Chapter 7 - Understanding Code

Question 1

For Questions 17 and 18, consider that inside the class Sky, we have already coded the following:

public class Sky

{

private Color color;

public Sky( Color c )

{

color = c;

}

}

17. Consider the following method header:
public Color getColor( )
Is this method a constructor, mutator, or accessor?

Answer 1

• The method getColor doesn’t take any argument and returns the Color object
• So getColor method is an accessor method of Sky class.
• The getColor is not a constructor since constructor has same name as class name.
• The getColor is not mutator method since mutator methods takes arguments and sets value to
the corresponding field.
• Therefore, getColor is an accessor method of Sky class.

Question 2

For Questions 17 and 18, consider that inside the class Sky, we have already coded the following:

public class Sky

{

private Color color;

public Sky( Color c )

{

color = c;

}

}

18. Consider the following method header:
public void setColor( Color c )​
Is this method a constructor, mutator, or accessor?

Answer 2

• The method setColor takes Color argument and return void as return type.
• So setColor method is a mutator method of Sky class.
• The setColor is not a constructor since constructor has same name as class name.
• The setColor is not accessor method since accessor doesn’t take argument.
• Therefore, setColor is a mutator method of Sky class.

Question 3

For Questions 19 through 24, consider that the class Airplane has two methods with the following method headers; we also have a default constructor already coded.

public static double foo1( String s )
public String foo2( char c )

19&20. What is the return type of method foo1 and foo2?

Answer 3

foo1 = double
foo2 = String

Question 4

For Questions 19 through 24, consider that the class Airplane has two methods with the following method headers; we also have a default constructor already coded.

public static double foo1( String s )
public String foo2( char c )

23. Write a line or two of code to call method foo1 from a client class.

Answer 4

foo1 = class (since static used)
foo2 = instance (since static not used)

Question 5

For Questions 19 through 24, consider that the class Airplane has two methods with the following method headers; we also have a default constructor already coded.

public static double foo1( String s )
public String foo2( char c )

23. Write a line or two of code to call method foo1 from a client class..

Answer 5

double d = Airplane.foo1( “Boeing” );

Question 6

For Questions 19 through 24, consider that the class Airplane has two methods with the following method headers; we also have a default constructor already coded.

public static double foo1( String s )
public String foo2( char c )

24. Write a line or two of code to call method foo2 from a client class. Assume we have instantiated an object named a1.

Answer 6

String s = a1.foo2( ‘A’ );

Question 7

25. Inside method main, we see code like

Airplane.foo3( 34.6 );

From this, reconstruct the header of method foo3 (which belongs to the class Airplane); make appropriate assumptions if necessary.

Answer 7

• From the statement, it is said that the Airplane is the name of the class.
• The class name, Airplane itself calling the method foo3 with an argument of type 34.6.
• Only static member methods can be called without using class object.
• So, the method foo3 is static method and return type is void since it is not returning any value.

o static void foo3();

• The argument to the static method is 34.6 is a double type value.
• Therefore, argument type is double is passed to the static method foo3.

o static void foo3(double value);

• Since it is called directly by the class name, it has public access specifier.
• public static void foo3(double value);
• Finally, the method header foo3 is as follows

o public void static foo3(double value);

Question 8

26. Inside method main, we see code like

Airplane a = new Airplane( );

int n = a.foo4( “Hello” );

From this, reconstruct the header of method foo4 (which belongs to class Airplane).

Answer 8

public int foo4( String s );

Question 9

27. If you have defined the following enum constants

enum Seasons { Winter, Spring, Summer, Fall };

what is the output of the following code sequence?

System.out.println( Seasons.Spring.ordinal( ) );

Answer 9

Output of the statement: 1

Explanation:

• From the statement, Seasons is the name of enum constant.
• The Enum class name allows accessing the constants declared.
• By default , the enum class constant starts from 0 ,1,2 and continues

Until user specifically set value of constant in the enum class.

• So that the constants in the Seasons enum class starts from Winter=0,Spring=1,Summer=2 and Fall=3.
• The method ordinal that returns the integer constant value set to the Spring value.
• Therefore, the Spring constant variable returns 1 when called ordinal method on the enum class Seasons.

Question 10

28. Declare two instance variables: grade, which is an integer, and letterGrade, which is a char.

// declare grade here
// declare letterGrade here

Answer 10

int grade;
char letterGrade;

Question 11

29. Declare a class field for a federal tax rate, a constant, with value 0.07.

Answer 11

public static final double FEDERAL_TAX_RATE=0.07;

Question 12

For Questions 30 through 37, we will assume that class TelevisionChannel has three fields: name, a String; number, an integer; and cable, a boolean, which represents whether the channel is a cable channel.

30. Code a default constructor for that class: initialize the fields to an empty string, 0, and false, respectively.

Answer 12

public TelevisionChannel()

{

//set empty string to name variable

name=””;

//set 0 to number variable

number=0;

//set false to cable

cable=false;

}

Question 13

For Questions 30 through 37, we will assume that class TelevisionChannel has three fields: name, a String; number, an integer; and cable, a boolean, which represents whether the channel is a cable channel.

31. Code a constructor for that class that takes three parameters.

Answer 13

Assume the following TelevisionChannel class declaration

//TelevisionChannel.java

public class TelevisionChannel

{

//name of type string

private String name;

//number of type integer

private int number;

//cable of type boolean

private boolean cable;

}

Parameterized TelevisionChannel constructor of TelevisionChannel class

/*Parameter constructor that set values

to class TelevisionChannel fields */

public TelevisionChannel(String n, int num, boolean c)

{

//set n to name field

name=n;

//set num to number variable

number=num;

//set c to cable boolean variable

cable=c;

}

Question 14

For Questions 30 through 37, we will assume that class TelevisionChannel has three fields: name, a String; number, an integer; and cable, a boolean, which represents whether the channel is a cable channel.

37. Code a method returning the word cable if the current object represents a cable channel and returning the word network if the current object does not represent a cable channel.

Answer 14

public String isCableOrNetwork()

{

//using conditional operator, ?:

//checking cable boolean value is true, return cable

//otherwise return Network

return cable==true? “Cable”: “Network”;

}

Question 15

For Questions 30 through 37, we will assume that class TelevisionChannel has three fields: name, a String; number, an integer; and cable, a boolean, which represents whether the channel is a cable channel.

36. Code a method returning the number of digits in the channel number. For instance, if the channel number is 21, the method returns 2; if the channel number is 412, the method returns 3.

Answer 15

public int numDigits()

{

int temp=number;

int count=0;

while(temp!=0)

{

count++;

temp=temp/10;

}

return count;

}

Question 16

For Questions 30 through 37, we will assume that class TelevisionChannel has three fields: name, a String; number, an integer; and cable, a boolean, which represents whether the channel is a cable channel.

35. Code the equals method.

Answer 16

public boolean equals(Object object)

{

//cast object to TelevisionChannel

TelevisionChannel other=(TelevisionChannel)object;

//Returns true if other object is contains same values

//instance fields of this class.

return name.equals(other.getName()) &&
number==other.getNumber() &&
cable==other.getCable();

}

Question 17

For Questions 30 through 37, we will assume that class TelevisionChannel has three fields: name, a String; number, an integer; and cable, a boolean, which represents whether the channel is a cable channel.

34. Code the toString method.

Answer 17

public String toString()

{

return “Name : “+name

+” Number : “+number

+” Cable : “+cable;

}

Question 18

For Questions 30 through 37, we will assume that class TelevisionChannel has three fields: name, a String; number, an integer; and cable, a boolean, which represents whether the channel is a cable channel.

33. Code the three mutators for that class.

Answer 18

public void setName(String n) { name=n;}

public void setNumber(int num) {number=num;}

public void setCable(boolean c) {cable=c;}