Wrap-Up

Answers to Self Review Exercises

3.1

a) object. b) public. c) class. d) new. e) type, name. f) default package. g) instance variable. h) float, double. i) double-precision. j) nexTDouble. k) access modifier. l) void. m) nextLine. n) java.lang. o) import declaration. p) floating-point number. q) single-precision. r) %f. s) primitive, reference.
3.2

a) False. By convention, method names begin with a lowercase first letter and all subsequent words in the name begin with a capital first letter. b) True. c) True. d) True. e) False. A primitive-type variable cannot be used to invoke a methoda reference to an object is required to invoke the object's methods. f) False. Such variables are called local variables and can be used only in the method in which they are declared. g) True. h) False. Primitive-type instance variables are initialized by default. i) True. j) True. k) True. l) False. Such literals are of type double by default.
 
3.3

A local variable is declared in the body of a method and can be used only from the point at which it is declared through the end of the method declaration. A field is declared in a class, but not in the body of any of the class's methods. Every object (instance) of a class has a separate copy of the class's fields. Also, fields are accessible to all methods of the class. (We will see an exception to this in Chapter 8, Classes and Objects: A Deeper Look.)
3.4

A parameter represents additional information that a method requires to perform its task. Each parameter required by a method is specified in the method's declaration. An argument is the actual value for a method parameter. When a method is called, the argument values are passed to the method so that it can perform its task.

Exercises

3.5

What is the purpose of keyword new? Explain what happens when this keyword is used in an application.
3.6

What is a default constructor? How are an object's instance variables initialized if a class has only a default constructor?
3.7

Explain the purpose of an instance variable.
3.8

Most classes need to be imported before they can be used in an application. Why is every application allowed to use classes System and String without first importing them?
3.9

Explain how a program could use class Scanner without importing the class from package java.util.
3.10

Explain why a class might provide a set method and a get method for an instance variable.
3.11

Modify class GradeBook (Fig. 3.10) as follows:

  1. Include a second String instance variable that represents the name of the course's instructor.

  2. Provide a set method to change the instructor's name and a get method to retrieve it.

  3. Modify the constructor to specify two parametersone for the course name and one for the instructor's name.

  4. Modify method displayMessage such that it first outputs the welcome message and course name, then outputs "This course is presented by: " followed by the instructor's name.

Use your modified class in a test application that demonstrates the class's new capabilities.
3.12

Modify class Account (Fig. 3.13) to provide a method called debit that withdraws money from an Account. Ensure that the debit amount does not exceed the Account's balance. If it does, the balance should be left unchanged and the method should print a message indicating "Debit amount exceeded account balance." Modify class AccountTest (Fig. 3.14) to test method debit.
3.13

Create a class called Invoice that a hardware store might use to represent an invoice for an item sold at the store. An Invoice should include four pieces of information as instance variablesa part number (type String), a part description (type String), a quantity of the item being purchased (type int) and a price per item (double). Your class should have a constructor that initializes the four instance variables. Provide a set and a get method for each instance variable. In addition, provide a method named getInvoiceAmount that calculates the invoice amount (i.e., multiplies the quantity by the price per item), then returns the amount as a double value. If the quantity is not positive, it should be set to 0. If the price per item is not positive, it should be set to 0.0. Write a test application named InvoiceTest that demonstrates class Invoice's capabilities.
3.14

Create a class called Employee that includes three pieces of information as instance variablesa first name (type String), a last name (type String) and a monthly salary (double). Your class should have a constructor that initializes the three instance variables. Provide a set and a get method for each instance variable. If the monthly salary is not positive, set it to 0.0. Write a test application named EmployeeTest that demonstrates class Employee's capabilities. Create two Employee objects and display each object's yearly salary. Then give each Employee a 10% raise and display each Employee's yearly salary again.
3.15

Create a class called Date that includes three pieces of information as instance variablesa month (type int), a day (type int) and a year (type int). Your class should have a constructor that initializes the three instance variables and assumes that the values provided are correct. Provide a set and a get method for each instance variable. Provide a method displayDate that displays the month, day and year separated by forward slashes (/). Write a test application named DateTest that demonstrates class Date's capabilities.
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