©2000, john wiley & sons, inc. horstmann/java essentials, 2/e 1 chapter 7: more about methods 1...

©2000, John Wiley & Sons, Inc.Horstmann/Java Essentials, 2/e

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Chapter 7: More about Methods1

Chapter 7

More about Methods


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Formal and actual parameters

• xFormal parameters:class BankAccount{ public void deposit(double amount) { balance += amount; } . . .}

• Actual parameters:harrysChecking.deposit(allowance - 200)


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Figure 1Parameter Passing


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Object parameters

• public void transfer(BankAccount other, double amount){ withdraw(amount); other.deposit(amount);}

• Call with an object and a number parameter:double allowance = 800;momsSavings.transfer(harrysChecking, allowance);


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Figure 2Object References andNumber Parameters


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Accessor and mutator methods

• Accessor: does not change the state of the implicit parameter (e.g. getBalance)

• Mutator: changes the state of the implicit parameter (e.g. deposit)

• Immutable class: all methods are accessors (e.g. String)


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Side effect

• Any observable change outside the implicit parameter

• Example: modify other parameter (e.g. transfer)

• Example: printing in method: public void deposit(double amount){ if (amount < 0) System.out.println("Bad value"); . . .}


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Minimize side effects

• Excellent: No modifications (getBalance)• Good: Mutators that only change the implicit

parameter (deposit)• Fair: Methods that change an explicit

parameter (transfer)• Poor: Methods with other side effects

(changing static variables, printing)


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Static methods

• Method with no implicit parameter• Doesn't act on an object• Typical example: all parameters are numbers:

Recall that x and y are approximately equal if |x - y| £ e max(|x|, |y|)

• public static boolean approxEqual (double x, double y){ return Math.abs(x - y) <= EPSILON * Math.max(Math.abs(x), Math.abs(y));}


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Static methods

• In Java, every method must be in a class:class Numeric{ public static boolean approxEqual(double x, double y) { . . . }}

• Call with class name instead of object:if (Numeric.approxEqual(a, b)) . . .


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Can't modify number parameters

• public static void updateBalance (double balance, double interestRate){ double interest = balance * interestRate / 100; balance = balance + interest;}

• Won't work:double savings = 10000;double rate = 5;updateBalance(savings, rate);


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Figure 3A Method Cannot ModifyNumeric Parameters


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Can modify object parameter state

• public static void updateBalance (BankAccount account, double interestRate){ double interest = account.getBalance() * interestRate / 100; account.deposit(interest);}

• BankAccount collegeFund = new BankAccount(10000);double rate = 5;updateBalance(collegeFund, rate);


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Figure 4A Method CanModify theState of ObjectParameters


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Can't modify object param content

• public static chooseAccount(BankAccount betterAccount, BankAccount candidate1, BankAccount candidate2){ if (candidate1.getBalance() > candidate2.getBalance()) betterAccount = candidate1; else betterAccount = candidate2;}

• BankAccount collegeFund = new BankAccount(10000);BankAccount momsSavings = new BankAccount(8000);BankAccount myAccount = null; // won't workchooseAccount(myAccount,momsSavings,collegeFund);


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Figure 5A Method Cannot ReplaceObject Parameters


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Static variables

• One variable per class, not per object• Example: last assigned account number—

class property, not object property• public class BankAccount{ . . . private double balance; private int accountNumber; private static int lastAssignedNumber;}


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Static variables

• public BankAccount(){ lastAssignedNumber++; accountNumber = lastAssignedNumber;}

• Static variable initialization:class BankAccount{ . . . private static int lastAssignedNumber = 0;}

• Static constants: Math.PI


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Figure 6Instance Variables and aStatic Variable


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Variable types

• Instance variables• Static variables• Local variables• Parameter variables


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Scope

• Class scope—defined in class (instance or static variable)

• Block scope—defined in block (local variable)• Shadowingclass BankAccount{ public BankAccount(double initialBal) { double balance = initialBal; } private double balance; }


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Method Comments

• /** Tests whether two floating-point numbers are equal except for roundoff @param x a floating-point number @param y a floating-point number @return true if x and y are approximately equal*/public static boolean approxEqual(double x, double y){ . . .}


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Class comments

• /** A class that stores the position and speed of a simulated cannon ball*/public class CannonBall{ . . .}

• Extract comments with javadoc utility:javadoc MyProg.java


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Figure 7An HTML Page Produced by thejavadoc Utility


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Program Fac.java

public class Fac{ public static void main(String[] args) { ConsoleReader console = new ConsoleReader(System.in); System.out.println("Please enter a number:"); int n = console.readInt(); System.out.println(n + "! = ” + factorial(n));}


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Preconditions

• What should a method do when it is called with bad parameters?Math.sqrt(-1);account.deposit(-1000);

• Do nothing?• Terminate program?• Return error condition to caller?


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Preconditions

• Establish precondition:@param amount the amount ofmoney to deposit; must be > 0

• Throw exception if precondition is violated:if (amount <= 0) throw new IllegalArgumentException();

• Caller's responsibility: call the methods with parameters that fulfill the precondition

• Your responsibility: compute the right results when the parameters fulfill the precondition


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Recursion

• n! = 1 ´ 2 ´ 3 ´ . . . ´ n• 0! = 1

1! = 12! = 23! = 64! = 245! = 120

• Number of permutations of n symbols• 3! = 6: car, rac, arc, acr, rca, cra


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Recursion

• No magic ... operation:public static int factorial(int ){ return 1 * 2 * 3 * ... * n;}

• That's not how I computed the table—I usedn! = (n - 1)! ´ n

• public static int factorial(int ){ return factorial(n - 1) * n;}

• Almost right—recursion must terminate.


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Recursion

• Every recursive call must simplify the computation in some way

• There must be special cases to handle the simplest computations

• factorial(4) calls factorial(3) factorial(3) calls factorial(2) factorial(2) calls factorial(1) factorial(1) calls factorial(0) factorial(0) returns 1 factorial(1) returns 1 * 1 = 1 factorial(2) returns 1 * 2 = 2 factorial(3) returns 2 * 3 = 6factorial(4) returns 6 * 4 = 24


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/** Computes the factorial of an integer. @param n an integer >= 0 @return n! */ public static int factorial(int n) { if (n == 0) return 1; else { int result = n * factorial(n - 1); return result; } }}


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Figure 11Towers of Hanoi

©2000, john wiley & sons, inc. horstmann/java essentials, 2/e 1 chapter 7: more about methods 1...

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