Small experiment for finding out the definition of the PCI mutation operator. I don't really see how downcasting in Java will change the behaviour in most cases: non-static methods are resolved using the actual type instead of the declared type, after all.

PCIMutationTest.java

class Parent {
  @Override
  public String toString() {
    return "Parent";
  }
}

class Child extends Parent {
  @Override
  public String toString() {
    return "Child";
  }
}

public class PCIMutationTest {
  public static void main(String[] args) {
    Child c = new Child();
    Parent p = c;
    System.out.println(p.toString());
    System.out.println(c.toString());
    System.out.println(((Child)p).toString());
  }
}