Diagnostic Quiz

Problems

Q2. More on private

The first two options are relatively easy to analyze, the problem lies in the third option b.y=1 and this is valid because foo belongs to class B, and thus has access to y of any instance of B.

Private fields are accessible to all methods within the same class, regardless of which instance is being accessed.

Q8. Subtype and Inheritance

Given the following code,

class P {
}

class Q extends P {
}

class R extends Q {
}

1. The inheritance relationship we can get is $P>Q>R$ (Here, let's denote $a>b$ as " $a$ is the parent of $b$.)

2. Then from the inheritance relationship, we should get this subtype relationship $P:>Q>:R$. And then use the basic knowledge of combinatorics, we know there will be 3 possible answers.

Q11. this and class method

Given the following code,

class Test {
   private int number = 2;

   public static int getNumber() {
     return this.number;
   }
}

Firstly, we notice that the method getNumber() is static. According to our learning in class method, we should deduce that the getNumber() method is associated to the class, instead of any instance from the class.

Also, in our learning, we know that a class method cannot access its instance fields or call other of its instance methods, and one corollory of this is that the keyword this is also not allowed in a class method (This is also covered in the lecture notes).

Q12. main() method

Same as we have seen in the lecture notes, the main() method must be declared as the following:

public static final void main(String[] args) {
}

Q13. Method Signature and Descriptor

For this problem, refer back to the lecture ntoes about method signature and method descriptor.

Q15. Overriding and Overloading

I will leave it when I learn Overloading.

Tips

1. Private fields are accessible to all methods within the same class, regardless of which instance is being accessed.

2. Suppose we have two classes $P$ and $Q$, if $Q$ inherits from $P$, then we can say $Q$ is the subtype of $P$ or $P:>Q$.

3. A class method (method with the keyword static) cannot access its instance fields or call other of its instance methods, and one corollory of this is that the keyword this is also not allowed in a class method. Otherwise, a compilation-error will be generated.

4. Include the method signature and method descriptor in the cheatsheet!