Question

The following code gets the first type parameter class declared generically in the interface SomeGenericInterface which gets concretely implemented in the class SomeClass.

This code actually works.

The question is: Does it work in any case, i.e. are the following two Class methods:

• getInterfaces()
• getGenericInterfaces()

guaranteed to always have the same number of elements with the same respective order of the interfaces returned by these methods?

Or is there some safer way to do this?

<!-- language: lang-java -->

Class clazz = SomeClass.class;

Class classes[] = clazz.getInterfaces();
Type types[] = clazz.getGenericInterfaces();
ParameterizedType found = null;

for (int i=0; i<classes.length; i++) {
   if (  classes[i] == SomeGenericInterface.class) {
      found = (ParameterizedType) types[i];
      break;
   }
}
if (found == null) {
     return null;
}
Class firstType = (Class) found.getActualTypeArguments()[0];

Answer 1

The javadoc for both methods states:

If this object represents a class, the return value is an array
containing objects representing all interfaces implemented by the
class. The order of the interface objects in the array corresponds to
the order of the interface names in the implements clause of the
declaration of the class represented by this object.

so the answer to both your questions is yes, the same number of elements and in the same order.