Method / Fungsi

7/30/2019 Method / Fungsi

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Lecture 5: Functional

decomposition

Prepared by: Ms Sandy Lim

BIT106 Introduction toProgramming in Java


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The Math class

We can use pre-defined methods fromthe Math class to perform calculations.

Method Return

type

Example Meaning

Math.abs(num) int /

double

int pValue = Math.abs(-5) pValue |-5| or 5

Math.max(num1, num2) int /

double

int larger = Math.max(5, 6)

double bigger = Math.max(2.3, 1.7)

larger 6

bigger 2.3

Math.min(num1, num2) int /

double

int min = Math.min(5, 1)

double small =Math.min(5.16, 5.17)

min 1

small 5.16

Math.pow(num1, num2) double double value = Math.pow(2, 3) value 2^3 or 8.0

Math.round(num) long long rounded1 = Math.round(2.16)

long rounded2 = Math.round(2.87)

rounded1 2

rounded2 3

Math.sqrt(num) double double sqrt1 = Math.sqrt(9)

sqrt1 9 or 3


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Review

Try each of the pre-defined methods fromthe Math class listed on the previous slide


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5

Review

Write a Java program that asks the user toenter two numbers, then find the absolute

value each of the numbers. Then find

square root of the larger of the two positivenumbers.


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Functional Decomposition

When our programs become larger, we may

want to break them down into parts.

This is done by using separate, independent

methods that have a specific purposeor

function.

The mainmethod will then be kept at a

reasonable size.


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Advantages of using methods

The mainmethod is smaller and more readable

Each method can be studied carefully and

debugged separately

Methods can be invoked (called) many timeswithout duplication of code

Methods can be modified without affecting other

parts of the program

Methods may even be used by other programs


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Using Methods in Programs

When we use methods, the mainmethod actsas a coordinator

Particular methods are called orinvokedas

the need arises.

The computer passes the control to the

method when the method is called.


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Method Structure

A Java method has four parts: A return type

indicates the type of data to be returned by the method

A meaningful name

indicates the purpose of the method An argument / parameter list

indicates data that are input to the method

A body contains the processing instructions for the method

The method is organized into the method header

the method body


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Example

Here's a method that calculates and returns the

total cost of an item given the unit price and quantity

purchased

double calcCost(double uPrice, int qty)

{

double cost = uPrice * qty;return cost;

}

return type

method name

parameter list

method bodymethod header


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Method Header

The method header is very important because it indicates the

input

output

purpose of the method

Can you determine the inputs, output and purpose of the followingmethods?


boolean equalsIgnoreCase(String anotherString)

double sqrt(double number)void displayErrorMessage()

int largest(int a, int b, int c)

String doSomething(char x, int y)


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Access Modifiers

An access modifier indicates how a method can beaccessed: public

private

protected

static

A public method indicates that it can be accessedfrom outside the class.

If no access modifier is specified, the default will bepublic access.


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Static methods

Static methods are methods which belong to a class

and do not require an object to be invoked.

Some methods have no meaningful connection to an object.

For example,

finding the maximum of two integers

computing a square root

converting a letter from lowercase to uppercase

generating a random number Such methods can be defined as static.


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Example

Consider the following static method:

publicstaticvoid printStars()

{

System.out.println("***********************");

}

static keyword used

void return type

indicates no value will be returned


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Example method invocation

publicclass Star

{

publicstaticvoid main(String[] args)

{

System.out.println("Here is a line of stars");

printStars();

System.out.println("Here is another!");

printStars();

}

publicstaticvoid printStars()

{System.out.println("***********************");

}

}

static keyword usedvoid return type

indicates no value will be returned

method invoked here

and here


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Invoking from another class

The static methods defined in one class can be

invoked from another class.

The name of the class where the method is defined

must be used.publicclass PrintName

{

publicstaticvoid main(String[] args)

{

System.out.println("My name is Jane!");Star.printStars();

}

}

The static method printStars() is found in the class Star


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Arguments

Arguments or parameters provide inputs to a

method.

A method definition contains a formal argument

list in the method headerArguments are defined with a type and a name and

are separated by commas.

Some methods may not receive any arguments.

double sqrt (double num)

boolean login (String username, String password)

char letterGrade(double examMark, int attendance)


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Invocation with arguments

A method invocation must correspond to the argument list of

the method:

number of arguments

type of arguments

order of arguments

Example: A method has the following header:

publicstaticvoid greeting(char gender, String name)

Which statement can beused to invoke thismethod?

greeting("m", "Joe");

greeting("Jane", 'f');

greeting("Joe");

greeting('f', "Joe");


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Overloading

We can create two methods in the Star class with the samename, printStars

However, the method headers must be different:

public static void printStars()

// this method prints a line of stars

publicstaticvoid printStars(int n)

// this method prints a line of n stars

Creating two methods with the same name is calledoverloading.

The method that is invoked will be based on thearguments.

Consider the Math class methods.


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Exercise

Using the two methods printStars() andprintStars(n), available in the Star class,

write a program that will print the following:

Starting************************************

*

**

*******

Finished:

*************************************


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Exercise

Write down the definition of a Java method

which calculates and displays the average

score obtained by a student given three

assignment scores. How do we call this method?


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Returning Data from Methods

Methods are often used to process inputs

(arguments).

The results of this processing can be returned to

the calling code(the code that invoked the method)

int bigger = Math.max(15, 6);

System.out.print("The larger value is " + bigger);

The result of this method invocation

is 15.

bigger gets the

value that is

returned


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Return Types

A method header must have a return type.


{

double cost = uPrice * qty;

return cost;}

return type is double

a value must be returnedthe returned value

must be a double


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Void Return Type

If the return type in a method header is void, thismeans that the method does not return any values.

// This method simply prints out a

greeting.publicstaticvoidgreeting(String name)

{

System.out.println("Hello," + name);

System.out.println("How are youtoday?");

}


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Using Returned Data

Data that is returned from a method can be: used directly in an expression, or

stored in a variable

Example: given the static method:

double price = 2.50; // price per item

System.out.println("The cost of 3 items " + cost(price, 3));

System.out.print("How many items do you want?");

int number = sc.nextInt();

double totalCost = cost(price, number);

System.out.println("The cost of " + number + " items is ");System.out.println(totalCost);

publicstaticdouble cost(double uPrice, int qty)

{

return uPrice * qty;

}

invocations:


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Exercise

Write a method that receives input as an integer

representing a time in minutes and returns a String

representing the time in hours and minutes.

Eg: input: 415

returned value: "6 hours 55 minutes"

Write a program to test this method.


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Exercise

Write two methods, calcTotal and findGrade.

The first method should calculate and return the total based on two

scores: assignment and exam.

the assignment is worth 30%

the exam is worth 70% find the total based on the weights

F< 50

D50 total < 60

C60 total < 70

B70 total < 80

A80 total 100

GradeRangeThe second method shouldreturn a letter grade based onthe table:

Place both methods in a classnamed Result and test them.


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Exercise

Now use the methods you defined to write the followingprogram:

Ask a student for her name and assignment and exam

scores, then display her results.

Enter your name :Margaret Lim

Enter your assignment score :60

Enter your exam score :90

FINAL RESULTSName Assignment Exam Total Grade

Margaret Lim 60.0 90.0 81.0 A

Press any key to continue...

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