Chapter 5 - Class Design

Question 1

What are the rules for a concrete class to extend an abstract class?

Answer 1

1. The concrete class should implement all the methods from the abstract class.

Abstract classes extending abstract classes do not need to implemet the methods from the class that is being extended.

Question 2

Can an abstract class be instantiated directly?

Answer 2

False

Question 3

Do abstract classes require abstract and no abstract methods to be defined?

Answer 3

No. Abstract classes may define any number or zero abstract or non-abstract methods.

Question 4

Can abstract classes may be marked as private, protected or final?

Answer 4

No. By any means abstract classes can be marked as private, protected or final.

Question 5

Does an abstract class that extends an abstract class inherits all of its abstract methods as is own abstract methods?

Answer 5

True

Question 6

Does the first concrete class that extends an abstract class must provide an implementation for all the inherited abstract methods?

Answer 6

True

Question 7

Must abstract methods only be defined in abstract classes?

Answer 7

Yes. Abstract methods cannot be defined in classes that are not abstract.

Question 8

Can abstract methods be declared private or final?

Answer 8

No. Abstract methods cannot be of the type private or final.

Question 9

What are the rules for implementing an abstract method?

Answer 9

1. The name and signature of the implemented method should be the same.
2. The visibility of the method in the subclass must be at least as accessible as the method in the parent class.

Question 10

What an interface is?

Answer 10

An interface is a specialized kind of abstract class. It shares many of the same properties and rules as an abstract class.

Question 11

Can an interface be intanciated directly?

Answer 11

No. This is the interfaces rule # 1. Interfaces as abstract classes cannot be directly instantiated.

Question 12

Must interfaces define methods?

Answer 12

No. An interface is not required to define methods.

Question 13

Can an interface be defined as final?

Answer 13

No. This is interfaces rule #3. An interface may not be marked as final.

Question 14

Must interfaces have the abstract keyword in them?

Answer 14

No. Top level interfaces are asume to be abstract whether the “abstract” keyword id present or not.

Question 15

What will happen if an interface method is marked as private, protected or final?

Answer 15

A compiler error will be emitted. All top level interfaces are assume to be abstract whether the keyword is present or not. Therefore it is not possible to make a method private, protected or final.

Question 16

Can a nondefault method in an interface be marked as private, protected or final?

Answer 16

No. This will trigger a compiler error. All nondefault methods in an interface are assume to have the modifiers abstract and public in their definition. So, defining methods as private, protected or final should not be done.

Question 17

What is the inheriting an interface rule # 1?

Answer 17

An interface that extends another interface, as well as an abstract class that implements an interface, inherits all the abstract methods as its own abstract methods.

Question 18

What is the inheriting an interface rule # 2?

Answer 18

The first concrete class that implements an interface, or extends an abstract class that implements an interface, must provide an implementation for all the inherited abstract methods.

Question 19

Can an abstract class extend several abstract classes at the same time?

Answer 19

No. Just one abstract class can be extended at a time.

Question 20

Can multiple interfaces be extended at the same time?

Answer 20

Yes. An interface may extend multiple interfaces.

Example:

public interface Seal extends HasTail, HasWhiskers{ }

Where HasTail and HasWiskers are interfaces.

Question 21

Do abstract classes must include ‘abstract’ keyword in their definition?

Answer 21

Yes, abstract classes must include abstract keyword.

Question 22

Do interfaces must include ‘abstract’ keyword in their definition?

Answer 22

No, they may or not include ‘abstract’ in their definition. If not included, Java includes it by default.

Question 23

Can a class extend an interface?

Answer 23

No. A class can only implement an interface.

Question 24

Can an interface implement another interface?

Answer 24

No. An interface can only extend another interface.

Question 25

Can a interface extend a class?

Answer 25

No. An interface cannot extend a class. This generates a compiler error.

Question 26

Can 2 methods in a class be defined with the same name and input parameters but different return type?

Answer 26

No. The code won’t compile.

Question 27

What is rule # 1 for Interface Variables?

Answer 27

Interface variables are assumed to be public, static and final.

Question 28

Can interface variables be private or protected?

Answer 28

No. This will trigger a compiler error.

Question 29

What is rule # 2 for Interface Variables?

Answer 29

The value of an interface variable must be set when it is declared since it is marked as final.

Interface variables are essentially constant variables defined on the instance level. Because they are assumed to be static, they are accessible even without an instance of the interface.

Question 30

Can default interface methods be declared within a class or abstract class?

Answer 30

No. A default method may only be declared within an interface.

Question 31

Must default methods have the ‘default’ keyword in them?

Answer 31

Yes. Default methods must be marked with the default keyword and must provide a method body.

Question 32

Can a default method be marked as static, final or abstract?

Answer 32

No. A default method is not assumed to be static, final or abstract because it may be used or overridden by a class that implements the interface.

Question 33

Can a default method be marked as private or protected?

Answer 33

No. Like all methods in interfaces, a default method is assumed to be public and will not compile if marked as private or protected.

Question 34

Can interface methods with a body be declared without the default keyword?

Answer 34

No. This will generate a compiler error. Interfaces may only contain methods with a body that are marked as default.

Question 35

Can a class implement 2 interfaces that have the default methods with the same name and signature?

Example:
public interface Walk{
  public default int getSpeed( ) {
      return 5;
     }
  }

public class Cat implements Walk, Run { 
  public static void main (String[] args)
  system.out.println(new Cat().getSpeed());
 }

}

Answer 35

No. That will generate a compiler error because Java won’t know which method to choose. To fix this issue the subclass can override the duplicate default method.

Question 36

What is the default access modifier for an Static Interface Method?

Answer 36

A static method in an interface is assumed to be public and will not compile if marked as private or protected.

Question 37

Can a static method defined in an interface be inherited by a class that implements the interface?

Answer 37

No. Static methods defined in an interface are not inherited in any class that implements the interface.

Question 38

What needs to be done to reference a static method?

Answer 38

A reference to the name of the interface the static method belongs to should be used.

Question 39

In lambda expressios, can variables be redeclared?

Answer 39

No. In lambda expressions variables cannot be redeclared.

Question 40

What the type of the object determines?

Answer 40

The type of the the object determines which properties exist within the object in memmory.

Question 41

What the reference type to the object determine?

Answer 41

The type of reference to the object determines which methods and variables are accesible.

Question 42

What is a Virtual Method?

Answer 42

A Virtual Method is a method in which the specific implementation is not determined until runtime.

Question 43

In Virtual Methods, what do you get if you call a method on an object that overrides a method? The parent o or the overriden method?

Answer 43

You get the overriden method, even if the call to the method is on a parent reference or within the parent class.

Question 44

What is the rule related with accessibility for overriden methods?

Answer 44

Overriden methods must be at least as accessible as the method it is overriding, otherwise the code won’t compile.

Question 45

What is the rule related with exceptions in polrmorphims and methods overriding?

Answer 45

A subclass cannot declared an overriden method with a new or broader exception.

Question 46

What rule is related with return types in polymorphims and method overriding?

Answer 46

Overriden methods may use covariant return types.

Question 47

What is one special rule for constructors and overriding methods that needs to be followed?

Answer 47

If the parent class does not include a non-argument constructor, an explicit call to a parent constructor must be provided in the child’s contructor.

Question 48

In a static method, should a body be provided?

Answer 48

Yes, in static methods a body needs to be provided, otherwise a compiler error will be thrown.

Question 49

Should an interface method that provides a body be marked as default or static explicitly?

Answer 49

Yes. An interface method that provides a body must be marked as static or default, otherwise a compiler error will be thrown.

Question 50

Can methods mark as final be overriden?

Answer 50

No. Methods marked as final cannot be overriden.

Question 51

Abstract classes methods should be implemented by concrete classes extending the abstract class. Should the methods be overriden or overloaded?

Answer 51

Abstract methods inherited by a concrete class must be overriden, not overloaded.

Question 52

Can null value always be passed as an object value?

Answer 52

Yes, null can be passed as an object value regardless of type.

Question 53

Can variables be overriden?

Answer 53

No. Variables can only be hidden. They cannot be overriden even if they are public.

Question 54

Which methods can be overriden? With which type of access modifiers?

Answer 54

Methods with access modifier public and protected can be overriden. Static or private methods can NOT be overriden.