Chapter 5 Methods Introducing Methods Declaring Methods

Calling Methods

Passing Parameters

Pass by Value

Overloading Methods

Method Abstraction

local variable, global variable

The Math Class (library functions)

Java Documentation

Introducing Methods

Method StructureA method is a collection of statements that are grouped together to perform an operation.

public static int max(int num1, int num2){

int result = 0;

if (num1 > num2) result = num1;else result = num2;

return result;}

modifier

returnValueType

methodName

parameters

return value

methodbody

methodheading

Declaring Methods

public static int max(int num1, int num2)

{ if (num1 > num2) return num1; else return num2;}

Calling Methods

Example 4.1 Testing the max method

This program demonstrates calling a method max to return the largest of the int values

TestMaxTestMax Run

Passing ParametersYou pass arguments to the corresponding

parameters in the method signatureFor e.g:

void nPrintln(String message, int n){ for (int i=0; i<n; i++) System.out.println(message);}

To call the above method:nPrintln(m, i); //where m is String and i is int

message and n are parametersm and i are arguments

Pass by ValuePass by value: the parameter is allocated the required memory storage and the value of the argument is copied to the parameter. There are 2 separate copies of the same values.

Example 4.2 Testing Pass by value

This program demonstrates passing values to the methods.

TestPassByValueTestPassByValue Run

Overloading MethodsMethods defined with the same name are overloaded. The

parameter lists are different, the return type is not relevant.

Example 4.3 Overloading the max Methoddouble max(double num1, double num2) { if (num1 > num2) return num1; else return num2;}

TestMethodOverloadingTestMethodOverloading Run

The scope of variables

Local variable

Global variable

Scope of variable

Method Abstraction

You can think of the method body as a black box that contains the detailed implementation for the method.

Method Signature

Method body

Black Box

Optional Input Optional returnvalue

The Math Class

Class constants:– PI– E (the base of natural logarithms)

Class methods: – Trigonometric Methods – Exponent Methods– Miscellaneous

Trigonometric Methods sin(double a)

cos(double a)

tan(double a)

acos(double a)

asin(double a)

atan(double a)

Exponent Methods exp(double a)

Returns e raised to the power of a.

log(double a)

Returns the natural logarithm of a.

pow(double a, double b)

Returns a raised to the power of b.

sqrt(double a)

Returns the square root of a.

Rounding Methods double ceil(double x)

Returns the smallest (closest to negative infinity) double value that is greater than or equal to the argument and is equal to a mathematical integer.

double floor(double x) Returns the largest (closest to positive infinity) double value that is

less than or equal to the argument and is equal to a mathematical integer.

double rint(double x)Returns the double value that is closest in value to the argument and is

equal to a mathematical integer. int round(float x)

Returns the closest int to the argument. Long round(double x)

Returns the closest long to the argument.

min, max, abs, and random max(a, b)and min(a, b)

Returns the maximum or minimum of two parameters.

abs(a)Returns the absolute value of the parameter.

random()Returns a double value with a positive sign, greater than or equal to 0.0 and less than 1.0. Returns a random double value in the range [0.0, 1.0).

Java Documentation

http://java.sun.com/javase/6/docs/api/

Using Math Methods

Example 4.4 Computing Mean and Standard Deviation

Generate 10 random numbers and compute the mean and standard deviation

ComputeMeanDeviationComputeMeanDeviation Run

n

xmean

n

ii

1

1

)(

1

2

12

nn

xx

deviation

n

i

n

ii

i

Case Studies

Example 4.5 Displaying Calendars

The program reads in the month and year and displays the calendar for a given month of the year.

PrintCalendarPrintCalendar Run

Design DiagramprintCalendar

(main)

readInput printMonth

printMonthTitle printMonthBody

getMonthName getStartDay

getTotalNumberOfDays

getNumberOfDaysInMonth

isLeapYear

Recursion

E.g. 4.6 Factorial

Factorial(0)=1; Factorial(n)=n*factorial(n-1)

Recursion

E.g. 4.7 fibonacci numbers

Fib(0)=1; Fib(1)=1; Fib(n)=Fib(n-2)+Fib(n-1), n>=2

Recursion

E.g. 4.8 Towers of Hanoi 3 Poles and n disks

– Move n disks from pole A to pole B.

– One disk can be moved at any time

– Big disk can’t placed on top of small disk

Recursion

Towers of Hanoi Solution

– If n=1, move disk 1 from A to B

– Else Move n-1 disks from A to C, B as temporary Move disk n from A to B Move n-1 disks from C to B, A as temporary