Deck 4: Writing Classes

Question 1

Consider a sequence of method invocations as follows: main calls m1, m1 calls m2, m2 calls m3 and then m2 calls m4, m3 calls m5. If m4 has just terminated, what method will resume execution?

A) m1
B) m2
C) m3
D) m5
E) main

Answer 1

B
Explanation: B) Once a method terminates, control resumes with the method that called that method. In this case, m2 calls m4, so that when m4 terminates, m2 is resumed.

Question 2

Instance data for a Java class

A) are limited to primitive types (e.g., int, float, char)
B) are limited to Strings
C) are limited to objects(e.g., Strings, classes defined by other programmers)
D) may be primitive types or objects, but objects must be defined to be private
E) may be primitive types or objects

Answer 2

E
Explanation: E) The instance data are the entities that make up the class and may be any type available whether primitive or object, and may be public or private. By using objects as instance data, it permits the class to be built upon other classes. This relationship where a class has instance data that are other classes is known as a has-a relationship.

Question 3

Consider a Rational class designed to represent rational numbers as a pair of int's, along with methods reduce (to reduce the rational to simplest form), gcd (to find the greatest common divisor of two int's), as well as methods for addition, subtraction, multiplication, and division. Why should the reduce and gcd methods be declared to be private.

A) Because they will never be used
B) Because they will only be called from methods inside of Rational
C) Because they will only be called from the constructor of Rational
D) Because they do not use any of Rational's instance data
E) Because it is a typo and they should be declared as public

Answer 3

B
Explanation: B) All items of a class that are declared to be private are only accessible to entities within that class, whether they are instance data or methods. In this case, since these two methods are only called from other methods (including the constructor) of Rational, they are declared private to promote information hiding to a greater degree. Note that answer C is not a correct answer because the reduce method calls the gcd method, so one of the methods is called from a method other than the constructor.

Question 4

To define a class that will represent a car, which of the following definitions is most appropriate?

A) private class car
B) public class car
C) public class Car
D) public class CAR
E) private class Car

Question 5

For the questions below, use the following class definition.
import java.text.DecimalFormat;
public class Student
{
private String name;
private String major;
private double gpa;
private int hours;
public Student(String newName, String newMajor, double newGPA, int newHours)
{
name = newName;
major = newMajor;
gpa = newGPA;
hours = newHours;
}
public String toString( )
{
DecimalFormat df = new DecimalFormat("xxxx"); // xxxx needs to be replaced
return name + "\n" + major + "\n" + df.format(gpa) + "\n" + hours
}
}
Assume that another method has been defined that will compute and return the student's class rank (Freshman, Sophomore, etc). It is defined as: public String getClassRank( )
Given that s1 is a student, which of the following would properly be used to get s1's class rank?

A) s1 = getClassRank( );
B) s1.toString( );
C) s1.getHours( );
D) s1.getClassRank( );
E) getClassRank(s1);

Question 6

A class' constructor usually defines

A) how an object is initialized
B) how an object is interfaced
C) the number of instance data in the class
D) the number of methods in the class
E) if the instance data are accessible outside of the object directly

Question 7

If a method does not have a return statement, then

A) it will produce a syntax error when compiled
B) it must be a void method
C) it can not be called from outside the class that defined the method
D) it must be defined to be a public method
E) it must be an int, double, float or String method

Question 8

Which of the following Applet methods is called automatically when the applet is first loaded?

A) Applet (the Applet's constructor)
B) init
C) start
D) getCodeBase
E) getImage

Question 9

For the questions below, use the following class definition.
import java.text.DecimalFormat;
public class Student
{
private String name;
private String major;
private double gpa;
private int hours;
public Student(String newName, String newMajor, double newGPA, int newHours)
{
name = newName;
major = newMajor;
gpa = newGPA;
hours = newHours;
}
public String toString( )
{
DecimalFormat df = new DecimalFormat("xxxx"); // xxxx needs to be replaced
return name + "\n" + major + "\n" + df.format(gpa) + "\n" + hours
}
}
Which of the following patterns should be used in place of "xxxx" when instantiating df so that the gpa to be output in typical form (like 3.810)?

A) "#.###"
B) "#.0"
C) "0.# "
D) "0.000"
E) "0.0##"

Question 10

Having multiple class methods of the same name where each method has a different number of or type of parameters is known as

A) encapsulation
B) information hiding
C) tokenizing
D) importing
E) method overloading

Question 11

StringTokenizer is a class in the java.util library that can divide a String based on some delimiter String (a delimiter is a separator). If the instruction StringTokener st = new StringTokenizer(str, "&&"); is executed, where str is some String, then st divides up str into separate Strings whenever

A) a blank space is found
B) two blank spaces are found
C) a single ampersand ("&") is found
D) two ampersands are found
E) two ampersands or the substring "and" or "AND" is found

Question 12

A variable whose scope is restricted to the method where it was declared is known as a(n)

A) parameter
B) global variable
C) local variable
D) public instance data
E) private instance data

Question 13

Another method that might be desired is one that updates the Student's number of credit hours. This method will receive a number of credit hours and add these to the Student's current hours. Which of the following methods would accomplish this?

A) public int updateHours( ) { return hours; }
B) public void updateHours( ) { hours++; }
C) public updateHours(int moreHours) { hours += moreHours; }
D) public void updateHours(int moreHours) { hours += moreHours; }
E) public int updateHours(int moreHours) { return hours + moreHours; }

Question 14

Consider a method defined with the header: public void foo(int a, int b). Which of the following method calls is legal?

A) foo(0, 0.1);
B) foo(0 / 1, 2 * 3);
C) foo(0);
D) foo( );
E) foo(1 + 2, 3 * 0.1);

Question 15

The behavior of an object is defined by the object's

A) instance data
B) constructor
C) visibility modifiers
D) methods
E) all of the above

Question 16

The relationship between a class and an object is best described as

A) classes are instances of objects
B) objects are instances of classes
C) objects and classes are the same thing
D) classes are programs while objects are variables
E) objects are the instance data of classes

Question 17

Which of the following reserved words in Java is used to create an instance of a class?

A) class
B) public
C) public or private, either could be used
D) import
E) new

Question 18

An example of passing message to a String where the message has a String parameter would occur in which of the following messages?

A) length
B) substring
C) equals
D) toUpperCase
E) none of the above, it is not possible to pass a String as a parameter in a message to a String

Question 19

For the questions below, use the following class definition.
import java.text.DecimalFormat;
public class Student
{
private String name;
private String major;
private double gpa;
private int hours;
public Student(String newName, String newMajor, double newGPA, int newHours)
{
name = newName;
major = newMajor;
gpa = newGPA;
hours = newHours;
}
public String toString( )
{
DecimalFormat df = new DecimalFormat("xxxx"); // xxxx needs to be replaced
return name + "\n" + major + "\n" + df.format(gpa) + "\n" + hours
}
}
Which of the following could be used to instantiate a new Student s1?

A) Student s1 = new Student( );
B) s1 = new Student( );
C) Student s1 = new Student("Jane Doe", "Computer Science", 3.333, 33);
D) new Student s1 = ("Jane Doe", "Computer Science", 3.333, 33);
E) new Student(s1);

Question 20

In order to preserve encapsulation of an object, we would do all of the following except for which one?

A) Make the instance data private
B) Define the methods in the class to access and manipulate the instance data
C) Make the methods of the class public
D) Make the class final
E) All of the above preserve encapsulation

Question 21

A GUI container is an object that defines a screen element.

Question 22

A constructor may contain a return statement so long as no value (or expression) is returned.

Question 23

Formal parameters are those that appear in the method call and actual parameters are those that appear in the method header.

Question 24

The interface of a class is based on those data instances and methods that are declared public.

Question 25

Every class definition must include a constructor.

Question 26

Defining formal parameters requires including each parameters type.

Question 27

What happens if you declare a class constructor to have a void return type?

A) You'll likely receive a syntax error
B) The program will compile with a warning, but you'll get a runtime error
C) There's nothing wrong with declaring a constructor to be void
D) The class' default constructor will be used instead of the one you're declaring
E) None of the above

Question 28

Java methods can return more than one item if they are modified with the reserved word continue, as in public continue int foo( ) { ... }

Question 29

The following method header definition will result in a syntax error: public void aMethod( );

Question 30

The expressions that are passed to a method in an invocation are called

A) actual parameters
B) formal parameters
C) formal arguments
D) formals
E) any of the above

Question 31

All Java classes must contain a main method which is the first method executed when the Java class is called upon.

Question 32

In a UML diagram for a class

A) classes are represented as rectangles
B) there may be a section containing the name of the class
C) there may be a section containing the attributes (data) of the class
D) there may be a section containing the methods of the class
E) all of the above

Question 33

Java methods can return only primitive types (int, double, float, char, boolean, etc).

Question 34

The software failure at the Denver International Airport's baggage handling system is a good example of

A) how a large system can be obsolete by the time it is developed
B) how designers sometimes have too much faith in the technology they are using
C) how failures are often a result of multiple variables caused by a system as a whole in actual operation
D) how the use of a centralized computer is really unrealistic in today's distributed processing
E) all of the above

Question 35

Listener objects "wait" for an event to occur.

Question 36

A method defined in a class can access the class' instance data without needing to pass them as parameters or declare them as local variables.

Question 37

Visibility modifiers include

A) public, private
B) public, private, protected
C) public, private, protected, final
D) public, protected, final, static
E) public, private, protected, static

Question 38

What are the objects needed in order to create a Java GUI (graphical user interface)?

A) components
B) events
C) listeners
D) all of the above
E) only classes from AWT and/or Swing are needed

Question 39

Consider a method defined with the header: public void doublefoo(double x). Which of the following method calls is legal?

A) doublefoo(0);
B) doublefoo(0.555);
C) doublefoo(0.1 + 0.2);
D) doublefoo(0.1, 0.2);
E) all of the above are legal except for D

Question 40

While multiple objects of the same class can exist, in a given program there can be only one version of each class.

Question 41

Write the statement to instantiate an Box object, blueBox, with a length of 6, width of 4, and height of 2.

Question 42

An Employee will have a name, social security number, position, and hourly wages. Define the instance data for this class.

Question 43

Write the constructor, which is passed the player's name and position.

Question 44

Write a statement to output the volume of the blue box.

Question 45

Accessors and mutators provide mechanisms for controlled access to a well-encapsulated class.

Question 46

Write a statement to instantiate a cube with a length of 3, width of 3, and height of 3.

Question 47

Write the constructor for this class.

Question 48

When reasoning about a car, we use such information as its make and model, year, color, cost (or worth), number of miles, gas mileage, insurance cost, and so forth. Consider a program that will attempt to determine how practical a car might be on a road trip. Which of these types of information might you use in your program as instance data?
Provide a justification for your selection.

Question 49

Assume method0 calls method1 and method2, method1 calls method3 which calls method4 and method5. Explain the chain of method calls (what order methods are called and from which method).

Question 50

Consider the "Push Me!" program described in section 4.7. As given the push counter starts at zero and is incremented each time the "Push Me!" button is pressed. Let's say that you want to add a class called ResetListener that resets the count to zero. Write the ResetListener class.

Question 51

Explain why it would be a poor decision to make instance data public instead of private. Give an example of why this could result in a problem.

Question 52

Write a toString method that returns the player's name, position and batting average formatted to have 1 digit to the left of the decimal and 3 digits to the right of the decimal. Assume DecimalFormat has been imported.

Question 53

What visibility modifiers would you use for the methods that move the elevator up one floor, move the elevator down one floor, that request the elevator, and that start the elevator moving?

Question 54

Add a constructor to class Box for a cube, which has only 1 parameter, side.

Question 55

An object should be encapsulated in order to guard its data and methods from inappropriate access.

Question 56

Regarding the software failure described at the Denver International Airport, it is critical to factor in errors and inefficiencies that always occur in a complex system.