Abstract Classes and Interfaces

Question 1

What is an abstract class?

Answer 1

An abstract class is a class that cannot be used to create objects. It contains abstract methods.

Question 2

How should constructors in abstract classes be created?

Answer 2

They should have the ‘protected’ modifier.

Question 3

Can an abstract method be contained in a concrete class?

Answer 3

No. If there’s an abstract method, the class must be abstract.

Question 4

What must the subclass do in regards to the abstract methods defined in the abstract superclass?

Answer 4

It must implement/override them all, or be defined as an abstract class itself.

Question 5

Why would you want constructors in an abstract class, since you cannot instantiate the class?

Answer 5

Because they may be invoked by the constructors of concrete subclasses.

Question 6

If GeometricObject is an abstract class, why can you do this?
GeometricObject[] objects = new GeometricObject[10];

Answer 6

Because you are not creating objects, you are creating 10 GeometricObject references inside the array. You can do that for the same reason you can do:
GeometricObject obj = new Circle();

Question 7

What is an Interface?

Answer 7

An interface is a class-like construct that contains only constants and abstract methods. It’s intent is to specify common behavior for objects of related classes or unrelated classes.

Question 8

Where is the compareTo method defined?

Answer 8

It’s defined in the Comparable interface, and in all classes that implement the comparable interface.

Question 9

What is the difference between abstract classes and interfaces in regards to variables?

Answer 9

Abstract classes has no restrictions in regards to variables, but interfaces can only have ‘public static final’ variables.

Question 10

What is the difference between abstract classes and interfaces in regards to constructors?

Answer 10

An abstract class cannot be instantiated using the new operator, but subclasses can invoke the constructors through their own constructors using the super keyword.
Interfaces have no constructors.

Question 11

What is the difference between abstract classes and interfaces in regards to methods.

Answer 11

Abstract classes have no method restrictions. They may have abstract methods and concrete methods.
Interfaces can only have public abstract non-static methods.

Question 12

What is the difference between abstract classes and interfaces in regards to inheritance?

Answer 12

Only single inheritance is allowed for abstract classes (a class may only extend one single class), but multiple inheritance is allowed for interfaces (a class may implement several interfaces).

Question 13

How is an abstract class defined? What is the syntax?

Answer 13

An abstract class has the keyword 'abstract' in the header, like this:
modifier abstract class className.

Question 14

How is an interface defined? What is the syntax?

Answer 14

An interface has the keyword ‘interface’ in the header, like this:
modifier interface interfaceName.

Question 15

How does a class make use of an interface?

Answer 15

By using the keyword 'implements' in the header, like this:
class className implements interfaceName.

Question 16

When should you use interfaces, and when should you use abstract classes?

Answer 16

In general, interfaces are preferred over abstract classes because an interface can define a common supertype for unrelated classes. Interfaces are more flexible than classes. However, by using abstract classes, you can inherit the implementation of other (non-abstract) methods. You can’t do that with interfaces - an interface cannot provide any method implementations.

Question 17

Which of the following class definitions defines a legal abstract class?
A. class A { abstract void unfinished() { } }
B. class A { abstract void unfinished(); }
C. abstract class A { abstract void unfinished(); }
D. public class abstract A { abstract void unfinished(); }

Answer 17

C. abstract class A { abstract void unfinished(); }

Question 18

Which of the following declares an abstract method in an abstract Java class?
A. public abstract method(); 
B. public abstract void method(); 
C. public void abstract Method(); 
D. public void method() {} 
E. public abstract void method() {}

Answer 18

B. public abstract void method();

Question 19

Which of the following statements regarding abstract methods are true?
A. An abstract class can have instances created using the constructor of the abstract class.
B. An abstract class can be extended.
C. A subclass of a non-abstract superclass can be abstract. 
D. A subclass can override a concrete method in a superclass to declare it abstract.
 E. An abstract class can be used as a data type.

Answer 19

All but A are true.

Question 20

Which of the following statements regarding abstract methods are true?
A. Abstract classes have constructors.
B. A class that contains abstract methods must be abstract.
C. It is possible to declare an abstract class that contains no abstract methods. 
D. An abstract method cannot be contained in a nonabstract class. 
E. A data field can be declared abstract.

Answer 20

All but E are true.

Question 21

Suppose A is an abstract class, B is a concrete subclass of A, and both A and B have a default constructor. Which of the following is correct?
A. A a = new A();
B. A a = new B();
C. B b = new A();
D. B b = new B();

Answer 21

B and D are true.

Question 22

What is the output of running class Test?

public class Test {
  public static void main(String[] args) {
    new Circle9();
  }
}

public abstract class GeometricObject {
  protected GeometricObject() {
    System.out.print("A");
  }

  protected GeometricObject(String color, boolean filled) {
    System.out.print("B");
  }
}

public class Circle9 extends GeometricObject {
  /** Default constructor */
  public Circle9() {
    this(1.0);
    System.out.print("C");
  }

  /** Construct circle with a specified radius */
  public Circle9(double radius) {
    this(radius, "white", false);
    System.out.print("D");
  }

  /** Construct a circle with specified radius, filled, and color */
  public Circle9(double radius, String color, boolean filled) {
    super(color, filled);
    System.out.print("E");
  }
}

Answer 22

BEDC

Question 23

Analyze the following code.

    Number[] numberArray = new Integer[2];
    numberArray[0] = new Double(1.5);
A. You cannot use Number as a data type since it is an abstract class.
B. Since each element of numberArray is of the Number type, you cannot assign an Integer object to it.
C. Since each element of numberArray is of the Number type, you cannot assign a Double object to it.
D. At runtime, new Integer[2] is assigned to numberArray. This makes each element of numberArray an Integer object. So you cannot assign a Double object to it.

Answer 23

D. At runtime, new Integer[2] is assigned to numberArray. This makes each element of numberArray an Integer object. So you cannot assign a Double object to it.

Question 24

Which of the following is a correct interface?
A. interface A { void print() { }; }
B. abstract interface A { print(); }
C. abstract interface A { abstract void print() { };}
D. interface A { void print();}

Answer 24

D. interface A { void print();}

Question 25

Which of the following is incorrect?
A. An abstract class contains constructors.
B. The constructors in an abstract class should be protected.
C. The constructors in an abstract class are private.
D. You may declare a final abstract class.
E. An interface may contain constructors.

Answer 25

C, D and E are true.

Question 26

\_\_ is a reference type.
A. A class type
B. An interface type
C. An array type
D. A primitive type

Answer 26

A, B and C

Question 27

Show the output of running the class Test in the following code lines:

interface A {
}

class C {  
}

class B extends D implements A {
}

public class Test {
  public static void main(String[] args) {
    B b = new B();
    if (b instanceof A)
      System.out.println("b is an instance of A");
    if (b instanceof C)
      System.out.println("b is an instance of C");
  }
}

class D extends C {  
}

Answer 27

b is an instance of A followed by b is an instance of C.

Question 28

Suppose A is an interface, B is a concrete class with a default constructor that implements A. Which of the following is correct?
A. A a = new A();
B. A a = new B();
C. B b = new A();
D. B b = new B();

Answer 28

B and D are true.