Chapter 2: Defining a Simple Class

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Chapter 2 Defining a Simple Class
2
Object Interaction Clients and Servers

• Objectives After studying this chapter you
  should understand the following
• the client-server relationship
• the purpose of a class specification
• the purpose of a class implementation
• the function of statements in a program
• the function of arithmetic expressions in a
  program
• the meaning and use of an assignment statement
• the role of parameters in the specification of a
  method
• the role of comments in programs.

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Object Interaction Clients and Servers

• Also, you should be able to
• write the specification of a simple class
• implement a simple class
• invoke an objects methods
• construct a simple static diagram
• construct a simple interaction diagram describing
  the interaction of two objects.
• write and evaluate arithmetic expressions
• write legal return statements
• write legal assignment statements
• develop a javadoc document for the client.

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Clients and Servers

• Recall
• an object has features queries and commands.
• objects cooperate to produce a problem solution.

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Client and Server relationship
query
Piece
Player
currentSquare
white
color
white
color
queen
rank
yes
userControlled
d1
d1
square


client
server
response
command
Piece
Player
white
color
moveTo(c2)
white
color
queen
rank
yes
userControlled
d1
square


client
Piece
white
color
queen
rank
c2
square

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Clients and Servers

• Object A uses object B
• object A is termed the client, and object B is
  the server.
• A client queries and commands a server.

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Clients and Servers

• Example chess playing program.
• Client Player, Server Piece
• Player object queries Piece object to determine
  its location,
• Player object commands Piece to move to new
  location on board.

8
Server specification and Implementation

• Client need only know servers features and use.
• Object specification (interface) definition of
  objects features, as seen by its clients.
• The implementation provides the internals that
  actually make up the features.

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Example of Client specification for Counter

• Start listing its responsibilities
• Class Counter
• queries
• currentCount the current value of count, a
  non-negative integer
• commands
• reset set the value of count to 0
• incrementCount increment the value of count by 1

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Defining class Counter in Java
package counters / A simple integer
counter. / public class Counter
Definitions of features goes here.
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Specifying a Method for a query
/ The number of items counted. / public
int currentCount ()
Name of method.
Type of value returned by query.
Method implementation goes here.
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Specifying a Method for a command
/ The number of items counted. / public
void incrementCount ()
Name of method.
Type of value returned by command.
Method implementation goes here.
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Class constructor

• constructor a class method used to create and
  initialize an object.

Name of method.
/ Create a new Counter, with the count
initialized to 0 / public Counter ()
Method implementation goes here.

• Note name of class constructor is the same as
  the name of the class.

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Static diagram of the class Counter
Counter

int
currentCount()

void
incrementCount()

void
reset()
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Invoking queries

• clients reference object to Counter myCounter
• To query myCounter for current value of count

myCounter.currentCount()
server
client
Counter

myCounter
4
int

count
myCounter.currentCount()
public
int

currentCount ()
.
.
4

2. object myCounter performs actions as
1. client invokes the method
prescribed by the method definition.
currentCount of the object myCounter.
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Invoking commands

• clients reference object to Counter myCounter
• To command myCounter to reset,

myCounter.reset()
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Interaction diagrams

• client object interacting with a Counter. object
  queries Counter for current count, and then give
  Counter command reset.

currentCount()
the object is active
executing the method
currentCount
count
time
reset()
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Implementing class data

• Counter needs current value of count.
• Define only one instance variable syntax to use

private variableType variableName

• Variable definition is included in class

public class Counter private int count //
current count
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Counter object showing instance variable
Counter
Value of variable at some time
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Type of variable
int

count
Name of variable
Instance variable
Counter
-
int
count

int
currentCount()

void
incrementCount()

void
reset()
Static diagram of class Counter, showing instance
variable
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Implementing functionality

• For each method provide algorithm
• set of instructions for processor to carrys out.
• Algorithm is described via Java statements
• statement a language construct that describes an
  action for the processor to execute.

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Implementing query currentCount

• Method must deliver value stored in variable
  count.
• Use return statement return count

• /
• The number of items counted.
• /
• public int currentCount ()
• return count

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Return statement and expressions

• The general form of a return statement is

return expression

• Expression language construct that describes
  how to compute a particular value.
• Processor evaluates expression to produce a
  value.

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Implementing simple commandsAssignment statement

• A command modifies state of the object.
• Use an assignment statement
• instructs processor to compute a value and store
  it in a variable.

• Processor executes assignment in two steps
• computes the value by the expression on RHS
• stores value in variable on LHS, replacing
  previous value.

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Implementing command reset

• want to store 0 in the variable count.

• /
• Reset the count to 0.
• /
• public void reset ()
• count 0

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Implementing the command incrementCount

• want to update count to current value in count
  plus one.
• Use count on LHS, and
• Use the expression count 1 on RHS

• /
• Increment the count by 1.
• /
• public void incrementCount ()
• count count 1

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Implementing constructor

• instance variable count must be set to 0 in the
  instance creation.

/ Create a new Counter, with the count
initialized to 0. / public Counter () count
0
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Arithmetic expressions

• Expressions that evaluate to integer and floating
  point values.
• Can be built by combining literals and variable
  names with arithmetic operators
• addition
• - subtraction
• multiplication
• / division
• remainder

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Unary operators and -

• assume that i1, is an int variables containing 10

3 ? 3 -3 ? -3 i1 ? 10 - i1 ? -10
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Division operator

• / denotes division when applied to two floating
  point operands, but integer quotient when applied
  to two integer operands.

2.0/4.0 ? 0.5 2/4 ? 0 5.0/4.0 ?
1.25 5/4 ? 1
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Remainder operator

• Primarily used with integer operands

10 5 ? 0 10 3 ? 1 10 6 ? 4 10 11
? 10
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Mixed type expressionsNumeric promotion

• Mixed Operands
• what happens if one operand is int and the other
  double,
• int operand is converted (promoted) to a double
  representing same value

7 / 2.0 ? 7.0 / 2.0 ? 3.5 i1 0.5 ? 10
0.5 ? 10.0 0.5 ? 5.0
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Operator precedence

• What is the order of evaluation in

5 10 2

• Unary and have higher precedence than binary
  operators.
• , /, have higher precedence than operators ,-

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Operator precedence

• If two operators have equal precedence,
  operations are performed left to right. i.e.

10 / 5 3 6

• Parentheses are used to override precedence. i.e.

10 / ( 5 3)
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Casting

• Occasionally must convert a value to a different
  type to perform certain operations.
• Syntax (type) expression

10/40 0 (double)10/(double)40 0.25
10.0/40.0 0.25 (int)10.0/(int)40.0 0

• Cast operators have higher precedence than
  arithmetic operators.

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A simple green-yellow-red Traffic signal

• Features

• public TrafficSignal ()
• Create a new TrafficSignal, initially green.
• public int light ()
• The light currently on.
• public void change ()
• Change to the next light.

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Traffic signal implementation

• Instance variable int light
• represent possible lights with integers
• 0 for green, 1 for yellow, 2 for red.
• To isolate client from choice of values for
  light, use named class constants

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Named class constants
public class TrafficSignal / The green
signal light. / public static final int GREEN
0 / The yellow signal light.
/ public static final int YELLOW 1 /
The red signal light. / public static final
int RED 2
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Implementing constructor
/ Create a new TrafficSignal, initially
green. / public TrafficSignal () light
TrafficSignal.GREEN
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Implementing command change
/ Change to the next light.
/ public void change () light (light 1)
3

• Note remainder by 3 will yield values 0, 1, or 2

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Methods with parameters

• Often client must provide additional data when
  invoking a constructor or method.
• Additional data provided are parameters.

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Methods with parameters

• Example model a Playing card.
• The constructor needs two parameters
• suit
• Rank
• As the client must specify which card to create.

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Constructor with parameters
public PlayingCard (int suit , int rank)
Parameter name
Parameter type
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Invoking constructor with parameters

• Client invokes constructor providing two int
  values
• One for suit
• Another for rank

new PlayingCard(4, 3)

• This invocation will create a PlayingCard with
• suit spades
• rank (value of card) 3

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Executing constructor with parameters Method
variables

• When client invokes constructor,

new PlayingCard(4, 3)

• Two int method variables are created
• named suit and rank
• initialized with 4, and 3 respectively.
• lifetime of method variables constructor
  execution .
• destroyed at completion of execution of
  constructor.
• Lifetime of objects instance variables object
  lifetime .

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Implementing PlayingCard constructor

• PlayingCard has two instance variables
• private int suit
• private int rank
• Initialize them in constructor, using clients
  values.
• Clients values are in method variables suit,
  rank

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Implementing PlayingCard constructor

• The following implementation will not work

public PlayingCard (int suit, int rank) suit
suit rank rank

• In body of method, names suit and rank refer
  to method variables only.

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Implementing PlayingCard constructor

• The keyword this refers to object being
  constructed.
• this.suit refers to its instance variable suit
• this.rank refers to its instance variable rank.

public PlayingCard (int suit, int rank)
this.suit suit this.rank rank
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Java in detail arithmethic expressions Simple
expressions

• Literals

0 7 23 0.5 2.0 3.14159 2.4e-23

• Variable names
• It denotes value currently stored in variable.

int i1 10 int i2 -20 int i3
30 double d1 2.5 double d2
0.5 //evaluating them produces their
values i1 ? 10 i2 ? -20 i3 ? 30
d1 ? 2.5 d2 ? 0.5
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Java in detail arithmethic expressions Operators

• Expressions can be combined with operators to
  form more complicated expressions.

i1 / -3 ? -3 -7 / 2 ? -3 i1 -3 ? 1 -7 2 ? -1
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Java in detail arithmethic expressions Numeric
promotion

• Operands for binary and unary operators are
  automatically converted to similar values of a
  different type when necessary.

7 / 2.0 ? 7.0 / 2.0 ? 3.5 i1 0.5 ? 10
0.5 ? 10.0 0.5 ? 5.0
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Java in detail arithmethic expressions Precedence

• Operators , /, and have higher precedence than
  binary operators and -.
• In an un-parenthesized expression multiplication
  is done before addition

Multiply before adding
i1 10 2 ? i1 20 ? 30
i1 10 2 ? 100 2 ? 102 10 / 2 1 ? 5
1 ? 6 5 6 / 10 ? 5 0 ? 5 - 5 i1 ? (-5)
10 ? 5
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Java in detail arithmethic expressions
Associativity

• Binary operators are left associative when
  expression contains two operators with equal
  precedence, operations are performed left to
  right.

Left operator before right
i1 / 5 2 ? 2 2 ? 4
10 - 4 - 3 ? 6 - 3 ? 3 i1/20 2 ? 10/20
2 ? 02 ? 0 2i1/20 ? 210/20 ? 20/20 ? 1 20 /
i12 ? 20 / 102 ? 22 ? 4
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Java in detail Concatenation

• For operands Strings binary operator denotes
  string concatenation.

string1 string2

• Evaluates to a String containing characters of
  string1 with characters of string2 appended.

"abc" "def" ? "abcdef"
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Java in detail Concatenation

• If one operands of is a String and the other
  isnt, the non-String operand will be converted
  to a String, and concatenation performed.

int i 23 "abc" i ? "abc23 i " " ? "23
""2i" 2i ? "2i46"2i"!" ? "25!" (
is left associative!)"!"2i ? "!223"
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Java in detail Casting
(type)expression (double)i1 ? (double)10 ? 10.0 (d
ouble)i1 / i3 ? 10.0 / 30 ? 0.333 (int)d3 ? 2(
int)d4 ? -2 (double)i1/i3 ? 10.0/30 ? 0.333 (do
uble)(i1/i3) ? (double)0 ? 0.0
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Java in detail basic organizational structure

• package a collection of closely related classes.
• compilation unit a file containing the
  definition of one or more classes of a package.

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Java in detail basic organizational structure
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Java in detail referring to classes in a
different package,import statements

• Via import statement, can refer to a class in
  another package with by name rather than with its
  fully qualified name.
• Two formats for an import statement

import packageName.className import
packageName. import javax.swing.border.LineBorde
r import javax.swing.border.
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Java in detail referring to classes in a
different package,import statements

• Instead of writing

javax.swing.border.LineBorder myBorder myBorder
new javax.swing.border.LineBorder(myColor)

• Write

import javax.swing.border.LineBorder import
javax.swing.border. LineBorder
myBorder myBorder new LineBorder(myColor)
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Java in detail package java.lang

• The predefined package java.lang contains the
  definitions of a number of commonly used classes.
  
• String is a member of java.lang.
• Do not need to explicitely import this package.
  Java does it by default.

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Summary

• Saw how to specify and implement a simple class
  using the programming language Java.
• Two objects are related in a fundamental way when
  one object uses the other
• one object queries and command the other.
• This relation is client-server relationship.
• The client object queries and commands the server
  object.

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Summary

• Definition of class of which the object is an
  instance, consists of two parts
• Specification a precise description of objects
  features as seen by a client.
• Implementation internal mechanisms that enable
  an object to behave according to its
  specification.

63
Summary

• Class specification is made up of
• method specifications
• constructor specifications.

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Summary

• Methods define
• the specifications
• the implementation of an objects features.
• A method that defines a query returns a value of
  a given type.
• A method that defines a command causes object to
  change state when it is invoked.
• Constructors are used to create objects and
  initialize their states.

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Summary

• Implementing a class involves
• writing data descriptions for data stored in
  class instances, and
• writing method bodies that define the actions the
  processor performs when the methods are invoked.

66
Summary

• An objects data are stored in instance
  variables.
• Instance variables are created when object is
  created.
• Instance variables are part of the objects
  implementation
• An instance variable is created with a variable
  declaration

private variableType variableName
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Summary

• A method body is made up of a sequence of
  statements that implement the method algorithm.
• Statements describe actions the processor
  performs when executing the method.
• When method is invoked, processor executes
  statements one after another, in the order in
  which they appear in the method body.

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Summary

• Return statement has the form

return expression

• Assignment statement has the form

variable expression
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Summary

• An expression describes how a value is to be
  computed.
• Arithmetic expressions, in particular, produce
  numeric values when they are evaluated.
• Both return statements and assignment statements
  include expressions.

70
Summary

• Parameters in method or constructor indicate
  that client provide information when the method
  or constructor is invoked.
• Arguments values client supplies when invoking
  method or constructor.

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Summary

• Method variables variables allocated for each
  parameter and initialized with the argument
  provided by the client.
• Method variables created when the method is
  invoked, and deallocated when execution of method
  is complete.

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Summary

• Packages groups of class definitions that make
  up a program.
• Classes defined in same package are related to
  each other instances of these classes can access
  each other by default.
• If a class is to be visible throughout the entire
  system, it must be explicitly labeled as public.
• Class definitions are written in source files.
• Each file can contain definition of at most one
  public class.