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Program Elements -- Introduction
• We can now examine the core elements of programming
• Chapter 3 focuses on:
–
–
–
–
–
Chapter 3
data types
variable declaration and use
operators and expressions
decisions and loops
input and output
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Primitive Data Types
• A data type is defined by a set of values and the
operators you can perform on them
• Each value stored in memory is associated with a
particular data type
• The Java language has several predefined types, called
primitive data types
• The following reserved words represent eight different
primitive types:
– byte, short, int, long, float, double, boolean, char
Chapter 3
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Variables
• A variable is an identifier that represents a location in
memory that holds a particular type of data
• Variables must be declared before they can be used
• The syntax of a variable declaration is:
data-type variable-name;
• For example:
int total;
Chapter 3
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Variables
• Multiple variables can be declared on the same line:
int total, count, sum;
• Variables can be initialized (given an initial value) in the
declaration:
int total = 0, count = 20;
float unit_price = 57.25;
• See Piano_Keys.java
Chapter 3
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Assignment Statements
• An assignment statement takes the following form:
variable-name = expression;
• The expression is evaluated and the result is stored in the
variable, overwriting the value currently stored in the
variable
• See United_States.java
• The expression can be a single value or a more
complicated calculation
Chapter 3
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Constants
• A constant is similar to a variable except that they keep
the same value throughout their existence
• They are specified using the reserved word final in
the declaration
• For example:
final double PI = 3.14159;
final int STUDENTS = 25;
Chapter 3
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Constants
• When appropriate, constants are better than variables
because:
– they prevent inadvertent errors because their value cannot
change
• They are better than literal values because:
– they make code more readable by giving meaning to a value
– they facilitate change because the value is only specified in one
place
Chapter 3
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Input and Output
• Java I/O is based on input streams and output streams
• There are three predefined standard streams:
Stream
Purpose
Default Device
System.in
System.out
System.err
reading input
writing output
writing errors
keyboard
monitor
monitor
• The print and println methods write to standard
output
Chapter 3
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Escape Sequences
• See Echo.java
• An escape sequence is a special sequence of characters
preceded by a backslash (\)
• They indicate some special purpose, such as:
Chapter 3
Escape Sequence
Meaning
\t
\n
\"
\'
\\
tab
new line
double quote
single quote
backslash
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Numeric Input
• Converting a string that holds an integer into the integer
value can be done with a method in the Integer
wrapper class:
value = Integer.parseInt (my_string);
• A value can be read and converted in one line:
num = Integer.parseInt (stdin.readLine());
• See Addition.java and Addition2.java
Chapter 3
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Program Development
• The creation of software involves four basic activities:
–
–
–
–
establishing the requirements
creating a design
implementing the code
testing the implementation
• The development process is much more involved that
this, but these basic steps are a good starting point
Chapter 3
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Requirements
• Requirements specify the tasks a program must
accomplish (what to do, not how to do it)
• They often address the user interface
• An initial set of requirements are often provided, but
usually must be critiqued, modified, and expanded
• It is often difficult to establish detailed, unambiguous,
complete requirements
• Careful attention to the requirements can save significant
time and money in the overall project
Chapter 3
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Design
• A program follows an algorithm, which is a step-by-step
process for solving a problem
• The design specifies the algorithms and data needed
• In object-oriented development, it establishes the classes,
objects, and methods that are required
• The details of a method may be expressed in pseudocode,
which is code-like, but does not necessarily follow any
specific syntax
Chapter 3
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Implementation
• Implementation is the process of translating a design into
source code
• Most novice programmers think that writing code is the
heart of software development, but it actually should be
the least creative
• Almost all important decisions are made during
requirements analysis and design
• Implementation should focus on coding details, including
style guidelines and documentation
Chapter 3
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Testing
• A program should be executed multiple times with
various input in an attempt to find errors
• Debugging is the process of discovering the cause of a
problem and fixing it
• Programmers often erroneously think that there is "only
one more bug" to fix
• Tests should focus on design details as well as overall
requirements
Chapter 3
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Program Development
• See Average.java
• Follow the process of requirements analysis, design,
implementation, and testing
• There are always multiple ways to design and implement
a program
• Any design has advantages and disadvantages; there are
always trade-offs
• See Average2.java
Chapter 3
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Objects and Classes -- Introduction
• Now that some low-level programming concepts have
been established, we can examine objects in more detail
• Chapter 4 focuses on:
–
–
–
–
–
–
–
–
Chapter 4
the concept of objects
the use of classes to create objects
using predefined classes
defining methods and passing parameters
defining classes
visibility modifiers
static variables and methods
method overloading
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Objects
• An object has:
– state - descriptive characteristics
– behaviors - what it can do (or be done to it)
• For example, a particular bank account
–
–
–
–
Chapter 4
has an account number
has a current balance
can be deposited into
can be withdrawn from
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Classes
• A class is a blueprint of an object
• It is the model or pattern from which objects are created
• A class defines the methods and types of data associated
with an object
• Creating an object from a class is called instantiation; an
object is an instance of a particular class
• For example, the Account class could describe many
bank accounts, but toms_savings is a particular
bank account with a particular balance
Chapter 4
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Creating Objects
• The new operator creates an object from a class:
Account toms_savings = new Account ();
• This declaration asserts that toms_savings is a
variable that refers to an object created from the
Account class
• It is initialized to the object created by the new
operator
• The newly created object is set up by a call to a
constructor of the class
Chapter 4
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Constructors
• A constructor is a special method used to set up an object
• It has the same name as the class
• It can take parameters, which are often used to initialize
some variables in the object
• For example, the Account constructor could be set up
to take a parameter specifying its initial balance:
Account toms_savings = new Account (125.89);
Chapter 4
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Object References
• The declaration of the object reference variable and the
creation of the object can be separate activities:
Account toms_savings;
toms_savings = new Account (125.89);
• Once an object exists, its methods can be invoked using
the dot operator:
toms_savings.deposit (35.00);
Chapter 4
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The String Class
• A character string in Java is an object, defined by the
String class
String name = new String ("Ken Arnold");
• Because strings are so common, Java allows an
abbreviated syntax:
String name = "Ken Arnold";
• Java strings are immutable; once a string object has a
value, it cannot be changed
Chapter 4
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The String Class
• A character in a string can be referred to by its position,
or index
• The index of the first character is zero
• The String class is defined in the java.lang
package (and is therefore automatically imported)
• Many helpful methods are defined in the String class
• See Carpe_Diem.java
Chapter 4
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The StringTokenizer Class
• The StringTokenizer class makes it easy to break
up a string into pieces called tokens
• By default, the delimiters for the tokens are the space,
tab, carriage return, and newline characters (white space)
• The StringTokenizer class is defined in the
java.util package
• See Int_Reader.java
Chapter 4
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The Random Class
• A program may need to produce a random number
• The Random class provides methods to simulate a
random number generator
• The nextInt method returns a random number from
the entire spectrum of int values
• Usually, the number must be scaled and shifted into a
particular range to be useful
• See Flip.java
Chapter 4
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Garbage Collection
• When an object no longer has any valid references to it,
it can no longer be accessed by the program
• It is useless, and therefore called garbage
• Java performs automatic garbage collection periodically,
returning an object's memory to the system for future use
• In other languages, the programmer has the responsibility
for performing garbage collection
Chapter 4
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Methods
• A class contains methods; prior to defining our own
classes, we must explore method definitions
• We've defined the main method many times
• All methods follow the same syntax:
return-type method-name ( parameter-list ) {
statement-list
}
Chapter 4
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Methods
• A method definition:
int third_power (int number) {
int cube;
cube = number * number * number;
return cube;
}
Chapter 4
// method third_power
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Methods
• A method may contain local declarations as well as
executable statements
• Variables declared locally can only be used locally
• The third_power method could be written without
any local variables:
int third_power (int number) {
return number * number * number;
}
Chapter 4
// method third_power
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The return Statement
• The return type of a method indicates the type of value
that the method sends back to the calling location
• A method that does not return a value (such as main)
has a void return type
• The return statement specifies the value that will be
returned
• Its expression must conform to the return type
Chapter 4
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Method Flow of Control
• The main method is invoked by the system when you
submit the bytecode to the interpreter
• Each method call returns to the place that called it
main
method1
method2
method2();
method1();
Chapter 4
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Defining Classes
• The syntax for defining a class is:
class class-name {
declarations
constructors
methods
}
• The variables, constructors, and methods of a class are
generically called members of the class
Chapter 4
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Defining Classes
class Account {
int account_number;
double balance;
Account (int account, double initial) {
account_number = account;
balance = initial;
} // constructor Account
void deposit (double amount) {
balance = balance + amount;
} // method deposit
}
Chapter 4
// class Account
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Constructors
• A constructor:
–
–
–
–
–
is a special method that is used to set up a newly created object
often sets the initial values of variables
has the same name as the class
does not return a value
has no return type, not even void
• The programmer does not have to define a constructor
for a class
Chapter 4
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Classes and Objects
• A class defines the data types for an object, but a class
does not store data values
• Each object has its own unique data space
• The variables defined in a class are called instance
variables because each instance of the class has its own
• All methods in a class have access to all instance
variables of the class
• Methods are shared among all objects of a class
Chapter 4
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Classes and Objects
Objects
account_number
2908371
Class
balance
573.21
int account_number
double balance
account_number
4113787
balance
9211.84
Chapter 4
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Encapsulation
• You can take one of two views of an object:
– internal - the structure of its data, the algorithms used by its
methods
– external - the interaction of the object with other objects in the
program
• From the external view, an object is an encapsulated
entity, providing a set of specific services
• These services define the interface to the object
Chapter 4
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Encapsulation
• An object should be self-governing; any changes to the
object's state (its variables) should be accomplished by
that object's methods
• We should make it difficult, if not impossible, for another
object to "reach in" and alter an object's state
• The user, or client, of an object can request its services,
but it should not have to be aware of how those services
are accomplished
Chapter 4
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Encapsulation
• An encapsulated object can be thought of as a black box;
its inner workings are hidden to the client
toms_savings
deposit
withdraw
add_interest
client
produce_statement
Chapter 4
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Abstraction
• Encapsulation is a powerful abstraction
• An abstraction hides the right details at the right time
• We use abstractions every day:
– driving a car
– using a computer
• Encapsulation makes an object easy to manage mentally
because its interaction with clients is limited to a set of
well-defined services
Chapter 4
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Visibility Modifiers
• We accomplish encapsulation through the appropriate
use of visibility modifiers
• A modifier is a Java reserved word that specifies
particular characteristics of a programming construct
• We've used the modifier final to define a constant
• Java has three visibility modifiers: public, private,
and protected
• We will discuss the protected modifier later
Chapter 4
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Visibility Modifiers
• Members of a class that are declared with public
visibility can be accessed from anywhere
• Members of a class that are declared with private
visibility can only be accessed from inside the class
• Members declared without a visibility modifier have
default visibility and can be accessed by any class in the
same package
• Java modifiers are discussed in detail in Appendix F
Chapter 4
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Visibility Modifiers
• As a general rule, no object's data should be declared
with public visibility
• Methods that provide the object's services are usually
declared with public visibility so that they can be
invoked by clients
• Public methods are also called service methods
• Other methods, called support methods, can be defined
that assist the service methods; they should not be
declared with public visibility
Chapter 4
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Classes and Objects
• See Tunes.java
music
add_cds
main
print
Chapter 4
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The static Modifier
• The static modifier can be applied to variables or
methods
• It associates a variable or method with the class rather
than an object
• This approach is a distinct departure from the normal
way of thinking about objects
Chapter 4
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Static Variables
• Normally, each object has its own data space
• If a variable is declared as static, only one copy of the
variable exists for all objects of the class
private static int count;
• Changing the value of a static variable in one object
changes it for all others
• Static variables are sometimes called class variables
Chapter 4
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Static Methods
• Normally, we invoke a method through an instance (an
object) of a class
• If a method is declared as static, it can be invoked
through the class name; no object needs to exist
• For example, the Math class in the java.lang
package contains several static mathematical operations
Math.abs (num) -- absolute value
Math.sqrt (num) -- square root
Chapter 4
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Static Methods
• The main method is static; it is invoked by the system
without creating an object
• Static methods cannot reference instance variables,
because instance variables don't exist until an object
exists
• However, they can reference static variables or local
variables
• Static methods are sometimes called class methods
Chapter 4
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Overloaded Methods
• Method overloading is the process of using the same
method name for multiple methods
• The signature of each overloaded method must be unique
• The signature is based on the number, type, and order of
the parameters
• The compiler must be able to determine which version of
the method is being invoked by analyzing the parameters
• The return type of the method is not part of the signature
Chapter 4
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Overloaded Methods
• The println method is overloaded:
println (String s)
println (int i)
println (double d)
etc.
• The lines
System.out.println ("The total is:");
System.out.println (total);
invoke different versions of the println method
Chapter 4
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Overloaded Methods
• Constructors are often overloaded to provide multiple
ways to set up a new object
Account (int account) {
account_number = account;
balance = 0.0;
}
// constructor Account
Account (int account, double initial) {
account_number = account;
balance = initial;
} // constructor Account
• See Casino.java
Chapter 4
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Classes and Objects
• See Storm.java
Chapter 4
current_size
4
Storm
current_size
18
current_size
12
drop1
drop2
drop3
current_size
7
drop4
drop5
current_size
24
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Objects for Organizing Data -- Introduction
• As our programs get more sophisticated, we need
assistance organizing large amounts of data
• Chapter 6 focuses on:
–
–
–
–
–
array declaration and use
arrays of objects
parameters and arrays
multidimensional arrays
the Vector class
– additional techniques for managing strings
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Arrays
• An array is an ordered list of values
• Each value has a numeric index
• An array of size N is indexed from zero to N-1
• The following array of integers has a size of 10 and is
indexed from 0 to 9
0
scores
Chapter 6
1
2
3
4
5
6
7
8
9
79 87 94 82 67 98 87 81 74 91
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Arrays
• A particular value in an array is referenced using the
array name followed by the index in brackets
• For example, the expression
scores[4]
refers to the value 67 (which is the 5th value in the array)
• That expression represents a place to store a single
integer, can can be used wherever an integer variable can
• For example, it can be assigned a value, printed, used in
a calculation
Chapter 6
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Arrays
• An array stores multiple values of the same type
• That type can be primitive types or objects
• Therefore, we can create an array of integers, or an array
of characters, or an array of String objects, etc.
• In Java, the array itself is an object
• Therefore the name of the array is a object reference
variable, and the array itself is instantiated separately
Chapter 6
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Declaring Arrays
• The scores array could be declared as follows:
int[] scores = new int[10];
• Note that the type of the array does not specify its size,
but each object of that type has a specific size
• The type of the variable scores is int[] (an array of
integers)
• It is set to a newly instantiated array of 10 integers
• See Basic_Array.java
Chapter 6
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Declaring Arrays
• Some examples of array declarations:
float[] prices = new float[500];
boolean[] flags;
flags = new boolean[20];
char[] codes = new char[1750];
Chapter 6
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Bounds Checking
• Once an array is created, it has a fixed size
• An index used in an array reference must specify a valid
element
• That is, they must be in bounds (0 to N-1)
• The Java interpreter will throw an exception if an array
index is out of bounds
• This is called automatic bounds checking
• Its common to inadvertently introduce off-by-one errors
when using arrays
Chapter 6
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Bounds Checking
• Each array object has a public constant called length
that stores the size of the array
• It is referenced through the array name (just like any
other object):
scores.length
• Note that length holds the number of elements, not the
largest index
• See Reverse_Numbers.java and
Adjust_Test_Scores.java
Chapter 6
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Array Declarations Revisited
• The brackets of the array type can be associated with the
element type or with the name of the array
• Therefore
float[] prices;
and
float prices[];
are essentially equivalent
• The first format is usually more readable
Chapter 6
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Initializer Lists
• An initializer list can be used to instantiate and initialize
an array in one step
• The values are delimited by braces and separated by
commas
• Examples:
int[] units = {147, 323, 89, 933, 540,
269, 97, 114, 298, 476};
char[] letter_grades = {'A', 'B', 'C',
'D', 'F'};
Chapter 6
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Initializer Lists
• Note that when an initializer list is used:
– the new operator is not used
– no size value is specified
• The size of the array is determined by the number of
items in the initializer list
• An initializer list can only be used in the declaration of
an array
• See Primes.java and Sales_Analysis.java
Chapter 6
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Arrays of Objects
• The elements of an array can be object references
• The declaration
String[] words = new String[25];
reserves space to store 25 references to String objects
• It does NOT create the String objects themselves
• Each object stored in an array must be instantiated
separately
Chapter 6
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Arrays of Objects
• See Children.java and Presidents.java
• Objects can have arrays as instance variables
• Therefore, fairly complex structures can be created
simply with arrays and objects
• The software designer must carefully determine an
organization of data and objects that makes sense for the
situation
• See Roll_Call.java
Chapter 6
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Arrays as Parameters
• An entire array can be passed to a method as a parameter
• Like any other object, the reference to the array is
passed, making the formal and actual parameters aliases
of each other
• Changing an array element in the method changes the
original
• An array element can be passed to a method as well, and
follow the parameter passing rules of that element's type
• See Array_Test.java
Chapter 6
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Multidimensional Arrays
• A one-dimensional array stores a simple list of values
• A two-dimensional array can be thought of as a table of
values, with rows and columns
• A two-dimensional array element is referenced using two
index values
• To be precise, a two-dimensional array in Java is an array
of arrays, therefore each row can have a different length
Chapter 6
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Multidimensional Arrays
• An initializer list can be used to create and set up a
multidimensional array
• Each element in the list is itself an initializer list
• Note that each array dimension has its own length
constant
• See Multi_Array_Test.java and
Soda_Survey.java
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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The Vector Class
• An object of class Vector is similar to an array in that
it stores multiple values
• However, a vector
– only stores objects
– does not have the indexing syntax that arrays have
• Service methods are used to interact with a vector
• The Vector class is part of the java.util package
• See Beatles.java and ZZ_Top.java
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
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The Vector Class
• An important difference between an array and a vector is
that a vector can be thought of as a dynamic, able to
change its size as needed
• Each vector initially has a certain amount of memory
space reserved for storing elements
• If an element is added that doesn't fit in the existing
space, more room is automatically acquired
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
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The Vector Class
• A vector is implemented using an array
• Whenever new space is required, a new, larger array is
created, and the values are copied from the original to the
new array
• To insert an element, existing elements are first copied,
one by one, to another position in the array
• Therefore, the implementation of Vector in the API is
not very efficient
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
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The StringTokenizer Class Revisited
• We've seen a StringTokenizer object separate a
string into separate tokens
• By default, those tokens are delimited by white space
• But by using other StringTokenizer constructors,
we can define the delimiters used to define a token
• We can also set whether we want the delimiters
themselves returned as tokens
• See Voltaire.java and URL_Tokens.java
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Java Software Solutions
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The StringBuffer Class
• Recall that the value of a String object is immutable;
once set it cannot be changed
• The StringBuffer class can be used to define a
character string whose value can change
• It's service methods include the ability to append and
insert characters
• See Money.java
• However, most functionality defined by the
StringBuffer class can be accomplished with
String objects and string concatenation
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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