Question

Me and my friends wondered if there really is a difference inside the JVM between interfaces and pure abstract classes, or if it is just really syntactic sugar.

I don’t really see why there would be difference, but it might not be so far-fetched.

Answer 1

As far as bytecode (.class file) is concerned, they are completely different:

From 4.1 The ClassFile Structure:

ClassFile {
    //...
    u2 constant_pool_count;
    cp_info constant_pool[constant_pool_count-1];
    //...
    u2 super_class;
    u2 interfaces_count;
    u2 interfaces[interfaces_count];
}

Clearly class can have a single superclass (abstract or not) and multiple implemented interfaces. It is a limitation of JVM, not a Java (the language) restriction.