3Introduction to Classes,
Objects, Methods and
Strings
Objectives
In this chapter you’ll:
■Declare a class and use it to
create an object.
■Implement a class’s behaviors
as methods.
■Implement a class’s attributes
as instance variables.
■Call an object’s methods to
make them perform their
tasks.
Self-Review Exercises 2
Self-Review Exercises
3.1 Fill in the blanks in each of the following:
a) Each class declaration that begins with keyword must be stored in a file that
has exactly the same name as the class and ends with the .java filename extension.
b) Keyword in a class declaration is followed immediately by the class’s name.
c) Keyword requests memory from the system to store an object, then calls the
corresponding class’s constructor to initialize the object.
d) Each parameter must specify both a(n) and a(n) .
e) By default, classes that are compiled in the same directory are considered to be in the
same package, known as the .
f) Java provides two primitive types for storing floating-point numbers in memory:
and .
g) Variables of type double represent floating-point numbers.
h) Scanner method returns a double value.
j) Return type indicates that a method will not return a value.
k) Scanner method reads characters until it encounters a newline character,
then returns those characters as a String.
l) Class String is in package .
m) A(n) is not required if you always refer to a class with its fully qualified class
name.
n) A(n) is a number with a decimal point, such as 7.33, 0.0975 or 1000.12345.
o) Variables of type float represent _ -precision floating-point numbers.
p) The format specifier is used to output values of type float or double.
q) Types in Java are divided into two categories— types and types.
3.2 State whether each of the following is true or false. If false, explain why.
a) By convention, method names begin with an uppercase first letter, and all subsequent
words in the name begin with a capital first letter.
b) An import declaration is not required when one class in a package uses another in the
same package.
3Chapter 3 Introduction to Classes, Objects, Methods and Strings
c) Empty parentheses following a method name in a method declaration indicate that the
method does not require any parameters to perform its task.
d) A primitive-type variable can be used to invoke a method.
e) Variables declared in the body of a particular method are known as instance variables
and can be used in all methods of the class.
f) Every method’s body is delimited by left and right braces ({ and }).
g) Primitive-type local variables are initialized by default.
h) Reference-type instance variables are initialized by default to the value null.
i) Any class that contains public static void main(String[] args) can be used to exe–
cute an app.
j) The number of arguments in the method call must match the number of parameters in
the method declaration’s parameter list.
k) Floating-point values that appear in source code are known as floating-point literals and
are type float by default.
3.3 What is the difference between a local variable and an instance variable?
3.4 Explain the purpose of a method parameter. What is the difference between a parameter
and an argument?
Exercises
NOTE: Solutions to the programming exercises are located in the ch03solutions folder.
Each exercise has its own folder named ex03_## where ## is a two-digit number represent-
ing the exercise number. For example, exercise 3.11’s solution is located in the folder ex-
03_11.
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3.5 (Keyword new) What’s the purpose of keyword new? Explain what happens when you use it.
3.6 (Default Constructors) What is a default constructor? How are an object’s instance variables
initialized if a class has only a default constructor?
3.7 (Instance Variables) Explain the purpose of an instance variable.
3.8 (Using Classes without Importing Them) Most classes need to be imported before they can
be used in an app. Why is every app allowed to use classes System and String without first importing
them?
3.9 (Using a Class without Importing It) Explain how a program could use class Scanner with-
out importing it.
3.10 (set and get Methods) Explain why a class might provide a set method and a get method for
an instance variable.