generics - Java Class.cast() vs. cast operator

Question

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.

1. Should Class.cast() be banished to Generics land? There it has quite a few legitimate uses.
2. Should compilers generate compile errors when Class.cast() is used and illegal conditions can be determined at compile time?
3. Should Java provide a cast operator as a language construct similar to C++?

Answer 1

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.