Android Gradle Fallback to Another Build Variant

I have four build variants, A, B, C and D. For a specific activity I have common code in a base class in the main source sets. I want to additional functionality such that A & B have the same additionally functionality and C & D have the same functionality which is different from A & B. I have been able to do this by copy and pasting the extending class in the source sets for A & B and C & D respectively; however, that makes maintenance an issue. I cannot merge variants A & B or C & D because they do have other important differences. How do I include the class such that it is only in A and B references it and only C and D references it. I tried adding java source sets but then it couldn't find any other classes or other modules.

I was able to solve this issue by creating two new directories E & F. Then I added source sets for A & B to be their current source sets and E and for C & D I added F.

sourceSets {
    A {
        java.srcDirs = ['src/A/java', 'src/public/E']
    }
    B {
        java.srcDirs = ['src/B/java', 'src/public/E']
    }
    C {
        java.srcDirs = ['src/C/java', 'src/internal/F']
    }
    D {
        java.srcDirs = ['src/D/java', 'src/internal/F']
    }
}