[] oa = (Object[]) o;
List li = new ArrayList();
li.add(new Integer(3));
// Unsound, but passes run time store check
oa[1] = li;

// Run-time error: ClassCastException.
String s = lsa[1].get(0);
If arrays of parameterized type were allowed, the previous example would compile without any unchecked warnings, and yet fail at run-time. We’ve had type-safety as a primary design goal of generics. In particular, the language is designed to guarantee that if your entire application has been compiled without unchecked warnings using javac -source 1.5, it is type safe.

However, you can still use wildcard arrays. The following variation on the previous code forgoes the use of both array objects and array types whose element type is parameterized. As a result, we have to cast explicitly to get a String out of the array.

// OK, array of unbounded wildcard type.
List

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