Question

Consider the following two Java files that contain a simplified version of my issue

#a.java
package a;
public interface I {
 //...
}

#b.java
package b;
public interface I {
 //... (different stuff from a.I)
}

You’ll notice within my project there are two interfaces named “I”. This cannot be changed.
I am in a situation where I need to use both types inside a single class. Now of course, I could just reference each of their types as a.I and b.I, but I’m trying to avoid it for nothing other than maintaining readability.

I want to do something like this

interface B extends b.I {

}

This could let me use the interface of I by using B and, and a.I as just I by importing. The problem is that this doesn’t work, let’s take this concrete example using

interface MyList extends List<String> {
}

MyList l = new ArrayList<String>();

This yields a type error. Why doesn’t Java “know” that MyList extends List?

Also, I’ve tried casting in the above example, but it generates a ClassCastException

Thoughts?

Answer 1

This is valid:

List<String> list = new MyList();

This is not:

MyList l = new ArrayList<String>();

ArrayList is not a MyList. They just have a common superinterface. That’s why it doesn’t work. Java “knows” MyList is a List but that doesn’t mean you can assign a List to it.

Consider:

public interface MyList extends List<String> {
  void foo();
}

MyList l = new ArrayList<String>();
l.foo();

Obviously ArrayList does not have the foo() method.