Question

I am building a java class to be used as a generic comparator based on the sample shown on Generic Comparator. The constructor I am building for the comparator is as follows:

public GenericComparator(java.lang.reflect.Field sortField, boolean ascending){
        ....   }

While the above constructor allows me to get the sort field, there is no way for the compiler to know if the field is implementing Comparable interface and a developer calling the constructor may pass a field that doesn’t implement the Comparableinterface.

I am wondering if there is a way to make this a compile time error and not handle it at Run time with the use of instanceof check to make sure the field has implemented the interface.

A similar implementation found in the Collections.sort(...) Type declaration.

public static <T extends Comparable<? super T>> void sort(List<T> list) {
    ...
}

Answer 1

Comparable.isAssignableFrom(field.getType())

does it at runtime, but there’s no way at all to make this a compile-time error. Fields don’t (and can’t, really) carry their type information at compile time — they’re runtime objects, and can be chosen at runtime. For example, if you had

random.nextBoolean() ? Foo.class.getField("bar") : Foo.class.getField("baz")

then no compiler, no matter how smart, could tell at compile time what type the resulting field would have.