4015: ICOM Advanced Programming
Lecture 8
Chapter Eight: Designing Classes
ICOM 4015 Fall 2008
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Chapter Eight: Designing Classes
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Chapter Goals To learn how to choose appropriate classes to implement To understand the concepts of cohesion and coupling To minimize the use of side effects To document the responsibilities of methods and their callers with preconditions and postconditions To understand the difference between instance methods and static methods To introduce the concept of static fields
Contined
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Chapter Goals (cont.) To understand the scope rules for local variables and instance fields To learn about packages To learn about unit testing frameworks
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Choosing Classes A class represents a single concept from the problem domain Name for a class should be a noun that describes concept Concepts from mathematics:
Point Rectangle Ellipse
Concepts from real life:
BankAccount CashRegister
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Choosing Classes Actors (end in -er, -or) objects do some kinds of work for you
Scanner Random // better name: RandomNumberGenerator
Utility classes no objects, only static methods and constants
Math
Program starters: only have a main method Don't turn actions into classes: Paycheck is a better name than ComputePaycheck
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.1 What is the rule of thumb for finding classes? Answer: Look for nouns in the problem description.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.2 Your job is to write a program that plays chess. Might ChessBoard be an appropriate class? How about MovePiece? Answer: Yes (ChessBoard) and no (MovePiece).
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Cohesion A class should represent a single concept The public interface of a class is cohesive if all of its features are related to the concept that the class represents This class lacks cohesion:
public class CashRegister { public void enterPayment(int dollars, int quarters, int dimes, int nickels, int pennies) . . . public static final double NICKEL_VALUE = 0.05; public static final double DIME_VALUE = 0.1; public static final double QUARTER_VALUE = 0.25; . . . }
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Cohesion
CashRegister,
as described above, involves two concepts: cash register and coin Solution: Make two classes:
public class Coin { public Coin(double aValue, String aName) { . . . } public double getValue() { . . . } . . . } public class CashRegister { public void enterPayment(int coinCount, Coin coinType) { . . . } . . . }
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Coupling A class depends on another if it uses objects of that class CashRegister depends on Coin to determine the value of the payment Coin does not depend on CashRegister High Coupling = many class dependencies Minimize coupling to minimize the impact of interface changes To visualize relationships draw class diagrams UML: Unified Modeling Language. Notation for object-oriented analysis and design
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Coupling
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
High and Low Coupling Between Classes
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.3 Why is the CashRegister class from Chapter 4 not cohesive? Answer: Some of its features deal with payments, others with coin values.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.4 Why does the Coin class not depend on the CashRegister class? Answer: None of the coin operations require the CashRegister class.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.5 Why should coupling be minimized between classes? Answer: If a class doesn't depend on another, it is not affected by interface changes in the other class.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Accessors, Mutators and Immutable Classes Accessor: does not change the state of the implicit parameter
double balance = account.getBalance();
Mutator: modifies the object on which it is invoked
account.deposit(1000);
Immutable class: has no mutator methods (e.g., String)
String name = "John Q. Public"; String uppercased = name.toUpperCase(); // name is not changed
It is safe to give out references to objects of immutable classes; no code can modify the object at an unexpected time
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.6 Is the substring method of the String class an accessor or a mutator? Answer: It is an accessor calling substring doesn't modify the string on which the method is invoked. In fact, all methods of the
String
class are accessors.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.7 Is the Rectangle class immutable? Answer: No translate is a mutator.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Side Effects Side effect of a method: any externally observable data modification
public void transfer(double amount, BankAccount other) { balance = balance - amount; other.balance = other.balance + amount; // Modifies explicit parameter }
Updating explicit parameter can be surprising to programmers; it is best to avoid it if possible
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Side Effects Another example of a side effect is output
public void printBalance() // Not recommended { System.out.println("The balance is now $" + balance); }
Bad idea: message is in English, and relies on System.out It is best to decouple input/output from the actual work of your classes You should minimize side effects that go beyond modification of the implicit parameter
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.8 If a refers to a bank account, then the call a.deposit(100) modifies the bank account object. Is that a side effect? Answer: No a side effect of a method is any change outside the implicit parameter.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.9 Consider the DataSet class of Chapter 6. Suppose we add a method
void read(Scanner in) { while (in.hasNextDouble()) add(in.nextDouble()); }
Does this method have a side effect? Answer: Yes the method affects the state of the Scanner parameter.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Common Error: Trying to Modify Primitive Type Parameters void transfer(double amount, double otherBalance)
{ balance = balance - amount; otherBalance = otherBalance + amount; }
Won't work Scenario:
double savingsBalance = 1000; harrysChecking.transfer(500, savingsBalance); System.out.println(savingsBalance);
In Java, a method can never change parameters of primitive type
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Common Error: Trying to Modify Primitive Type Parameters
double savingsBalance = 1000; harrysChecking.transfer(500, savingsBalance); System.out.println(savingsBalance); ... void transfer(double amount, double otherBalance) { balance = balance - amount; otherBalance = otherBalance + amount; }
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Common Error: Trying to Modify Primitive Type Parameters
double savingsBalance = 1000; harrysChecking.transfer(500, savingsBalance); System.out.println(savingsBalance); ... void transfer(double amount, double otherBalance) { balance = balance - amount; otherBalance = otherBalance + amount; }
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Common Error: Trying to Modify Primitive Type Parameters
double savingsBalance = 1000; harrysChecking.transfer(500, savingsBalance); System.out.println(savingsBalance); ... void transfer(double amount, double otherBalance) { balance = balance - amount; otherBalance = otherBalance + amount; }
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Common Error: Trying to Modify Primitive Type Parameters (cont.)
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Common Error: Trying to Modify Primitive Type Parameters
double savingsBalance = 1000; harrysChecking.transfer(500, savingsBalance); System.out.println(savingsBalance); ... void transfer(double amount, double otherBalance) { balance = balance - amount; otherBalance = otherBalance + amount; }
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Common Error: Trying to Modify Primitive Type Parameters (cont.)
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Animation 8.1 Trying to Modify Primitive Type Parameters
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Call by Value and Call by Reference Call by value: Method parameters are copied into the parameter variables when a method starts Call by reference: Methods can modify parameters Java has call by value A method can change state of object reference parameters, but cannot replace an object reference with another
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Call by Value and Call by Reference (cont.)
public class BankAccount { public void transfer(double amount, BankAccount otherAccount) { balance = balance - amount; double newBalance = otherAccount.balance + amount; otherAccount = new BankAccount(newBalance); // Won't work } }
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Call by Value Example
harrysChecking.transfer(500, savingsAccount);
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Preconditions Precondition: Requirement that the caller of a method must meet Publish preconditions so the caller won't call methods with bad parameters
/** Deposits money into this account. @param amount the amount of money to deposit (Precondition: amount >= 0) */
Typical use:
To restrict the parameters of a method To require that a method is only called when the object is in an appropriate state
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Preconditions (cont.) If precondition is violated, method is not responsible for computing the correct result. It is free to do anything
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Preconditions Method may throw exception if precondition violated more in Chapter 11
if (amount < 0) throw new IllegalArgumentException(); balance = balance + amount;
Method doesn't have to test for precondition. (Test may be costly)
// if this makes the balance negative, it's the caller's fault balance = balance + amount;
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Preconditions Method can do an assertion check
assert amount >= 0; balance = balance + amount;
To enable assertion checking:
java -enableassertions MyProg
You can turn assertions off after you have tested your program, so that it runs at maximum speed Many beginning programmers silently return to the caller
if (amount < 0) return; // Not recommended; hard to debug balance = balance + amount;
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Syntax 8.1 Assertion
assert condition;
Example:
assert amount >= 0;
Purpose: To assert that a condition is fulfilled. If assertion checking is enabled and the condition is false, an assertion error is thrown.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Postconditions Condition that is true after a method has completed If method call is in accordance with preconditions, it must ensure that postconditions are valid There are two kinds of postconditions:
The return value is computed correctly The object is in a certain state after the method call is completed /** Deposits money into this account. (Postcondition: getBalance() >= 0) @param amount the amount of money to deposit (Precondition: amount >= 0) */
Don't document trivial postconditions that repeat the @return clause
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Postconditions (cont.)
amount <= getBalance() // this is the way to state a postcondition amount <= balance // wrong postcondition formulation
Contract: If caller fulfills precondition, method must fulfill postcondition
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.10 Why might you want to add a precondition a to method that you provide for other programmers? Answer: Then you don't have to worry about checking for invalid values it becomes the caller's responsibility.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.11 When you implement a method with a precondition and you notice that the caller did not fulfill the precondition, do you have to notify the caller? Answer: No you can take any action that is convenient for you.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Static Methods Every method must be in a class A static method is not invoked on an object Why write a method that does not operate on an object? Common reason: encapsulate some computation that involves only numbers. Numbers aren't objects, you can't invoke methods on them. E.g., x.sqrt() can never be legal in Java public class Financial
{ public static double percentOf(double p, double a) { return (p / 100) * a; } // More financial methods can be added here. }
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Static Methods (cont.) Call with class name instead of object:
double tax = Financial.percentOf(taxRate, total);
main is static there aren't any objects yet
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.12 Suppose Java had no static methods. Then all methods of the Math class would be instance methods. How would you compute the square root of x? Answer:
Math m = new Math(); y = m.sqrt(x);
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.13 Harry turns in his homework assignment, a program that plays tictac-toe. His solution consists of a single class with many static methods. Why is this not an object-oriented solution? Answer: In an object-oriented solution, the main method would construct objects of classes Game, Player, and the like. Most methods would be instance methods that depend on the state of these objects.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Static Fields A static field belongs to the class, not to any object of the class. Also called class field public class BankAccount
{ . . . private double balance; private int accountNumber; private static int lastAssignedNumber = 1000; }
If lastAssignedNumber was not static, each instance of BankAccount would have its own value of lastAssignedNumber
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Static Fields (cont.)
public BankAccount() { // Generates next account number to be assigned lastAssignedNumber++; // Updates the static field // Assigns field to account number of this bank account accountNumber = lastAssignedNumber; // Sets the instance field }
Minimize the use of static fields (static final fields are ok)
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Static Fields Three ways to initialize:
1. Do nothing. Field is initialized with 0 (for numbers), false (for boolean values), or null (for objects) 2. Use an explicit initializer, such as public class BankAccount { . . . private static int lastAssignedNumber = 1000; // Executed once, // when class is loaded } 1. Use a static initialization block
Static fields should always be declared as private
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Static Fields (cont.) Exception: Static constants, which may be either private or public
public class BankAccount { . . . public static final double OVERDRAFT_FEE = 5; // Refer to it as // BankAccount.OVERDRAFT_FEE }
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
A Static Field and Instance Fields
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.14 Name two static fields of the System class. Answer: System.in and System.out.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.15 Harry tells you that he has found a great way to avoid those pesky objects: Put all code into a single class and declare all methods and fields static. Then main can call the other static methods, and all of them can access the static fields. Will Harry's plan work? Is it a good idea? Answer: Yes, it works. Static methods can access static fields of the same class. But it is a terrible idea. As your programming tasks get more complex, you will want to use objects and classes to organize your programs.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Scope of Local Variables Scope of variable: Region of program in which the variable can be accessed Scope of a local variable extends from its declaration to end of the block that encloses it
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Scope of Local Variables (cont.) Sometimes the same variable name is used in two methods:
public class RectangleTester { public static double area(Rectangle rect) { double r = rect.getWidth() * rect.getHeight(); return r; } public static void main(String[] args) { Rectangle r = new Rectangle(5, 10, 20, 30); double a = area(r); System.out.println(r); } }
These variables are independent from each other; their scopes are disjoint Big Java by Cay Horstmann
Copyright 2008 by John Wiley & Sons. All rights reserved.
Scope of Local Variables Scope of a local variable cannot contain the definition of another variable with the same name
Rectangle r = new Rectangle(5, 10, 20, 30); if (x >= 0) { double r = Math.sqrt(x); // Error - can't declare another variable called r here . . . }
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Scope of Local Variables (cont.) However, can have local variables with identical names if scopes do not overlap
if (x >= 0) { double r = Math.sqrt(x); . . . } // Scope of r ends here else { Rectangle r = new Rectangle(5, 10, 20, 30); // OK - it is legal to declare another r here . . . }
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Scope of Class Members Private members have class scope: You can access all members in any method of the class Must qualify public members outside scope
Math.sqrt harrysChecking.getBalance
Inside a method, no need to qualify fields or methods that belong to the same class
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Scope of Class Members (cont.) An unqualified instance field or method name refers to the this parameter
public class BankAccount { public void transfer(double amount, BankAccount other) { withdraw(amount); // i.e., this.withdraw(amount); other.deposit(amount); } . . . }
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Overlapping Scope A local variable can shadow a field with the same name Local scope wins over class scope
public class Coin { . . . public double getExchangeValue(double exchangeRate) { double value; // Local variable . . . return value; } private String name; private double value; // Field with the same name }
Continued
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Overlapping Scope (cont.) Access shadowed fields by qualifying them with the this reference
value = this.value * exchangeRate;
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.16 Consider the deposit method of the BankAccount class. What is the scope of the variables amount and newBalance? Answer: The scope of amount is the entire deposit method. The scope of newBalance starts at the point at which the variable is defined and extends to the end of the method.
Big Java by Cay Horstmann Copyright 2008 by John Wiley & Sons. All rights reserved.
Self Check 8.17 What is the scope of the balance field of the BankAccount class? Answer: It starts at the beginning of the class a...