Is it possible to use Class<?> with the new Pattern-Matching switch?

Question

As part of an investigation into the parameters of a method, I tried the new Pattern Matching for switch (Preview). Using a traditional condition, it works perfectly:

Method firstMethod = BitSet.class.getDeclaredMethods()[0];
Parameter firstParameter = firstMethod.getParameters()[0];
if (firstParameter.getType() == Integer.class) {
        System.out.println("Integer");
    }

When I tried to refactor it to use a switch statement, it did not compile:

Method firstMethod = BitSet.class.getDeclaredMethods()[0];
Parameter firstParameter = firstMethod.getParameters()[0];
switch (firstParameter.getType()) {
    case Integer.class: System.out.println("Integer");
    case int.class: System.out.println("int");
    default: System.out.println("other");
}

The error is:

 error: incompatible types: Class<Integer> cannot be converted to Class<CAP#1>
        case Integer.class: System.out.println("Integer");
                    ^
 where CAP#1 is a fresh type-variable:
   CAP#1 extends Object from capture of ?

Is this something that can't be done, or is it just a syntax error?

Answer 1

This is not how switching over types work. You can switch over an object’s actual type and have to specify type names, rather than Class literals.

Object o = 42;
switch(o) {
    case Integer i: System.out.println("Integer " + i); break;
    case String s: System.out.println("String " + s); break;
    default: System.out.println("other");
}

Note that with pattern matching, there is no fall-through support, so specifying break is mandatory. Or you use the new syntax which has no fall-through in the first place

Object o = 42;
switch(o) {
    case Integer i -> System.out.println("Integer " + i);
    case String s -> System.out.println("String " + s);
    default -> System.out.println("other");
}

The object returned by getType() is always an instance of java.lang.Class, so branching by its type makes no sense. This doesn’t mean that comparing the actual values with a switch statement or expression was impossible. The comparison can be performed with a guarded pattern:

Method firstMethodWithParam = Arrays.stream(BitSet.class.getDeclaredMethods())
        .filter(m -> m.getParameterCount() > 0)
        .findAny().orElseThrow();
switch(firstMethodWithParam.getParameterTypes()[0]) {
    case Class<?> cl when cl == Integer.class -> System.out.println("Integer");
    case Class<?> cl when cl == int.class -> System.out.println("int");
    case Class<?> cl when cl == String.class -> System.out.println("String");
    case Class<?> cl when cl == long.class -> System.out.println("long");
    case Class<?> cl when BitSet.class.isAssignableFrom(cl)
                                  -> System.out.println("BitSet or subtype");
    default -> System.out.println("other");
}

^{Note that the JDK 17 preview used && instead of when}

but that’s only for completeness. I think it’s obvious that this is no improvement over if statements or a map from Class to handler.