Answers to Self-Review Exercises

a) -d . b) format . c) shadows. d) finalize . e) single-type-import. f) default constructor. g) toString . h) accessor methods, query methods . i) predicate. j) values . k) has-a . l) enum . m) static . n) single static import. o) principle of least privilege. p) final . q) abstract data type (ADT). r) package declaration. s) type-import-on-demand. t) class loader. u) -classpath , CLASSPATH . v) mutator methods. w) static import on demand. x) public services, public interface. y) gc . z) consistent data.

Explain the notion of package access in Java. Explain the negative aspects of package access.

What happens when a return type, even void, is specified for a constructor?

(Rectangle Class) Create a class Rectangle . The class has attributes length and width , each of which defaults to 1. It has methods that calculate the perimeter and the area of the rectangle. It has set and get methods for both length and width . The set methods should verify that length and width are each floating-point numbers larger than 0.0 and less than 20.0. Write a program to test class Rectangle .

(Modifying the Internal Data Representation of a Class) It would be perfectly reasonable for the Time2 class of Fig. 8.5 to represent the time internally as the number of seconds since midnight rather than the three integer values hour , minute and second . Clients could use the same public methods and get the same results. Modify the Time2 class of Fig. 8.5 to implement the Time2 as the number of seconds since midnight and show that no change is visible to the clients of the class.

(Savings Account Class) Create class SavingsAccount . Use a static variable annualInterestRate to store the annual interest rate for all account holders. Each object of the class contains a private instance variable savingsBalance indicating the amount the saver currently has on deposit. Provide method calculateMonthlyInterest to calculate the monthly interest by multiplying the savingsBalance by annualInterestRate divided by 12this interest should be added to savingsBalance . Provide a static method modifyInterestRate that sets the annualInterestRate to a new value. Write a program to test class SavingsAccount . Instantiate two savingsAccount objects, saver1 and saver2 , with balances of $2000.00 and $3000.00, respectively. Set annualInterestRate to 4%, then calculate the monthly interest and print the new balances for both savers. Then set the annualInterestRate to 5%, calculate the next month's interest and print the new balances for both savers.

(Enhancing Class Time2 ) Modify class Time2 of Fig. 8.5 to include a tick method that increments the time stored in a Time2 object by one second. Provide method incrementMinute to increment the minute and method incrementHour to increment the hour. The Time2 object should always remain in a consistent state. Write a program that tests the tick method, the increment-Minute method and the incrementHour method to ensure that they work correctly. Be sure to test the following cases:

1. incrementing into the next minute,

2. incrementing into the next hour and

3. incrementing into the next day (i.e., 11:59:59 PM to 12:00:00 AM).

(Enhancing Class Date) Modify class Date of Fig. 8.7 to perform error checking on the initializer values for instance variables month, day and year (currently it validates only the month and day). Provide a method nexTDay to increment the day by one. The Date object should always remain in a consistent state. Write a program that tests the nexTDay method in a loop that prints the date during each iteration of the loop to illustrate that the nexTDay method works correctly. Test the following cases:

1. incrementing into the next month and

2. incrementing into the next year.

(Returning Error Indicators from Methods) Modify the set methods in class Time2 of Fig. 8.5 to return appropriate error values if an attempt is made to set one of the instance variables hour, minute or second of an object of class Time to an invalid value. [ Hint: Use boolean return types on each method.] Write a program that tests these new set methods and outputs error messages when incorrect values are supplied.

Rewrite Fig. 8.14 to use a separate import declaration for each static member of class Math that is used in the example.

Write an enum type TRafficLight , whose constants ( RED, GREEN, YELLOW ) take one parameterthe duration of the light. Write a program to test the trafficLight enum so that it displays the enum constants and their durations.

(Complex Numbers) Create a class called Complex for performing arithmetic with complex numbers. Complex numbers have the form

where i is

Write a program to test your class. Use floating-point variables to represent the private data of the class. Provide a constructor that enables an object of this class to be initialized when it is declared. Provide a no-argument constructor with default values in case no initializers are provided. Provide public methods that perform the following operations:

1. Add two Complex numbers: The real parts are added together and the imaginary parts are added together.

2. Subtract two Complex numbers: The real part of the right operand is subtracted from the real part of the left operand, and the imaginary part of the right operand is subtracted from the imaginary part of the left operand.

3. Print Complex numbers in the form ( a, b ), where a is the real part and b is the imaginary part.

(Date and Time Class) Create class DateAndTime that combines the modified Time2 class of Exercise 8.7 and the modified Date class of Exercise 8.8. Modify method incrementHour to call method nextday if the time is incremented into the next day. Modify methods toStandardString and toUniversalString to output the date in addition to the time. Write a program to test the new class DateAndTime . Specifically, test incrementing the time to the next day.

(Enhanced Rectangle Class) Create a more sophisticated Rectangle class than the one you created in Exercise 8.4. This class stores only the Cartesian coordinates of the four corners of the rectangle. The constructor calls a set method that accepts four sets of coordinates and verifies that each of these is in the first quadrant with no single x- or y -coordinate larger than 20.0. The set method also verifies that the supplied coordinates specify a rectangle. Provide methods to calculate the length , width , perimeter and area . The length is the larger of the two dimensions. Include a predicate method isSquare which determines whether the rectangle is a square. Write a program to test class Rectangle .

(Set of Integers) Create class IntegerSet . Each IntegerSet object can hold integers in the range 0100. The set is represented by an array of boolean s. Array element a[i] is true if integer i is in the set. Array element a[j] is false if integer j is not in the set. The no-argument constructor initializes the Java array to the "empty set" (i.e., a set whose array representation contains all false values).

Provide the following methods: Method union creates a third set that is the set- theoretic union of two existing sets (i.e., an element of the third set's array is set to TRue if that element is true in either or both of the existing setsotherwise, the element of the third set is set to false ). Method intersection creates a third set which is the set-theoretic intersection of two existing sets (i.e., an element of the third set's array is set to false if that element is false in either or both of the existing setsotherwise, the element of the third set is set to true ). Method insertElement inserts a new integer k into a set (by setting a[k] to true ). Method deleteElement deletes integer m (by setting a[m] to false ). Method toSetString returns a string containing a set as a list of numbers separated by spaces. Include only those elements that are present in the set. Use --- to represent an empty set. Method isEqualTo determines whether two sets are equal. Write a program to test class IntegerSet . Instantiate several IntegerSet objects. Test that all your methods work properly.

(Date Class) Create class Date with the following capabilities:

1. Output the date in multiple formats, such as

   MM/DD/YYYY
    June 14, 1992
    DDD YYYY
   

2. Use overloaded constructors to create Date objects initialized with dates of the formats in part (a). In the first case the constructor should receive three integer values. In the second case it should receive a String and two integer values. In the third case it should receive two integer values, the first of which represents the day number in the year. [ Hint: To convert the string representation of the month to a numeric value, compare strings using the equals method. For example, if s1 and s2 are strings, the method call s1.equals( s2 ) returns true if the strings are identical and otherwise returns false .]

(Rational Numbers) Create a class called Rational for performing arithmetic with fractions. Write a program to test your class. Use integer variables to represent the private instance variables of the classthe numerator and the denominator . Provide a constructor that enables an object of this class to be initialized when it is declared. The constructor should store the fraction in reduced form. The fraction

2/4

is equivalent to 1/2 and would be stored in the object as 1 in the numerator and 2 in the denominator . Provide a no-argument constructor with default values in case no initializers are provided. Provide public methods that perform each of the following operations:

1. Add two Rational numbers: The result of the addition should be stored in reduced form.

2. Subtract two Rational numbers: The result of the subtraction should be stored in reduced form.

3. Multiply two Rational numbers: The result of the multiplication should be stored in reduced form.

4. Divide two Rational numbers: The result of the division should be stored in reduced form.

5. Print Rational numbers in the form a/b , where a is the numerator and b is the denominator .

6. Print Rational numbers in floating-point format. (Consider providing formatting capabilities that enable the user of the class to specify the number of digits of precision to the right of the decimal point.)

(Huge Integer Class) Create a class HugeInteger which uses a 40-element array of digits to store integers as large as 40 digits each. Provide methods input , output , add and subtract . For comparing HugeInteger objects, provide the following methods: isEqualTo , isNotEqualTo , isGreaterThan , isLessThan , isGreaterThanOrEqualTo and isLessThanOrEqualTo . Each of these is a predicate method that returns true if the relationship holds between the two HugeInteger objects and returns false if the relationship does not hold. Provide a predicate method isZero . If you feel ambitious, also provide methods multiply , divide and remainder . [ Note: Primitive boolean values can be output as the word "true" or the word "false" with format specifier %b .]

(Tic-Tac-Toe) Create a class TicTacToe that will enable you to write a complete program to play the game of Tic-Tac-Toe. The class contains a private 3-by-3 two-dimensional array of integers. The constructor should initialize the empty board to all zeros. Allow two human players. Wherever the first player moves, place a 1 in the specified square, and place a 2 wherever the second player moves. Each move must be to an empty square. After each move, determine whether the game has been won and whether it is a draw. If you feel ambitious, modify your program so that the computer makes the moves for one of the players. Also, allow the player to specify whether he or she wants to go first or second. If you feel exceptionally ambitious, develop a program that will play three-dimensional Tic-Tac-Toe on a 4-by-4-by-4 board [ Note: This is a challenging project that could take many weeks of effort!].