Having being taught during my C++ days about evils of the C-style cast operator I was pleased at first to find that in Java 5 java.lang.Class
had acquired a cast
method.
I thought that finally we have an OO way of dealing with casting.
Turns out Class.cast
is not the same as static_cast
in C++. It is more like reinterpret_cast
. It will not generate a compilation error where it is expected and instead will defer to runtime. Here is a simple test case to demonstrate different behaviors.
package test;
import static org.junit.Assert.assertTrue;
import org.junit.Test;
public class TestCast
{
static final class Foo
{
}
static class Bar
{
}
static final class BarSubclass
extends Bar
{
}
@Test
public void test ( )
{
final Foo foo = new Foo( );
final Bar bar = new Bar( );
final BarSubclass bar_subclass = new BarSubclass( );
{
final Bar bar_ref = bar;
}
{
// Compilation error
final Bar bar_ref = foo;
}
{
// Compilation error
final Bar bar_ref = (Bar) foo;
}
try
{
// !!! Compiles fine, runtime exception
Bar.class.cast( foo );
}
catch ( final ClassCastException ex )
{
assertTrue( true );
}
{
final Bar bar_ref = bar_subclass;
}
try
{
// Compiles fine, runtime exception, equivalent of C++ dynamic_cast
final BarSubclass bar_subclass_ref = (BarSubclass) bar;
}
catch ( final ClassCastException ex )
{
assertTrue( true );
}
}
}
So, these are my questions.
Class.cast()
be banished to Generics land? There it has quite a few legitimate uses.Class.cast()
is used and illegal conditions can be determined at compile time?I've only ever used Class.cast(Object)
to avoid warnings in "generics land". I often see methods doing things like this:
@SuppressWarnings("unchecked")
<T> T doSomething() {
Object o;
// snip
return (T) o;
}
It's often best to replace it by:
<T> T doSomething(Class<T> cls) {
Object o;
// snip
return cls.cast(o);
}
That's the only use case for Class.cast(Object)
I've ever come across.
Regarding compiler warnings: I suspect that Class.cast(Object)
isn't special to the compiler. It could be optimized when used statically (i.e. Foo.class.cast(o)
rather than cls.cast(o)
) but I've never seen anybody using it - which makes the effort of building this optimization into the compiler somewhat worthless.