SD1 examination summer 2021

Question

Q:

We consider the following inheritance hierarchy:

abstract public class Parent {
  public void print(){
    System.out.println("Parent");
  }
}

public class A extends Parent {
  @Override
  public void print() {
    System.out.println("A");
  }
}

public class B extends Parent {
  @Override
  public void print() {
    System.out.println("B");
  }
}

There is nothing wrong here syntactically. All classes do compile well and allow for e.g.:

void someMethod(Parent p) {
  p.print();
}

However some code here is completely unnecessary.

Answer 1

A:

Class Parent class is abstract. Therefore we cannot create any instance of Parent. Due to runtime polymorphism Parent#print() will thus never be executed directly.

Classes A and B both override Parent.print(). And both child classes do not call Parent.print() using super.print(). Thus Parent.print() will never be executed.
Solution: Parent.print() should be declared abstract thus omitting its implementation:
```
abstract public class Parent {
  abstract public void print();
}
```
Note

We cannot simply omit Parent.print() altogether. Otherwise the p.print() call in someMethod(Parent p) would be impossible.

Prev	Up	Next
Spoiled execution	Home	SD1 examination winter 2020/1