CS100J%20Lecture%207

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CS100J Lecture 7

• Previous Lecture
• Computation and computational power
• Abstraction
• Classes, Objects, and Methods
• References and aliases
• Reading
• Lewis Loftus, Chapter 4 and Section 5.1
• Savitch, Chapter 4
• This Lecture
• Programming Concepts
• Classes, Objects, and Methods --- reiterated
• Accessors
• Encapsulation
• Java Constructs
• return statement
• Visibility modifiers
• public
• private

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Class Definition

• A class is a named group of declarations and
  methods
• Class definition
• class class-name
• declarations-and-methods
• A method is a named parameterized group of
  declarations and statements
• Method definition
• return-type method-name( parameter-list )
• declaration-and-statement-list

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Class Definition Example

• class Account
• int balance // current balance
• int deposits // deposits to date
• int withdrawals // withdrawals to date
• // deposit d to account
• void deposit(int d)
• balance balance d
• deposits deposits d
• // withdraw w from account
• void withdraw(int w)
• balance balance - w
• withdrawals withdrawals w

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Object Instantiation

• The value of expression
• new class-name()
• is a reference to a new object of the given
  class-name

5
Object Instance Example

• An object is a collection of instance variables
  (also known as fields) and methods
• An object can be referred to as a unit

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Access to Fields and Methods

• If
• r is an expression whose value refers to an
  object o of class c
• f is a field of o
• m is a method of o
• then
• r.f is a variable of object o
• r.m is a method of o

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Application Setting

• Process bank account transactions
• The input consists of zero or more non-zero
  integers signifying transactions, followed by
  0.
• A positive value is a deposit.
• A negative value is a withdrawal.
• Assume the account has an initial balance of 0,
  process all transactions, and output
• Final balance
• Total of all deposits
• Total of all withdrawals
• Sample input data
• 100 200 -50 0
• Sample output
• Balance 250
• Deposits 300
• Withdrawals 50

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Application Example

• int amount // current transaction
• Account account new Account()
• / Read and process all transactions. /
• amount in.readInt()
• while ( amount ! 0 )
• if ( amount gt 0 )
• account.deposit(amount)
• else
• account.withdraw(-amount)
• amount in.readInt()
• / Output summary information. /
• System.out.println(
• Balance account.balance
• )

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Application Example, cont.

• The code for the application would be in a
  different class from Account, say
• / Process banking stransactions. /
• import java.io.
• public class TrivialApplication
• public static void main(String args)
• declarations
• // Initialize Text object in to read
• // from standard input.
• TokenReader in new TokenReader(System.in)
• statements

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Methods that Return Values

• Method invocation
• object-expression . method-name(expression-list)
• Method execution
• initialize the parameters with the values of the
  argument expressions
• execute the statement-list of the method
• thereafter, continue where you were before
• Non-void methods with return-type t can return a
  value of type t using the statement
• return expression
• A method that just returns the value of a field
  is called an field accessor.

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Class Definition Example, Version 2

• class Account
• int balance // current balance
• int deposits // deposits to date
• int withdrawals // withdrawals to date
• // deposit d to account
• void deposit(int d)
• balance balance d
• deposits deposits d
• // withdraw w from account
• void withdraw(int w)
• balance balance - w
• withdrawals withdrawals w
• // field accessors

methods for accessing field values
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Application Example, Version 2

• int amount // current transaction
• Account account new Account()
• / Read and process all transactions. /
• amount in.readInt()
• while ( amount ! 0 )
• if ( amount gt 0 )
• account.deposit(amount)
• else
• account.withdraw(-amount)
• amount in.readInt()
• / Output summary information. /
• System.out.println(
• Balance account.getBalance()
• )
• System.out.println(

field values accessed via methods
field values accessed via methods
field values accessed via methods
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Field Modifiers

• A field declarations and method definitions can
  have a modifier
• Declarations
• modifier type name
• Method definitions
• modifier
• return-type method-name( parameter-list )
• statement-list
• Possible modifiers are
• public
• private
• others later

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Class Definition Example, Version 3
fields hidden from clients

• class Account
• private int balance // current balance
• private int deposits // deposits to date
• private int withdrawals // withdrawals to date
• // deposit d to account
• void deposit(int d)
• balance balance d
• deposits deposits d
• // withdraw w from account
• void withdraw(int w)
• balance balance - w
• withdrawals withdrawals w
• // field accessors

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Class Definition Example, Version 4
balance field deleted

• class Account
• private int deposits // deposits to date
• private int withdrawals // withdrawals to date
• // deposit d to account
• void deposit(int d)
• deposits deposits d
• // withdraw w from account
• void withdraw(int w)
• withdrawals withdrawals w
• // return computed balance
• int getBalance()
• return deposits - withdrawals

balance computed
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Encapsulation and Information Hiding

• The users of a class and its objects are called
  its clients.
• The author of a class is called its implementor.
• Information hiding principle. Implementors hide
  implementation details inside a class definition
  so clients cant see them. In particular, they
  make all fields private.
• The methods of a class provide services to
  clients. The interface of a class is the contract
  in which the implementor commits to deliver
  certain services to the clients.
• Implementors are free to change the
  implementation of a class provided the interface
  doesnt change, i.e. provided clients cant see
  any change in services.