test.java

// Test.java:

class Test<LineT extends Line> {
    private LineT line;

    private void test() {
        Serial serial = new Serial();
        line.serials.add(serial); // warning: unchceked call to add(E) ...
    }
}

// Line.java:

import java.util.List;

class Line<Foobar extends String> {
    List<Serial> serials;
}

// Serial.java:

class Serial {
}