Java OCA: Inheritance

Question 1

Which base class members AREN’T inherited by a class?

Answer 1

• private and default access members

* constructors of the base class

Question 2

Can a base class without abstract methods be defined as an abstract class?

Answer 2

Yes

Question 3

Can you ever create objects of an abstract class?

Answer 3

No

Question 4

What happens if a derived class doesn’t implement all the abstract methods of its base class?

Answer 4

It will fail to compile UNLESS it is defined as an abstract class.

Question 5

What are the implicit modifiers of an interface and its components?

Answer 5

• interface: public

* interface variables: public, static, final.

Question 6

What is the explicit way of writing
    interface Cheetah{
    double distance = 21;
    int height(); }

Answer 6

interface Cheetah{
public static final double distance = 21;
public abstract int height(); }

Question 7

Can interfaces include class names?

Answer 7

No, an interface can never extend any class.

Question 8

Can you define a top-level, protected Interface?

Answer 8

No, only possibilities are
• public
• no modifier (default access)
[ But inner or nested types can have any access level ]

Question 9

Which, if any, lines fail to compile,

1. interface MyInterface {
2. private int number = 10;
3. protected void aMethod();
4. interface interface2{}
5. public interface interface4{}
6. }

Answer 9

2+3: “illegal combination of modifiers: public and private” All members of an interface are inherently public.

Question 10

What are valid, non-access modifiers for an interface?

Answer 10

• abstract

* strictfp

Question 11

Which of the following compile successfully?

1. final interface MyInterface{}
2. static interface MyInterface{}
3. transient interface MyInterface{}
4. synchronized interface MyInterface{}
5. volatile interface MyInterface{}

Answer 11

None of them. Valid top-level interface modifiers include
• no modifier, public (access)
• abstract, strictfp (nonaccess)

Question 12

What kinds of methods can be defined in an interface?

Answer 12

• abstract
• default
• static

Question 13

What kind of method is eatPotato()?

interface Interviewer { void eatPotato(); }

Answer 13

• abstract

Question 14

Which of these is a valid default method?

1. interface Interviewer { default void submitInterviewStatus(); }
2. interface Interviewer { default void submitInterviewStatus(); { return 0; } }

Answer 14

2. Declaration of a default method must be followed by the method body marked using {}.

Question 15

How can a developer add methods to an interface without breaking existing implementations?

Answer 15

Only by adding default methods. When a class implements and interface with abstract methods, the class must implement all of the methods or else it won’t compile.

Question 16

Given
interface Interviewer {
abstract void conductInterview();
default void submitInterviewerStatus() {
System.out.println(“Accept”);
}
static void bookConferenceRoom(LocalDateTime dateTime, int duration) {
System.out.println(“Interview scheduled on:” + dateTime);
System.out.println(“Book conference room for: “ + duration + “ hrs”);
}
}
Which of the following lines, if any, fail to compile?

1. class Manager implements Interviewer {}
2. class Project {
3. public static void main(String [] args) {
4. Interviewer inv = new Manager();
5. inv.bookConferenceRoom(LocalDateTime.now(), 2);
6. Manager mgr = new Manager();
7. mgr.bookConferenceRoom(LocalDateTime.now(), 2);
   8.Interviewer.bookConferenceRoom(LocalDateTime.now(), 2);
   }
   }

Answer 16

5, 7: A static method in an interface can’t be called using a reference variable, it must be called using the interface name.

[Note: but it is fine to call a static variable defined in a CLASS using either the reference variables or by the name of the class.]

Question 17

Which of the following compile successfully?
1.  interface Interviewer {
         abstract int interviewConducted();
     }
     class Manager implements Interviewer {}
2. interface Interviewer {
         abstract int interviewConducted();
     }
     class Manager implements Interviewer {
          int interviewConducted();
          return 1;
     }
3. interface Interviewer {
         abstract Object interviewConducted();
     }
     class Manager implements Interviewer {
          public String interviewConducted() {
          return null;
          }
     }
4. interface Interviewer {
         abstract Object interviewConducted();
     }
     class Manager implements Interviewer {
          public Integer interviewConducted() {
          return 1;
          }
     }

Answer 17

1. No: doesn’t implement interviewConducted()
2. No: attempts to assign weaker access privilege to interviewConducted()
3. Compiles successfully
4. No: return type Integer isn’t compatible with int