Transcript Chapter 4: Writing Classes - University at Albany, SUNY

INF 523Q
Chapter 4:
Writing Classes
Writing Classes
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We've been using predefined classes. Now we will learn to
write our own classes to define new objects
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Chapter 4 focuses on:
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class declarations
method declarations
instance variables
encapsulation
method overloading
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Objects
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An object has:
• state - descriptive characteristics
• behaviors - what it can do (or be done to it)
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For example, consider a coin that can be flipped so that it's
face shows either "heads" or "tails"
The state of the coin is its current face (heads or tails)
The behavior of the coin is that it can be flipped
Note that the behavior of the coin might change its state
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Classes
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A class is a blueprint of an object
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It is the model or pattern from which objects are created
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For example, the String class is used to define String
objects
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Each String object contains specific characters (its state)
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Each String object can perform services (behaviors) such
as toUpperCase
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Classes
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The String class was provided for us by the Java
standard class library
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But we can also write our own classes that define specific
objects that we need
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For example, suppose we wanted to write a program that
simulates the flipping of a coin
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We could write a Coin class to represent a coin object
Classes
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A class contains data declarations and method declarations
int x, y;
char ch;
Data declarations
Method declarations
Data Scope
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The scope of data is the area in a program in which that
data can be used (referenced)
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Data declared at the class level can be used by all methods
in that class
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Data declared within a method can only be used in that
method
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Data declared within a method is called local data
Writing Methods
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A method declaration specifies the code that will be executed
when the method is invoked (or called)
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When a method is invoked, the flow of control jumps to the
method and executes its code
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When complete, the flow returns to the place where the
method was called and continues
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The invocation may or may not return a value, depending
on how the method was defined
Method Control Flow
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The called method could be within the same class, in which
case only the method name is needed
compute
myMethod();
myMethod
Method Control Flow
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The called method could be part of another class or object
main
obj.doIt();
doIt
helpMe();
helpMe
The Coin Class
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In our Coin class we could define the following data:
• face, an integer that represents the current face
• HEADS and TAILS, integer constants that represent the two
possible states
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We might also define the following methods:
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a Coin constructor, to set up the object
a flip method, to flip the coin
a getFace method, to return the current face
a toString method, to return a string description for printing
See Coin.java (page 180)
See CountFlips.java (page 179)
Coin.java
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//******************************************************************
// Coin.java
Author: Lewis and Loftus
// Represents a coin with two sides that can be flipped.
//******************************************************************
public class Coin
{
public final int HEADS = 0;
public final int TAILS = 1;
private int face;
// Sets up the coin by flipping it initially.
public Coin ()
{
flip();
}
// Flips the coin by randomly choosing a face.
public void flip ()
{
face = (int) (Math.random() * 2);
}
Coin.java (cont.)
// Returns the current face of the coin as an integer.
public int getFace ()
{
return face;
}
// Returns the current face of the coin as a string.
public String toString()
{
String faceName;
if (face == HEADS)
faceName = "Heads";
else
faceName = "Tails";
return faceName;
}
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}
CountFlips.java
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//******************************************************************
// CountFlips.java
Author: Lewis and Loftus
// Demonstrates the use of a programmer-defined class.
//******************************************************************
import Coin;
public class CountFlips
{
// Flips a coin multiple times and counts the number of heads and tails that result.
public static void main (String[] args)
{
final int NUM_FLIPS = 1000;
int heads = 0, tails = 0;
Coin myCoin = new Coin(); // instantiate the Coin object
for (int count=1; count <= NUM_FLIPS; count++)
CountFlips.java (cont.)
{
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myCoin.flip();
if (myCoin.getFace() == myCoin.HEADS)
heads++;
else
tails++;
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}
System.out.println ("The number flips: " + NUM_FLIPS);
System.out.println ("The number of heads: " + heads);
System.out.println ("The number of tails: " + tails);
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}
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}
The Coin Class
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Once the Coin class has been defined, we can use it again
in other programs as needed
Note that the CountFlips program did not use the
toString method
A program will not necessarily use every service provided
by an object
Instance Data
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The face variable in the Coin class is called instance data
because each instance (object) of the Coin class has its own
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A class declares the type of the data, but it does not reserve
any memory space for it
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Every time a Coin object is created, a new face variable
is created as well
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The objects of a class share the method definitions, but they
have unique data space
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That's the only way two objects can have different states
Instance Data
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See FlipRace.java (page 182)
class Coin
int face;
coin1
face
0
coin2
face
1
FlipRace.java
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//********************************************************************
// FlipRace.java
Author: Lewis and Loftus
// Demonstrates the existence of separate data space in multiple
// instantiations of a programmer-defined class.
//********************************************************************
import Coin;
public class FlipRace
{
// Flips two coins until one of them comes up heads a set number of times in a row.
public static void main (String[] args)
{
final int GOAL = 3;
int count1 = 0, count2 = 0;
// Create two separate coin objects
Coin coin1 = new Coin();
Coin coin2 = new Coin();
while (count1 < GOAL && count2 < GOAL)
{
coin1.flip();
coin2.flip();
FlipRace.java (cont.)
// Print the flip results (uses Coin's toString method)
System.out.print ("Coin 1: " + coin1);
System.out.println (" Coin 2: " + coin2);
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// Increment or reset the counters
count1 = (coin1.getFace() == coin1.HEADS) ? count1+1 : 0;
count2 = (coin2.getFace() == coin2.HEADS) ? count2+1 : 0;
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}
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// Determine the winner
if (count1 < GOAL)
System.out.println ("Coin 2 Wins!");
else
if (count2 < GOAL)
System.out.println ("Coin 1 Wins!");
else
System.out.println ("It's a TIE!");
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}
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}
Encapsulation
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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
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From the external view, an object is an encapsulated entity,
providing a set of specific services
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These services define the interface to the object
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Recall from Chapter 2 that an object is an abstraction,
hiding details from the rest of the system
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Encapsulation
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An object should be self-governing
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Any changes to the object's state (its variables) should be
accomplished by that object's methods
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We should make it difficult, if not impossible, for one object
to "reach in" and alter another object's state
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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
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Encapsulation
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An encapsulated object can be thought of as a black box
Its inner workings are hidden to the client, which only
invokes the interface methods
Client
Methods
Data
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Visibility Modifiers
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In Java, we accomplish encapsulation through the
appropriate use of visibility modifiers
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A modifier is a Java reserved word that specifies particular
characteristics of a method or data value
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We've used the modifier final to define a constant
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Java has three visibility modifiers: public, private,
and protected
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We will discuss the protected modifier later
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Visibility Modifiers
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Members of a class that are declared with public visibility
can be accessed from anywhere
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Members of a class that are declared with private visibility
can only be accessed from inside the class
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Members declared without a visibility modifier have default
visibility and can be accessed by any class in the same
package
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Java modifiers are discussed in detail in Appendix F
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Visibility Modifiers
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As a general rule, no object's data should be declared with
public visibility
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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
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A method created simply to assist a service method is called
a support method
Since a support method is not intended to be called by a
client, it should not be declared with public visibility
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Method Declarations Revisited
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A method declaration begins with a method header
char calc (int num1, int num2, String message)
method
name
return
type
parameter list
The parameter list specifies the type
and name of each parameter
The name of a parameter in the method
declaration is called a formal argument
Method Declarations
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The method header is followed by the method body
char calc (int num1, int num2, String message)
{
int sum = num1 + num2;
char result = message.charAt (sum);
return result;
}
The return expression must be
consistent with the return type
sum and result
are local data
They are created each
time the method is called,
and are destroyed when
it finishes executing
The return Statement
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The return type of a method indicates the type of value that
the method sends back to the calling location
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A method that does not return a value has a void return
type
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The return statement specifies the value that will be
returned
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Its expression must conform to the return type
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Parameters
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Each time a method is called, the actual arguments in the
invocation are copied into the formal arguments
ch = obj.calc (25, count, "Hello");
char calc (int num1, int num2, String message)
{
int sum = num1 + num2;
char result = message.charAt (sum);
return result;
}
Constructors Revisited
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Recall that a constructor is a special method that is used to
set up a newly created object
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When writing a constructor, remember that:
• it has the same name as the class
• it does not return a value
• it has no return type, not even void
• it often sets the initial values of instance variables
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The programmer does not have to define a constructor for
a class
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Writing Classes
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See Account.java (page 189)
See BankAccounts.java (page 188)
Account.java
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//********************************************************************
// Account.java
Author: Lewis and Loftus
// Represents a bank account with basic services such as deposit and withdraw.
//********************************************************************
import java.text.NumberFormat;
public class Account
{
private NumberFormat fmt = NumberFormat.getCurrencyInstance();
private final double RATE = 0.045; // interest rate of 4.5%
private long acctNumber;
private double balance;
private String name;
// Sets up the account by defining its owner, account number,
// and initial balance.
public Account (String owner, long account, double initial)
{
name = owner;
acctNumber = account;
balance = initial;
}
Account.java (cont.)
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// Validates the transaction, then deposits the specified amount
// into the account. Returns the new balance.
public double deposit (double amount)
{
if (amount < 0) // deposit value is negative
{
System.out.println ();
System.out.println ("Error: Deposit amount is invalid.");
System.out.println (acctNumber + " " + fmt.format(amount));
}
else
balance = balance + amount;
return balance;
}
// Validates the transaction, then withdraws the specified amount
// from the account. Returns the new balance.
public double withdraw (double amount, double fee)
{
amount += fee;
Account.java (cont.)
if (amount < 0) // withdraw value is negative
{
System.out.println ();
System.out.println ("Error: Withdraw amount is invalid.");
System.out.println ("Account: " + acctNumber);
System.out.println ("Requested: " + fmt.format(amount));
}
else
if (amount > balance) // withdraw value exceeds balance
{
System.out.println ();
System.out.println ("Error: Insufficient funds.");
System.out.println ("Account: " + acctNumber);
System.out.println ("Requested: " + fmt.format(amount));
System.out.println ("Available: " + fmt.format(balance));
}
else
balance = balance - amount;
return balance;
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}
Account.java (cont.)
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// Adds interest to the account and returns the new balance.
public double addInterest ()
{
balance += (balance * RATE);
return balance;
}
// Returns the current balance of the account.
public double getBalance ()
{
return balance;
}
// Returns the account number.
public long getAccountNumber ()
{
return acctNumber;
}
// Returns a one-line description of the account as a string.
public String toString ()
{
return (acctNumber + "\t" + name + "\t" + fmt.format(balance)); } }
BankAccounts.java
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//********************************************************************
// BankAccounts.java
Author: Lewis and Loftus
// Driver to exercise the use of multiple Account objects.
//********************************************************************
import Account;
public class BankAccounts
{
// Creates some bank accounts and requests various services.
public static void main (String[] args)
{
Account acct1 = new Account ("Ted Murphy", 72354, 102.56);
Account acct2 = new Account ("Jane Smith", 69713, 40.00);
Account acct3 = new Account ("Edward Demsey", 93757, 759.32);
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acct1.deposit (25.85);
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double smithBalance = acct2.deposit (500.00);
System.out.println ("Smith balance after deposit: " +
smithBalance);
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BankAccounts.java (cont.)
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System.out.println ("Smith balance after withdrawal: " +
acct2.withdraw (430.75, 1.50));
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acct3.withdraw (800.00, 0.0); // exceeds balance
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acct1.addInterest();
acct2.addInterest();
acct3.addInterest();
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System.out.println ();
System.out.println (acct1);
System.out.println (acct2);
System.out.println (acct3);
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}
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}
Writing Classes
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An aggregate object is an object that contains references to
other objects
An Account object is an aggregate object because it
contains a reference to a String object (that holds the
owner's name)
An aggregate object represents a has-a relationship
A bank account has a name
Writing Classes
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Sometimes an object has to interact with other objects of
the same type
For example, we might add two Rational number objects
together as follows:
r3 = r1.add(r2);
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One object (r1) is executing the method and another (r2) is
passed as a parameter
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See Rational.java (page 197)
See RationalNumbers.java (page 196)
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Rational.java
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//********************************************************************
// Rational.java
Author: Lewis and Loftus
// Represents one rational number with a numerator and denominator.
//********************************************************************
public class Rational
{
private int numerator, denominator;
// Sets up the rational number by ensuring a nonzero denominator
// and making only the numerator signed.
public Rational (int numer, int denom)
{
if (denom == 0)
denom = 1;
// Make the numerator "store" the sign
if (denom < 0)
{
numer = numer * -1;
denom = denom * -1;
}
numerator = numer;
denominator = denom;
reduce(); }
Rational.java (cont.)
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// Adds this rational number to the one passed as a parameter.
// A common denominator is found by multiplying the individual
// denominators.
public Rational add (Rational op2)
{
int commonDenominator = denominator * op2.getDenominator();
int numerator1 = numerator * op2.getDenominator();
int numerator2 = op2.getNumerator() * denominator;
int sum = numerator1 + numerator2;
return new Rational (sum, commonDenominator);
}
// Subtracts the rational number passed as a parameter from this
// rational number.
public Rational subtract (Rational op2)
{
int commonDenominator = denominator * op2.getDenominator();
int numerator1 = numerator * op2.getDenominator();
int numerator2 = op2.getNumerator() * denominator;
int difference = numerator1 - numerator2;
return new Rational (difference, commonDenominator);
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}
Rational.java (cont.)
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// Multiplies this rational number by the one passed as a parameter.
public Rational multiply (Rational op2)
{
int numer = numerator * op2.getNumerator();
int denom = denominator * op2.getDenominator();
return new Rational (numer, denom);
}
// Divides this rational number by the one passed as a parameter
// by multiplying by the reciprocal of the second rational.
public Rational divide (Rational op2)
{
return multiply (op2.reciprocal());
}
// Returns the reciprocal of this rational number.
public Rational reciprocal ()
{
return new Rational (denominator, numerator);
}
// Returns the numerator of this rational number.
public int getNumerator ()
{ return numerator; }
Rational.java (cont.)
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// Returns the denominator of this rational number.
public int getDenominator ()
{
return denominator; }
// Determines if this rational number is equal to the one passed
// as a parameter. Assumes they are both reduced.
public boolean equals (Rational op2)
{
return ( numerator == op2.getNumerator() &&
denominator == op2.getDenominator() );
}
// Returns this rational number as a string.
public String toString ()
{
String result;
if (numerator == 0)
result = "0";
else
if (denominator == 1)
result = numerator + "";
else
result = numerator + "/" + denominator;
return result; }
Rational.java (cont.)
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// Reduces this rational number by dividing both the numerator
// and the denominator by their greatest common divisor.
private void reduce ()
{
if (numerator != 0)
{
int common = gcd (Math.abs(numerator), denominator);
numerator = numerator / common;
denominator = denominator / common;
}
}
// Computes and returns the greatest common divisor of the two
// positive parameters. Uses Euclid's algorithm.
private int gcd (int num1, int num2)
{
while (num1 != num2)
if (num1 > num2)
num1 = num1 - num2;
else
num2 = num2 - num1;
return num1;
} }
RationalNumbers.java
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//********************************************************************
// RationalNumbers.java
Author: Lewis and Loftus
// Driver to exercise the use of multiple Rational objects.
//********************************************************************
import Rational;
public class RationalNumbers
{
// Creates some rational number objects and performs various
// operations on them.
public static void main (String[] args)
{
Rational r1 = new Rational (6, 8);
Rational r2 = new Rational (1, 3);
System.out.println ("First rational number: " + r1);
System.out.println ("Second rational number: " + r2);
RationalNumbers.java (cont.)
if (r1.equals(r2))
System.out.println ("r1 and r2 are equal.");
else
System.out.println ("r1 and r2 are NOT equal.");
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Rational r3 = r1.add(r2);
Rational r4 = r1.subtract(r2);
Rational r5 = r1.multiply(r2);
Rational r6 = r1.divide(r2);
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System.out.println ("r1 + r2: " + r3);
System.out.println ("r1 - r2: " + r4);
System.out.println ("r1 * r2: " + r5);
System.out.println ("r1 / r2: " + r6);
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}
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}
Overloading Methods
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Method overloading is the process of using the same method
name for multiple methods
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The signature of each overloaded method must be unique
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The signature includes the number, type, and order of the
parameters
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The compiler must be able to determine which version of
the method is being invoked by analyzing the parameters
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The return type of the method is not part of the signature
48
Overloading Methods
Version 1
Version 2
float tryMe (int x)
{
return x + .375;
}
float tryMe (int x, float y)
{
return x*y;
}
Invocation
result = tryMe (25, 4.32)
Overloaded Methods
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The println method is overloaded:
println (String s)
println (int i)
println (double d)
etc.
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The following lines invoke different versions of the
println method:
System.out.println ("The total is:");
System.out.println (total);
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Overloading Methods
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Constructors can be overloaded
An overloaded constructor provides multiple ways to set up
a new object
See Die.java (page 204)
See SnakeEyes.java (page 203)
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Die.java
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//********************************************************************
// Die.java
Author: Lewis and Loftus
// Represents one die (singular of dice) with faces showing values
// between 1 and the number of faces on the die.
//********************************************************************
public class Die
{
private final int MIN_FACES = 4;
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private int numFaces; // number of sides on the die
private int faceValue; // current value showing on the die
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// Defaults to a six-sided die, initially showing 1.
public Die ()
{
numFaces = 6;
faceValue = 1;
}
Die.java (cont.)
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// Explicitly sets the size of the die. Defaults to a size of
// six if the parameter is invalid. Initial face is 1.
public Die (int faces)
{
if (faces < MIN_FACES)
numFaces = 6;
else
numFaces = faces;
faceValue = 1;
}
// Rolls the die and returns the result.
public int roll ()
{
faceValue = (int) (Math.random() * numFaces) + 1;
return faceValue;
}
// Returns the current die value.
public int getFaceValue ()
{
return faceValue; } }
SnakeEyes.java
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//********************************************************************
// SnakeEyes.java
Author: Lewis and Loftus
// Demonstrates the use of a class with overloaded constructors.
//********************************************************************
import Die;
public class SnakeEyes
{ // Creates two die objects, then rolls both dice a set number of
// times, counting the number of snake eyes that occur.
public static void main (String[] args)
{ final int ROLLS = 500;
int snakeEyes = 0, num1, num2;
Die die1 = new Die(); // creates a six-sided die
Die die2 = new Die(20); // creates a twenty-sided die
for (int roll = 1; roll <= ROLLS; roll++)
{ num1 = die1.roll();
num2 = die2.roll();
if (num1 == 1 && num2 == 1) // check for snake eyes
snakeEyes++;
}
System.out.println ("Number of rolls: " + ROLLS);
System.out.println ("Number of snake eyes: " + snakeEyes);
System.out.println ("Ratio: " + (float)snakeEyes/ROLLS); } }
The StringTokenizer Class
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The next example makes use of the StringTokenizer
class, which is defined in the java.util package
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A StringTokenizer object separates a string into
smaller substrings (tokens)
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By default, the tokenizer separates the string at white space
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The StringTokenizer constructor takes the original
string to be separated as a parameter
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Each call to the nextToken method returns the next token
in the string
Method Decomposition
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A method should be relatively small, so that it can be
readily understood as a single entity
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A potentially large method should be decomposed into
several smaller methods as needed for clarity
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Therefore, a service method of an object may call one or
more support methods to accomplish its goal
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See PigLatinTranslator.java (page 208)
See PigLatin.java (page 207)
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PigLatinTranslator.java
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//********************************************************************
// PigLatinTranslator.java
Author: Lewis and Loftus
// Represents a translation system from English to Pig Latin.
// Demonstrates method decomposition and the use of StringTokenizer.
//********************************************************************
import java.util.StringTokenizer;
public class PigLatinTranslator
{
// Translates a sentence of words into Pig Latin.
public String translate (String sentence)
{
String result = "";
sentence = sentence.toLowerCase();
StringTokenizer tokenizer = new StringTokenizer (sentence);
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while (tokenizer.hasMoreTokens())
{
result += translateWord (tokenizer.nextToken());
result += " ";
}
return result; }
PigLatinTranslator.java (cont.)
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// Translates one word into Pig Latin. If the word begins with a vowel, the suffix "yay“
// is appended to the word. Otherwise, the first letter or two are moved to the end of
// the word, and "ay" is appended.
private String translateWord (String word)
{
String result = "";
if (beginsWithVowel(word))
result = word + "yay";
else
if (beginsWithPrefix(word))
result = word.substring(2) + word.substring(0,2) + "ay";
else
result = word.substring(1) + word.charAt(0) + "ay";
return result;
}
// Determines if the specified word begins with a vowel.
private boolean beginsWithVowel (String word)
{
String vowels = "aeiouAEIOU";
char letter = word.charAt(0);
return (vowels.indexOf(letter) != -1); }
PigLatinTranslator.java (cont.)
// Determines if the specified word begins with a particular two-character prefix.
private boolean beginsWithPrefix (String str)
{
return ( str.startsWith ("bl") || str.startsWith ("pl") ||
str.startsWith ("br") || str.startsWith ("pr") ||
str.startsWith ("ch") || str.startsWith ("sh") ||
str.startsWith ("cl") || str.startsWith ("sl") ||
str.startsWith ("cr") || str.startsWith ("sp") ||
str.startsWith ("dr") || str.startsWith ("sr") ||
str.startsWith ("fl") || str.startsWith ("st") ||
str.startsWith ("fr") || str.startsWith ("th") ||
str.startsWith ("gl") || str.startsWith ("tr") ||
str.startsWith ("gr") || str.startsWith ("wh") ||
str.startsWith ("kl") || str.startsWith ("wr") ||
str.startsWith ("ph") );
}
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}
PigLatin.java
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//******************************************************************
// PigLatin.java
Author: Lewis and Loftus
// Driver to exercise the PigLatinTranslator class.
//******************************************************************
import PigLatinTranslator;
import cs1.Keyboard;
public class PigLatin
{
// Reads sentences and translates them into Pig Latin.
public static void main (String[] args)
{
String sentence, result, another;
PigLatinTranslator translator = new PigLatinTranslator();
PigLatin.java (cont.)
do
{
System.out.println ();
System.out.println ("Enter a sentence (no punctuation):");
sentence = Keyboard.readString();
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System.out.println ();
result = translator.translate (sentence);
System.out.println ("That sentence in Pig Latin is:");
System.out.println (result);
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System.out.println ();
System.out.print ("Translate another sentence (y/n)? ");
another = Keyboard.readString();
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}
while (another.equalsIgnoreCase("y"));
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}
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}