Chapter 3 Implementing Classes
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Transcript of Chapter 3 Implementing Classes
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Chapter 3 Implementing Classes 1
Chapter 3
Implementing Classes
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Chapter Goals To become familiar with the process of
implementing classes
To be able to implement simple methods
To understand the purpose and use of constructors
To understand how to access instance fields and local variables
To appreciate the importance of documentation comments
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Black Boxes
A black box magically does something
Hides its inner workings
Encapsulation: the hiding of “unimportant” details
What is the right concept for each black box?
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Black Boxes
Concepts are discovered through abstraction
Abstraction: taking away inessential features, until only the essence of the concept remains
In object-oriented programming the black boxes from which a program is manufactured are called objects
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Levels of Abstraction: A Real-Life Example
Black boxes in a car: transmission, electronic control module, etc.
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Levels of Abstraction: A Real- Life Example
Users of a car do not need to understand how black boxes work
Interaction of a black box with outside world is well-defined Drivers interact with car using pedals, buttons, etc. Mechanic can test that engine control module sends the
right firing signals to the spark plugs For engine control module manufacturers, transistors
and capacitors are black boxes magically produced by an electronics component manufacturer
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Levels of Abstraction: A Real- Life Example
Encapsulation leads to efficiency
Mechanic deals only with car components (e.g. electronic module), not with sensors and transistors However, it may be important for mechanic to
understand how things actually work…
Driver worries only about interaction with car (e.g., putting gas in the tank), not about engine electronics
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Levels of Abstraction: Software Design
Figure 2:Levels of Abstraction in Software Design
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Levels of Abstraction: Software Design
Good old days: computer programs manipulated primitive types (numbers, chars)
Manipulating too many of these primitive quantities increases complexity for programmers and leads to errors
Solution: Encapsulate routine computations in (software) black boxes
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Software and Complexity
How complex is software today?
Netscape 17,000,000
Space shuttle 10,000,000
Linux 1,500,000
Windows XP 40,000,000
Boeing 777 7,000,000
system Lines of code (LOC)
A new car contains more LOC than was required to land the Apollo astronauts on the moon
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Levels of Abstraction: Software Design
Abstraction used to invent higher-level data types
In object-oriented programming, objects are black boxes
Encapsulation: Programmer using an object knows about its behavior, but not about its internal structure (recall auto mechanic…)
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Levels of Abstraction: Software Design
In meatspace, possible abstractions are limited by physical world “Right” abstraction is usually easy
In software, easy to design bad abstractions, understanding good design is an important part of software engineering
First, define behavior of a class, then implement it
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Self Check
1. In Chapters 1 and 2, you used System.out as a black box to cause output to appear on the screen. Who designed and implemented System.out?
2. Suppose you are working in a company that produces personal finance software. You are asked to design and implement a class for representing bank accounts. Who will be the users of your class?
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Answers
1. The programmers who designed and implemented the Java library
2. Other programmers who work on the personal finance application
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Designing the Public Interface of a Class
Behavior of bank account (abstraction) deposit money withdraw money get balance
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Designing the Public Interface of a Class: Methods
Methods of BankAccount class
We want to support method calls such as
harrysChecking.deposit(2000);harrysChecking.withdraw(500);System.out.println(harrysChecking.getBalance());
deposit withdraw getBalance
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Designing the Public Interface of a Class: Method Definition
access specifier (such as public)
return type (such as String or void)
method name (such as deposit)
list of parameters (double amount for deposit)
method body enclosed in brackets: { }
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Designing the Public Interface of a Class: Method Definition
public void deposit(double amount) { . . . } public void withdraw(double amount) { . . . } public double getBalance() { . . . }
Examples
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Syntax 3.1: Method Definition
accessSpecifier returnType methodName(parameterType parameterName, . . .){ method body }
Example: public void deposit(double amount) { . . . }
Purpose:To define the behavior of a method
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Designing the Public Interface of a Class: Constructor Definition
A constructor initializes the instance variables (the “state” of an object) Also known as instance fields or just fields
Constructor name is same as class name
public BankAccount(){ // body--filled in later }
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Designing the Public Interface of a Class: Constructor Definition
Constructor body is executed when new object is created
Statements in constructor body will set the instance variables of the object that is constructed
All constructors of a class have the same name There may be multiple constructors
Compiler can tell constructors apart because they take different parameters
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Syntax 3.2: Constructor Definition
accessSpecifier ClassName(parameterType parameterName, . . .) { constructor body }
Example: public BankAccount(double initialBalance) { . . . }
Purpose:To define the behavior of a constructor
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BankAccount Public Interface The public constructors and methods of a
class form the public interface of the class. public class BankAccount{ // Constructors public BankAccount() { // body--filled in later } public BankAccount(double initialBalance) { // body--filled in later }
// Methods public void deposit(double amount) Continued…
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BankAccount Public Interface
{ // body--filled in later }
public void withdraw(double amount) { // body--filled in later } public double getBalance() { // body--filled in later } // private fields--filled in later }
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Syntax 3.3: Class Definition accessSpecifier class ClassName { constructors methods fields }
Example: public class BankAccount { public BankAccount(double initialBalance) { . . . } public void deposit(double amount) { . . . } . . . }
Purpose:To define a class, its public interface, and its implementation details
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Self Check
3. How can you use the methods of the public interface to empty the harrysChecking bank account?
4. Suppose you want a more powerful bank account abstraction that keeps track of an account number in addition to the balance. How would you change the public interface to accommodate this enhancement?
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Answers
3.
4. Add an accountNumber parameter to the constructors, and add a getAccountNumber method. There is no need for a setAccountNumber method–the account number never changes after construction.
harrysChecking.withdraw(harrysChecking.getBalance())
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Commenting the Public Interface/** Withdraws money from the bank account. @param the amount to withdraw*/public void withdraw(double amount){ // implementation filled in later }
/** Gets the current balance of the bank account. @return the current balance*/public double getBalance(){ // implementation filled in later }
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Class Comment
/** A bank account has a balance that can be changed by deposits and withdrawals.*/public class BankAccount{ . . .}
Provide documentation comments for every…
… class, method, parameter, return value
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Javadoc Method Summary
Figure 3:A Method Summary Generated by javadoc
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Javadoc Method Detail
Figure 4:Method Detail Generated by javadoc
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Self Check
5. Suppose we enhance the BankAccount class so that each account has an account number. Supply a documentation comment for the constructor BankAccount(int accountNumber, double initialBalance)
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Self Check
6. Why is the following documentation comment questionable?
/** Each account has an account number. @return the account number of this account.*/int getAccountNumber()
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Answers
6. The first sentence of the method description should describe the method (it is displayed in isolation in the summary table)
/** Constructs a new bank account with a given initial balance. @param accountNumber the account number for this account @param initialBalance the initial balance for this account*/
5.
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Instance Fields An object stores its data in instance fields (that
is, instance variables) “Field” is a technical term for a storage location inside a
block of memory
An instance of a class is an object of the class The class declaration specifies the instance fields
public class BankAccount{ . . . private double balance;}
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Instance Fields
An instance field declaration consists of access specifier (such as private) type of variable (such as double) name of variable (such as balance)
Each object of a class has its own set of instance fields
Declare all instance fields as private
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Instance Fields
Figure 5:Instance Fields
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Syntax 3.4: Instance Field Declaration
accessSpecifier class ClassName { . . . accessSpecifier fieldType fieldName; . . . }Example: public class BankAccount { . . . private double balance; . . . }Purpose:To define a field that is present in every object of a class
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Accessing Instance Fields
The deposit method of the BankAccount class can access the private instance field
public void deposit(double amount){ double newBalance = balance + amount; balance = newBalance;}
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Accessing Instance Fields Methods in other classes cannot access private instance fields
Encapsulation hiding data and providing access through methods
public class BankRobber{ public static void main(String[] args) { BankAccount momsSavings = new BankAccount(1000); . . . momsSavings.balance = -1000; // ERROR }}
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Self Check
7. Suppose we modify the BankAccount class so that each bank account has an account number. How does this change affect the instance fields?
8. What are the instance fields of the Rectangle class?
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Answers
7. An instance field, such as
needs to be added to the class
8. private int x;private int y;private int width;private int height;
private int accountNumber;
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Implementing Constructors
Constructors contain instructions to initialize the instance fields of an object
public BankAccount(){ balance = 0;}public BankAccount(double initialBalance){ balance = initialBalance;}
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Constructor Call Example
Creates a new object of type BankAccount Calls the second constructor (why?) Sets the initialBalance to 1000 Sets the balance instance field of the newly
created object to initialBalance Returns an object reference, the memory
location of the object, as the value of the new expression
Stores that object reference in the harrysChecking variable
BankAccount harrysChecking = new BankAccount(1000);
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Implementing Methods Some methods do not return a value
Some methods return an output value
public void withdraw(double amount){ double newBalance = balance - amount; balance = newBalance;}
public double getBalance(){ return balance;}
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Method Call Example
Set the parameter variable amount to 500 Fetch the balance field of the object whose location is stored in harrysChecking Add the value of amount to balance and store the result in the variable newBalance Store the value of newBalance in the balance instance field, overwriting the old value
harrysChecking.deposit(500);
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Syntax 3.5: The return Statement
return expression; or return;
Example: return balance;
Purpose:To specify the value that a method returns, and exit the method immediately. The return value becomes the value of the method call expression.
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File BankAccount.java01: /**02: A bank account has a balance that can be changed by 03: deposits and withdrawals.04: */05: public class BankAccount06: { 07: /**08: Constructs a bank account with a zero balance.09: */10: public BankAccount()11: { 12: balance = 0;13: }14: 15: /**16: Constructs a bank account with a given balance.17: @param initialBalance the initial balance 18: */ Contnued…
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File BankAccount.java19: public BankAccount(double initialBalance)20: { 21: balance = initialBalance;22: }23: 24: /**25: Deposits money into the bank account.26: @param amount the amount to deposit27: */28: public void deposit(double amount)29: { 30: double newBalance = balance + amount;31: balance = newBalance;32: }33: 34: /**35: Withdraws money from the bank account.36: @param amount the amount to withdraw Contnued…
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File BankAccount.java37: */38: public void withdraw(double amount)39: { 40: double newBalance = balance - amount;41: balance = newBalance;42: }43: 44: /**45: Gets the current balance of the bank account.46: @return the current balance47: */48: public double getBalance()49: { 50: return balance;51: }52: 53: private double balance;54: }
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Self Check
9. How is the getWidth method of the Rectangle class implemented?
10. How is the translate method of the Rectangle class implemented?
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Answers
9.
10. One possible implementation is
public int getWidth(){ return width;}
public void translate(int dx, int dy){ int newx = x + dx; x = newx; int newy = y + dy; y = newy;}
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Testing a Class
Test class: a class with a main method that contains statements to test another class Also known as a “tester” class
Typically carries out the following steps 1. Construct one or more objects of the class
that is being tested 2. Invoke one or more methods 3. Print out one or more results
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Testing a Class
Details for building test program vary
In most environments, you need to 1. Make a new subfolder for your program 2. Make two files, one for each class 3. Compile both files 4. Run the test program
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File BankAccountTester.java01: /**02: A class to test the BankAccount class.03: */04: public class BankAccountTester05: {06: /**07: Tests the methods of the BankAccount class.08: @param args not used09: */10: public static void main(String[] args)11: {12: BankAccount harrysChecking = new BankAccount();13: harrysChecking.deposit(2000);14: harrysChecking.withdraw(500);15: System.out.println(harrysChecking.getBalance());16: }17: }
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Testing with Bluej
Figure 6:The Return Value of the getBalance Method in Bluej
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Self Check
11. When you run the BankAccountTester program, how many objects of class BankAccount are constructed? How many objects of type BankAccountTester?
12. Why is the BankAccountTester class unnecessary in development environments such as BlueJ?
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Answers
11. One BankAccount object, no BankAccountTester object. The purpose of the BankAccountTester class is merely to hold the main method
12. In those environments, you can issue interactive commands to construct BankAccount objects, invoke methods, and display their return values
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Categories of Variables
Categories of variables Instance fields (balance in BankAccount) Local variables (newBalance in deposit
method) Parameter variables (amount in deposit
method)
An instance field belongs to an object
The fields stay alive until no method uses the object any longer
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Categories of Variables
In Java, the garbage collector periodically reclaims objects when they are no longer used
Local and parameter variables belong to a method
Instance fields are initialized to a default value, but you must initialize local variables
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Lifetime of Variables
harrysChecking.deposit(500);double newBalance = balance + amount;balance = newBalance;
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Lifetime of Variables
Figure 7: Lifetime of Variables
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Lifetime of Variables
Figure 7: Lifetime of Variables
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Self Check
13. What do local variables and parameter variables have in common? In which essential aspect do they differ?
14. During execution of the BankAccountTester program in the preceding section, how many instance fields, local variables, and parameter variables were created, and what were their names?
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Answers
13. Variables of both categories belong to methods. They come alive when the method is called, and they die when the method exits. They differ in their initialization. Parameter variables are initialized with the call values; local variables must be explicitly initialized.
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Answers
14. One instance field, named balance. Three local variables, one named harrysChecking and two named newBalance (in the deposit and withdraw methods); two parameter variables, both named amount (in the deposit and withdraw methods).
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Implicit and Explicit Method Parameters
The implicit parameter of a method is the object on which the method is invoked
The this reference denotes the implicit parameter
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Implicit and Explicit Method Parameters
Use of an instance field name in a method denotes the instance field of the implicit parameter
public void withdraw(double amount){ double newBalance = balance - amount; balance = newBalance;}
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Implicit and Explicit Method Parameters
balance refers to the balance of the object to the left of the dot, so, for example,
means
double newBalance = momsSavings.balance - amount;momsSavings.balance = newBalance;
momsSavings.withdraw(500);
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Implicit Parameters and this
Every method has one implicit parameter
The implicit parameter is always called this
Exception: static methods do not have an implicit parameter (more in Chapter 8)
double newBalance = balance + amount;// actually meansdouble newBalance = this.balance + amount;
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Implicit Parameters and this
When you refer to an instance field in a method, the compiler automatically applies it to the this parameter
momsSavings.deposit(500);
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Implicit Parameters and this
Figure 8:The Implicit Parameter of a Method Call
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Self Check
15. How many implicit and explicit parameters does the withdraw method of the BankAccount class have, and what are their names and types?
16. In the deposit method, what is the meaning of this.amount? Or, if the expression has no meaning, why not?
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Self Check
17. How many implicit and explicit parameters does the main method of the BankAccountTester class have, and what are they called?
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Answers
15. One implicit parameter, called this, of type BankAccount, and one explicit parameter, called amount, of type double
16. It is not a legal expression since this is of type BankAccount and the BankAccount class has no field named amount
17. No implicit parameter (the method is static) and one explicit parameter, args
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Electronic Voting Machines
Figure 9:Punch Card Ballot
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Electronic Voting Machines
Figure 10:Touch Screen Voting Machine