Chapter 9 Objects and Classes
Section 9.2 Defining Classes for Objects
1. represents an entity in the real world that can be distinctly identified.
a. A class
b. An object
c. A method
d. A data field
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2. is a template that defines objects of the same type.
a. A class
b. An object
c. A method
d. A data field
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3. An object is an instance of a .
a. program
b. class
c. method
d. data
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4. The keyword is required to declare a class.
a. public
b. private
c. class
d. All of the above.
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Section 9.3 Example: Defining Classes and Creating Objects
16. If TestCircle.java in Listing 9.1 is compiled successfully, .
a. TestCircle.class is created.
b. Circle.class is created.
c. TestCircle.class and Circle.class are created.
d. No class files are created.
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Section 9.4 Constructing Objects Using Constructors
5. is invoked to create an object.
a. A constructor
b. The main method
c. A method with a return type
d. A method with the void return type
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7. Which of the following statements are true?
a. Multiple constructors can be defined in a class.
b. Constructors do not have a return type, not even void.
c. Constructors must have the same name as the class itself.
d. Constructors are invoked using the new operator when an object is created.
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9. Analyze the following code.
class TempClass {
int i;
public void TempClass(int j) {
int i = j;
}
}
public class C {
public static void main(String[] args) {
TempClass temp = new TempClass(2);
}
}
a. The program has a compile error because TempClass does not have a default constructor.
b. The program has a compile error because TempClass does not have a constructor with an int argument.
c. The program compiles fine, but it does not run because class C is not public.
d. The program compiles and runs fine.
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6. Which of the following statements are true?
a. A default constructor is provided automatically if no constructors are explicitly declared in the class.
b. At least one constructor must always be defined explicitly.
c. Every class has a default constructor.
d. The default constructor is a no-arg constructor.
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8. Analyze the following code:
public class Test {
public static void main(String[] args) {
A a = new A();
a.print();
}
}
class A {
String s;
A(String s) {
this.s = s;
}
void print() {
System.out.println(s);
}
}
a. The program has a compile error because class A is not a public class.
b. The program has a compile error because class A does not have a default constructor.
c. The program compiles and runs fine and prints nothing.
d. The program would compile and run if you change A a = new A() to A a = new A(“5”).
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Section 9.5 Accessing Objects via Reference Variables
10. Given the declaration Circle x = new Circle(), which of the following statement is most accurate.
a. x contains an int value.
b. x contains an object of the Circle type.
c. x contains a reference to a Circle object.
d. You can assign an int value to x.
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17. Which of the following statements are correct?
a. A reference variable is an object.
b. A reference variable references to an object.
c. A data field in a class must be of a primitive type.
d. A data field in a class can be of an object type.
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Section 9.5.1 Accessing an Object’s Data and Methods
15. Analyze the following code.
public class Test {
int x;
public Test(String t) {
System.out.println(“Test”);
}
public static void main(String[] args) {
Test test = new Test();
System.out.println(test.x);
}
}
a. The program has a compile error because System.out.println method cannot be invoked from the constructor.
b. The program has a compile error because x has not been initialized.
c. The program has a compile error because you cannot create an object from the class that defines the object.
d. The program has a compile error because Test does not have a default constructor.
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Section 9.5.2 Reference Data Fields and the null Value
11. Analyze the following code.
public class Test {
int x;
public Test(String t) {
System.out.println(“Test”);
}
public static void main(String[] args) {
Test test = null;
System.out.println(test.x);
}
}
a. The program has a compile error because test is not initialized.
b. The program has a compile error because x has not been initialized.
c. The program has a compile error because you cannot create an object from the class that defines the object.
d. The program has a compile error because Test does not have a default constructor.
e. The program has a runtime NullPointerException because test is null while executing test.x.
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12. The default value for data field of a boolean type, numeric type, object type is , respectively.
a. true, 1, Null
b. false, 0, null
c. true, 0, null
d. true, 1, null
e. false, 1, null
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13. Which of the following statements are true?
a. Local variables do not have default values.
b. Data fields have default values.
c. A variable of a primitive type holds a value of the primitive type.
d. A variable of a reference type holds a reference to where an object is stored in the memory.
e. You may assign an int value to a reference variable.
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Section 9.5.3 Differences between Variables of Primitive Types and Reference Types
14. Analyze the following code:
public class Test {
public static void main(String[] args) {
double radius;
final double PI = 3.15169;
double area = radius * radius * PI;
System.out.println(“Area is ” + area);
}
}
a. The program has compile errors because the variable radius is not initialized.
b. The program has a compile error because a constant PI is defined inside a method.
c. The program has no compile errors but will get a runtime error because radius is not initialized.
d. The program compiles and runs fine.
Key:a radius is a local variable. It is used in radius * radius * PI, but not initialized.
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Section 9.6 Using Classes From the Java Library
18. Which of the following creates an object of the Date class:
A:
public class Test {
public Test() {
new java.util.Date();
}
}
B:
public class Test {
public Test() {
java.util.Date date = new java.util.Date();
}
}
a. A.
b. B.
c. Neither
d. Both A and B
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19. Which of the following statements are correct?
a. When creating a Random object, you have to specify the seed or use the default seed.
b. If two Random objects have the same seed, the sequence of the random numbers obtained from these two objects
are identical.
c. The nextInt() method in the Random class returns the next random int value.
d. The nextDouble() method in the Random class returns the next random double value.
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20. To obtain the distance between the points (40, 50) and (5.5, 4.4), use .
a. distance(40, 50, 5.5, 4.4)
b. new Point2D(40, 50).distance(5.5, 4.4)
c. new Point2D(40, 50).distance(new Point2D(5.5, 4.4))
d. new Point2D(5.5, 4.4).distance(40, 50)
e. new Point2D(5.5, 4.4).distance(new Point2D(40, 50))
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Section 9.7 Static Variables, Constants, and Methods
22. You should add the static keyword in the place of ? in Line in the following code:
Line 1 public class Test {
Line 2 private int age;
Line 3
Line 4 public ? int square(int n) {
Line 5 return n * n;
Line 6 }
Line 7
Line 8 public ? int getAge() {
Line 9 return age;
Line 10 }
Line 11 }
a. in line 4
b. in line 8
c. in both line 4 and line 8
d. none
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26. What is the output of the second println statement in the main method?
public class Foo {
int i;
static int s;
public static void main(String[] args) {
Foo f1 = new Foo();
System.out.println(“f1.i is ” + f1.i + ” f1.s is ” + f1.s);
Foo f2 = new Foo();
System.out.println(“f2.i is ” + f2.i + ” f2.s is ” + f2.s);
Foo f3 = new Foo();
System.out.println(“f3.i is ” + f3.i + ” f3.s is ” + f3.s);
}
public Foo() {
i++;
s++;
}
}
a. f2.i is 1 f2.s is 1
b. f2.i is 1 f2.s is 2
c. f2.i is 2 f2.s is 2
d. f2.i is 2 f2.s is 1
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28. What code may be filled in the blank without causing syntax or runtime errors:
public class Test {
java.util.Date date;
public static void main(String[] args) {
Test test = new Test();
System.out.println( );
}
}
a. test.date
b. date
c. test.date.toString()
d. date.toString()
Key:a b and d cause compile errors because date is an instance variable and cannot be accessed from static context. c
is wrong because test.date is null, causing NullPointerException.
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23. A method that is associated with an individual object is called .
a. a static method
b. a class method
c. an instance method
d. an object method
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24. To declare a constant MAX_LENGTH as a member of the class, you write
a. final static MAX_LENGTH = 99.98;
b. final static float MAX_LENGTH = 99.98;
c. static double MAX_LENGTH = 99.98;
d. final double MAX_LENGTH = 99.98;
e. final static double MAX_LENGTH = 99.98;
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25. Analyze the following code.
public class Test {
public static void main(String[] args) {
int n = 2;
xMethod(n);
System.out.println(“n is ” + n);
}
void xMethod(int n) {
n++;
}
}
a. The code has a compile error because xMethod does not return a value.
b. The code has a compile error because xMethod is not declared static.
c. The code prints n is 1.
d. The code prints n is 2.
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27. What is the output of the third println statement in the main method?
public class Foo {
int i;
static int s;
public static void main(String[] args) {
Foo f1 = new Foo();
System.out.println(“f1.i is ” + f1.i + ” f1.s is ” + f1.s);
Foo f2 = new Foo();
System.out.println(“f2.i is ” + f2.i + ” f2.s is ” + f2.s);
Foo f3 = new Foo();
System.out.println(“f3.i is ” + f3.i + ” f3.s is ” + f3.s);
}
public Foo() {
i++;
s++;
}
}
a. f3.i is 1 f3.s is 1
b. f3.i is 1 f3.s is 2
c. f3.i is 1 f3.s is 3
d. f3.i is 3 f3.s is 1
e. f3.i is 3 f3.s is 3
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29. Suppose the xMethod() is invoked in the following constructor in a class, xMethod() is in the class.
public MyClass() {
xMethod();
}
a. a static method
b. an instance method
c. a static method or an instance method
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30. Suppose the xMethod() is invoked from a main method in a class as follows, xMethod() is in the
class.
public static void main(String[] args) {
xMethod();
}
a. a static method
b. an instance method
c. a static method or an instance method
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21. Variables that are shared by every instance of a class are .
a. public variables
b. private variables
c. instance variables
d. class variables
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Section 9.8 Visibility Modifiers
32. Analyze the following code:
public class Test {
public static void main(String args[]) {
NClass nc = new NClass();
nc.t = nc.t++;
}
}
class NClass {
int t;
private NClass() {
}
}
a. The program has a compile error because the NClass class has a private constructor.
b. The program does not compile because the parameter list of the main method is wrong.
c. The program compiles, but has a runtime error because t has no initial value.
d. The program compiles and runs fine.
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33. Analyze the following code:
public class Test {
private int t;
public static void main(String[] args) {
int x;
System.out.println(t);
}
}
a. The variable t is not initialized and therefore causes errors.
b. The variable t is private and therefore cannot be accessed in the main method.
c. t is non-static and it cannot be referenced in a static context in the main method.
d. The variable x is not initialized and therefore causes errors.
e. The program compiles and runs fine.
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34. Analyze the following code and choose the best answer:
public class Foo {
private int x;
public static void main(String[] args) {
Foo foo = new Foo();
System.out.println(foo.x);
}
}
a. Since x is private, it cannot be accessed from an object foo.
b. Since x is defined in the class Foo, it can be accessed by any method inside the class without using an object. You
can write the code to access x without creating an object such as foo in this code.
c. Since x is an instance variable, it cannot be directly used inside a main method. However, it can be accessed through
an object such as foo in this code.
d. You cannot create a self–referenced object; that is, foo is created inside the class Foo.
non-static, it cannot be accessed in the main method without creating an object. (D) is incorrect, since it is permissible
to create an instance of the class within the class. The best choice is (C).
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31. To prevent a class from being instantiated,
a. don’t use any modifiers on the constructor.
b. use the public modifier on the constructor.
c. use the private modifier on the constructor.
d. use the static modifier on the constructor.
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Section 9.9 Data Field Encapsulation
37. Which is the advantage of encapsulation?
a. Only public methods are needed.
b. Making the class final causes no consequential changes to other code.
c. It enables changes to the implementation without changing a class’s contract and causes no consequential changes
to other code.
d. It enables changes to a class’s contract without changing the implementation and causes no consequential changes
to other code.
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35. Which of the following statements are true?
a. Use the private modifier to encapsulate data fields.
b. Encapsulating data fields makes the program easy to maintain.
c. Encapsulating data fields makes the program short.
d. Encapsulating data fields helps prevent programming errors.
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36. Suppose you wish to provide an accessor method for a boolean property finished, what should the signature of this
method?
a. public void getFinished()
b. public boolean getFinished()
c. public boolean isFinished()
d. public void isFinished()
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65. 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
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Section 9.10 Passing Objects to Methods
39. What is the value of myCount.count displayed?
public class Test {
public static void main(String[] args) {
Count myCount = new Count();
int times = 0;
for (int i = 0; i < 100; i++)
increment(myCount, times);
System.out.println(
“myCount.count = ” + myCount.count);
System.out.println(“times = “+ times);
}
public static void increment(Count c, int times) {
c.count++;
times++;
}
}
class Count {
int count;
Count(int c) {
count = c;
}
Count() {
count = 1;
}
}
a. 101
b. 100
c. 99
d. 98
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40. What is the value of times displayed?
public class Test {
public static void main(String[] args) {
Count myCount = new Count();
int times = 0;
for (int i = 0; i < 100; i++)
increment(myCount, times);
System.out.println(
“myCount.count = ” + myCount.count);
System.out.println(“times = “+ times);
}
public static void increment(Count c, int times) {
c.count++;
times++;
}
}
}
a.
1234567 1234567
b.
1234567 7654321
c.
7654321 1234567
d.
7654321 7654321
class Count {
int count;
Count(int c) {
count = c;
}
Count() {
count = 1;
}
}
a. 101
b. 100
c. 99
d. 98
e. 0
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41. What is the output of the following program?
import java.util.Date;
public class Test {
public static void main(String[] args) {
Date date = new Date(1234567);
m1(date);
System.out.print(date.getTime() + ” “);
m2(date);
System.out.println(date.getTime());
}
public static void m1(Date date) {
date = new Date(7654321);
}
public static void m2(Date date) {
date.setTime(7654321);
}
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38. When invoking a method with an object argument, is passed.
a. the contents of the object
b. a copy of the object
c. the reference of the object
d. the object is copied, then the reference of the copied object
Key: c See the Key Point.
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Section 9.11 Array of Objects
42. Given the declaration Circle[] x = new Circle[10], which of the following statement is most accurate?
a. x contains an array of ten int values.
b. x contains an array of ten objects of the Circle type.
c. x contains a reference to an array and each element in the array can hold a reference to a Circle object.
d. x contains a reference to an array and each element in the array can hold a Circle object.
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43. Assume java.util.Date[] dates = new java.util.Date[10], which of the following statements are true?
a. dates is null.
b. dates[0] is null.
c. dates = new java.util.Date[5] is fine, which assigns a new array to dates.
d. dates = new Date() is fine, which creates a new Date object and assigns to dates.
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Section 9.12 Immutable Objects and Classes
44. Which of the following statements are true about an immutable object?
a. The contents of an immutable object cannot be modified.
b. All properties of an immutable object must be private.
c. All properties of an immutable object must be of primitive types.
d. A readable object type property in an immutable object must also be immutable.
e. An immutable object contains no mutator methods.
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Section 9.13 Scope of Variables
45. What is the output for the first print statement in the main method?
public class Foo {
static int i = 0;
static int j = 0;
public static void main(String[] args) {
int i = 2;
int k = 3;
{
int j = 3;
System.out.println(“i + j is ” + i + j);
}
k = i + j;
System.out.println(“k is ” + k);
System.out.println(“j is ” + j);
}
}
d. i + j is 23
Key:d The first + operator in the expression “i + j is ” + i + j is evaluated.
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46. What is the output for the second print statement in the main method?
public class Foo {
static int i = 0;
static int j = 0;
public static void main(String[] args) {
int i = 2;
int k = 3;
{
int j = 3;
System.out.println(“i + j is ” + i + j);
}
k = i + j;
System.out.println(“k is ” + k);
System.out.println(“j is ” + j);
}
}
a. k is 0
b. k is 1
c. k is 2
d. k is 3
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47. What is the output for the third print statement in the main method?
public class Foo {
static int i = 0;
static int j = 0;
public static void main(String[] args) {
int i = 2;
int k = 3;
{
int j = 3;
System.out.println(“i + j is ” + i + j);
}
k = i + j;
System.out.println(“k is ” + k);
System.out.println(“j is ” + j);
}
}
a. j is 0
b. j is 1
c. j is 2
d. j is 3
the local variable ends at the end of the block before it is printed. So, when it is printed in the third print statement, the
static variable j is used.
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48. You can declare two variables with the same name in .
a. a method, one as a formal parameter and the other as a local variable
b. a block
c. two nested blocks in a method (two nested blocks means one being inside the other)
d. different methods in a class
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Section 9.14 The this Keyword
49. Analyze the following code:
class Circle {
private double radius;
public Circle(double radius) {
radius = radius;
}
}
a. The program has a compile error because it does not have a main method.
b. The program will compile, but you cannot create an object of Circle with a specified radius. The object will
always have radius 0.
c. The program has a compile error because you cannot assign radius to radius.
d. The program does not compile because Circle does not have a default constructor.
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Section 9.14.1 Using this to Reference Data Fileds
51. Which of the following can be placed in the blank line in the following code?
public class Test {
private int id;
public void m1() {
.id = 45;
}
}
a. this
b. Test
c. self
d. This
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Section 9.14.2 Using this to Invoke a Constructor
50. Analyze the following code:
class Test {
private double i;
public Test(double i) {
this.t();
this.i = i;
}
public Test() {
System.out.println(“Default constructor”);
this(1);
}
public void t() {
System.out.println(“Invoking t”);
}
}
a. this.t() may be replaced by t().
b. this.i may be replaced by i.
c. this(1) must be called before System.out.println(“Default constructor”).
d. this(1) must be replaced by this(1.0).