call generic method from non generic method

Question

I have two methods look like this. One is a generic method and the other is not.

<T> void a(final Class<T> type, final T instance) {
}

void b(final Class<?> type, final Object instance) {

    if (!Objects.requireNotNull(type).isInstance(instance)) {
        throw new IllegalArgumentException(instance + " is not an instance of " + type);
    }

    // How can I call a(type, instance)?
}

How can I call a() with type and instance from b()?

Answer 1

Use a generic helper method:

void b(final Class<?> type, final Object instance) {

    if (!type.isInstance(instance)) {
        // throw exception
    }

    bHelper(type, instance);
}

private <T> void bHelper(final Class<T> type, final Object instance) {
    final T t = type.cast(instance);
    a(type, t);
}

Class.cast will throw a ClassCastException if instance is not a T (so your earlier check may not be needed).

Answer 2

for example like this

a(String.class, new String("heloo"));