Introduction to Java Programming, Sixth Edition, Y. Daniel Liang

Chapter 9 Inheritance and Polymorphism


Section 9.2 Superclasses and Subclasses
1  Object-oriented programming allows you to derive new classes from existing classes. This is called ____________.

A. encapsulation
B. inheritance
C. abstraction
D. generalization

2  Which of the following statements are true?

A. A subclass is a subset of a superclass.
B. A subclass is usually extended to contain more functions and more detailed information than its superclass.
C. "class A extends B" means A is a subclass of B.
D. "class A extends B" means B is a subclass of A.

Section 9.3 Using the super KeywordSection 9.3.1 Calling Superclass Constructors
3  Suppose you create a class Cylinder to be a subclass of Circle. Analyze the following code:

class Cylinder extends Circle {
   double length;
  
   Cylinder(double radius) {
      Circle(radius);
   }
}

A. The program compiles fine, but you cannot create an instance of Cylinder because the constructor does not specify the length of the cylinder.
B. The program has a syntax error because you attempted to invoke the Circle class's constructor illegally.
C. The program compiles fine, but it has a runtime error because of invoking the Circle class's constructor illegally.

4  Analyze the following code:

import java.util.StringTokenizer;

public class A extends StringTokenizer {
}

A. The program has a compilation error because A does not have a default constructor.
B. The program has a compilation error because the default constructor of A invokes the default constructor of StringTokenizer, but StringTokenizer does not have a default constructor.
C. The program would compile fine if you add the following constructor into A: A(String s) { }
D. The program would compile fine if you add the following constructor into A: A(String s) { super(s); }

5  Analyze the following code:

public class Test extends A {
   public static void main(String[] args) {
     Test t = new Test();
     t.print();
   }
}

class A {
   String s;

   A(String s) {
     this.s = s;
   }

   public void print() {
     System.out.println(s);
   }
}

A. The program does not compile because Test does not have a default constructor Test().
B. The program has an implicit default constructor Test(), but it cannot be compiled, because its super class does not have a default constructor. The program would compile if the constructor in the class A were removed.
C. The program would compile if a default constructor A(){ } is added to class A explicitly.
D. The program compiles, but it has a runtime error due to the conflict on the method name print.

Section 9.3.2 Constructor Chaining
6  What is the output of running class C?

class A {
   public A() {
     System.out.println(
       "The default constructor of A is invoked");
   }
}

class B extends A {
   public B() {
     System.out.println(
       "The default constructor of B is invoked");
   }
}

public class C {
   public static void main(String[] args) {
     B b = new B();
   }
}

A. none
B. "The default constructor of B is invoked"
C. "The default constructor of A is invoked""The default constructor of B is invoked"
D. "The default constructor of B is invoked""The default constructor of A is invoked"
E. "The default constructor of A is invoked"

Section 9.3.3 Calling Superclass Methods
7  Which of the statements regarding the super keyword is incorrect?

A. You can use super to invoke a super class constructor.
B. You can use super to invoke a super class method.
C. You can use super.super.p to invoke a method in superclass?s parent class.
D. You cannot invoke a method in superclass?s parent class.

Section 9.4 Overriding Methods
8  Analyze the following code:

public class Test {
   public static void main(String[] args) {
     B b = new B();
     b.m(5);
     System.out.println("i is " + b.i);
   }
}

class A {
   int i;

   public void m(int i) {
     this.i = i;
   }
}

class B extends A {
   public void m(String s) {
   }
}

A. The program has a compilation error, because m is overridden with a different signature in B.
B. The program has a compilation error, because b.m(5) cannot be invoked since the method m(int) is hidden in B.
C. The program has a runtime error on b.i, because i is not accessible from b.
D. The method m is not overridden in B. B inherits the method m from A and defines an overloaded method m in B.

9  Which of the following statements are true?

A. To override a method, the method must be defined in the subclass using the same signature and return type as in its superclass.
B. Overloading a method is to provide more than one method with the same name but with different signatures to distinguish them.
C. It is a compilation error if two methods differ only in return type.
D. A private method cannot be overridden. If a method defined in a subclass is private in its superclass, the two methods are completely unrelated.
E. A static method cannot be overridden. If a static method defined in the superclass is redefined in a subclass, the method defined in the superclass is hidden.

Section 9.5 Overriding vs. Overloading
10  Which of the following statements are true?

A. A method can be overloaded in the same class.
B. A method can be overridden in the same class.
C. If a method overloads another method, these two methods must have the same signature.
D. If a method overrides another method, these two methods must have the same signature.

Section 9.6 The Object Class and Its toString() Method
11  Analyze the following code:

public class Test {
   public static void main(String[] args) {
     Object a1 = new A();
     Object a2 = new Object();
     System.out.println(a1);
     System.out.println(a2);
   }
}

class A {
   int x;

   public String toString() {
     return "A's x is " + x;
   }
}

A. The program cannot be compiled, because System.out.println(a1) is wrong and it should be replaced by System.out.println(a1.toString());
B. When executing System.out.println(a1), the toString() method in the Object class is invoked.
C. When executing System.out.println(a2), the toString() method in the Object class is invoked.
D. When executing System.out.println(a1), the toString() method in the A class is invoked.

Section 9.7 Polymorphism, Dynamic Binding, and Generic Programming
12  Which of the following statements are true?

A. You can always pass an instance of a subclass to a parameter of its superclass type. This feature is known as polymorphism.
B. The compiler finds a matching method according to parameter type, number of parameters, and order of the parameters at compilation time.
C. A method may be implemented in several subclasses. The Java Virtual Machine dynamically binds the implementation of the method at runtime.
D. Dynamic binding can apply to static methods.
E. Dynamic binding can apply to instance methods.

13  The equals method is defined in the Object class. Which of the following is correct to override it in the String class?

A. public boolean equals(String other)
B. public boolean equals(Object other)
C. public static boolean equals(String other)
D. public static boolean equals(Object other)

14  Analyze the following code.

// Program 1:
public class Test {
   public static void main(String[] args) {
     Object a1 = new A();
     Object a2 = new A();
     System.out.println(a1.equals(a2));
   }
}

class A {
   int x;

   public boolean equals(Object a) {
     return this.x == ((A)a)x;
   }
}


// Program 2:
public class Test {
   public static void main(String[] args) {
     Object a1 = new A();
     Object a2 = new A();
     System.out.println(a1.equals(a2));
   }
}

class A {
   int x;

   public boolean equals(A a) {
     return this.x == a.x;
   }
}

A. Program 1 displays true and Program 2 displays true
B. Program 1 displays false and Program 2 displays true
C. Program 1 displays true and Program 2 displays flase
D. Program 1 displays false and Program 2 displays false

15  Analyze the following code.

// Program 1:
public class Test {
   public static void main(String[] args) {
     Object a1 = new A();
     Object a2 = new A();
     System.out.println(a1.equals(a2));
   }
}

class A {
   int x;

   public boolean equals(A a) {
     return this.x == a.x;
   }
}


// Program 2:
public class Test {
   public static void main(String[] args) {
     A a1 = new A();
     A a2 = new A();
     System.out.println(a1.equals(a2));
   }
}

class A {
   int x;

   public boolean equals(A a) {
     return this.x == a.x;
   }
}

A. Program 1 displays true and Program 2 displays true
B. Program 1 displays false and Program 2 displays true
C. Program 1 displays true and Program 2 displays flase
D. Program 1 displays false and Program 2 displays false

16  Analyze the following code.

// Program 1
public class Test {
   public static void main(String[] args) {
     Object a1 = new A();
     Object a2 = new A();
     System.out.println(((A)a1).equals((A)a2));
   }
}

class A {
   int x;

   public boolean equals(A a) {
     return this.x == a.x;
   }
}


// Program 2
public class Test {
   public static void main(String[] args) {
     A a1 = new A();
     A a2 = new A();
     System.out.println(a1.equals(a2));
   }
}

class A {
   int x;

   public boolean equals(A a) {
     return this.x == a.x;
   }
}

A. Program 1 displays true and Program 2 displays true
B. Program 1 displays false and Program 2 displays true
C. Program 1 displays true and Program 2 displays flase
D. Program 1 displays false and Program 2 displays false

17  Given the following code, find the syntax error?

public class Test {
   public static void main(String[] args) {
     m(new GraduateStudent());
     m(new Student());
     m(new Person());
     m(new Object());
   }

   public static void m(Student x) {
     System.out.println(x.toString());
   }
}

class GraduateStudent extends Student {
}

class Student extends Person {
   public String toString() {
     return "Student";
   }
}

class Person extends Object {
   public String toString() {
     return "Person";
   }
}

A. m(new GraduateStudent()) causes an error
B. m(new Student()) causes an error
C. m(new Person()) causes an error
D. m(new Object()) causes an error

Section 9.8 Casting Objects and the instanceof Operator
18  Which of the following are Java keywords?

A. instanceOf
B. instanceof
C. cast
D. casting

19  Analyze the following code:

Cylinder cy = new Cylinder(1, 1);
Circle c = cy;

A. The code has a syntax error.
B. The code has a runtime error.
C. The code is fine.

20  Analyze the following code:

Circle c = new Circle (5);
Cylinder c = cy;

A. The code has a syntax error.
B. The code has a runtime error.
C. The code is fine.

21  Given the following classes and their objects:

class C1 {};
class C2 extends C1 {};
class C3 extends C1 {};

C2 c2 = new C2();
C3 c3 = new C3();

Analyze the following statement:

c2 = (C2)((C1)c3);

A. c3 is cast into c2 successfully.
B. You will get a runtime error because you cannot cast objects from sibling classes.
C. You will get a runtime error because the Java runtime system cannot perform multiple casting in nested form.
D. The statement is correct.

22  Given the following code:

class C1 {}
class C2 extends C1 { }
class C3 extends C2 { }
class C4 extends C1 {}

C1 c1 = new C1();
C2 c2 = new C2();
C3 c3 = new C3();
C4 c4 = new C4();

Which of the following expressions evaluates to false?

A. c1 instanceof C1
B. c2 instanceof C1
C. c3 instanceof C1
D. c4 instanceof C2

23  Analyze the following code:

public class Test {
   public static void main(String[] args) {
     String s = new String("Welcome to Java");
     Object o = s;
     String d = (String)o;
   }
}

A. When assigning s to o in Object o = s, a new object is created.
B. When casting o to s in String d = (String)o, a new object is created.
C. When casting o to s in String d = (String)o, the contents of o is changed.
D. s, o, and d reference the same String object.

24  You can assign _________ to a variable of Object[] type.

A. new char[100]
B. new int[100]
C. new double[100]
D. new String[100]
E. new java.util.Date[100]

Section 9.9 The ArrayList Class
25  You can create an ArrayList using _________.

A. new ArrayList[]
B. new ArrayList[100]
C. new ArrayList()
D. ArrayList()

26  Invoking _________ removes all elements in an ArrayList x.

A. x.remove()
B. x.clean()
C. x.delete()
D. x.empty()
E. x.clear()

27  Suppose ArrayList x contains two strings [Beijing, Singapore]. Which of the following method will cause the list to become [Beijing, Chicago, Singapore]?

A. x.add("Chicago")
B. x.add(0, "Chicago")
C. x.add(1, "Chicago")
D. x.add(2, "Chicago")

28  Suppose ArrayList x contains two strings [Beijing, Singapore]. Which of the following method will cause the list to become [Beijing]?

A. x.remove("Singapore")
B. x.remove(0)
C. x.remove(1)
D. x.remove(2)

29  Suppose ArrayList x contains two strings [Beijing, Singapore]. Which of the following method will cause runtime errors?

A. x.get(1)
B. x.set(2, "New York");
C. x.get(2)
D. x.remove(2)
E. x.size()

30  Invoking _________ returns the first element in an ArrayList x.

A. x.first()
B. x.get(0)
C. x.get(1)
D. x.get()

31  Invoking _________ returns the number of the elements in an ArrayList x.

A. x.getSize()
B. x.getLength(0)
C. x.length(1)
D. x.size()

Section 9.11 The protected Data and Methods
32  What modifier should you use on a class so that a class in the same package can access it but a class in a different package cannot access it?

A. public
B. private
C. protected
D. Use the default modifier.

33  What modifier should you use on the members of a class so that they are not accessible to another class in a different package, but are accessible to any subclasses in any package?

A. public
B. private
C. protected
D. Use the default modifier.

34  The visibility of these modifiers increases in this order:

A. private, protected, none (if no modifier is used), and public.
B. private, none (if no modifier is used), protected, and public.
C. none (if no modifier is used), private, protected, and public.
D. none (if no modifier is used), protected, private, and public.

35  A class design requires that a particular member variable must be accessible by any subclasses of this class, but otherwise not by classes which are not members of the same package. What should be done to achieve this?

A. The variable should be marked public.
B. The variable should be marked private.
C. The variable should be marked protected.
D. The variable should have no special access modifier.
E. The variable should be marked private and an accessor method provided.

36  Which of the following statements is false?

A. A public class can be accessed by a class from a different package.
B. A private method cannot be accessed by a class in a different package.
C. A protected method can be accessed by a subclass in a different package.
D. A method with no visibility modifier can be accessed by a class in a different package.

Section 9.12 The final Classes, Methods, and Variables
37  Which of the following classes cannot be extended?

A. class A { }
B. class A {   private A();}
C. final class A { }
D. class A {   protected A();}

Section 9.13 Methods in the Object Class (Optional)
38  Which of the following statement is correct?

A. Every class should override the finalized method.
B. You should invoke the finalize method to finalize an object.
C. Every object can be cloned using the clone() method.
D. You can invoke the clone() method on an array object to create another array.

39  What is the output of the following code:

public class Test {
   public static void main(String[] args) {
     Object o1 = new Object();
     Object o2 = new Object();
     System.out.print((o1 == o2) + " " + (o1.equals(o2)));
   }
}

A. false false
B. true true
C. false true
D. true false

40  What is the output of the following code:

public class Test {
   public static void main(String[] args) {
     String s1 = new String("Java");
     String s2 = new String("Java");
     System.out.print((s1 == s2) + " " + (s1.equals(s2)));
   }
}
 
A. false false
B. true true
C. false true
D. true false

41  What is the output of the following code:

import java.util.Date;

public class Test {
   public static void main(String[] args) {
     Date date1 = new Date();
     Date date2 = new Date();
     System.out.print((date1 == date2) + " " + (date1.getClass() == date2.getClass()));
   }
}

A. false false
B. true true
C. false true
D. true false

Section 9.14 (Optional) Hiding Fields and Static Methods
42  What is the output of running A?
public class A extends B {
   int x = 1;
   public static void main(String[] args) {
     System.out.print(new A().x);
     System.out.print(new B().x);
   }
}

class B {
   int x = 5;
}

A. 15
B. 51
C. 11
D. 55

Section 9.15 (Optional) Initialization Blocks
43  Analyze the following code:

public class Test {
   int x;

   { x++; }
}

A. The program cannot be compiled, because the statement x++ must be placed inside a method or a constructor.
B. You cannot construct an instance of Test, because it does not have a constructor.
C. When you construct an instance of Test, the value of x becomes 0;
D. When you construct an instance of Test, the value of x becomes 1;

44  Analyze the following code:

public class Test {
   int x;

   static { x++; }
}

A. The program cannot be compiled, because the statement x++ must be placed inside a method or a constructor.
B. The program cannot be compiled, because x is non-static, but is used in a static initialization block.
C. When you construct an instance of Test, the value of x becomes 0;
D. When you construct an instance of Test, the value of x becomes 1;

45  What is the output of the following program?
public class Test {
   public static void main(String[] args) {
     new A();
   }
  
   {
     System.out.print("Z");
   }
}

class A extends B {
   A() {
     System.out.print("A");
   }
  
   {
     System.out.print("X");
   }
}

class B {
   B() {
     System.out.print("B");
   }
  
   {
     System.out.print("Y");
   }
}

A. XYAB
B. ABXYZ
C. YBXA
D. BXAY
E. YBXAZ

46  What is the output of the following program?
public class Test extends Object {
   public static void main(String[] args) {
     Test test = new Test();
   }

   public Test() {
     System.out.print("A");
   }

   {
     System.out.print("B");
   }

   static {
     System.out.print("C");
   }
}

A. ABC
B. BAC
C. ACB
D. CBA
E. CAB

47  When you run the following program,

public class Test {
   public static void main(String[] args) {
     if (5 > 6)
       new java.util.Date();
   }
}

A. class java.lang.Object will be loaded.
B. class Test will be loaded.
C. class java.util.Date will be loaded.
D. class java.lang.String will be loaded.

Comprehensive
48  Polymorphism means ______________.

A. that data fields should be declared private.
B. that a class can extend another class.
C. that a variable of supertype can refer to a subtype object.
D. that a class can contain another class.

49  Encapsulation means ______________.

A. that data fields should be declared private.
B. that a class can extend another class.
C. that a variable of supertype can refer to a subtype object.
D. that a class can contain another class.

50  Inheritance means ______________.

A. that data fields should be declared private.
B. that a class can extend another class.
C. that a variable of supertype can refer to a subtype object.
D. that a class can contain another class.

51  Composition means ______________.
A. that data fields should be declared private.
B. that a class can extend another class.
C. that a variable of supertype can refer to a subtype object.
D. that a class can contain another class.