Ifi7184 lesson4

IFI7184.DT – Lesson 4

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Review - Lesson 3Class Libraries

The Random and Math Classes

Formatting Output

Condition statements

The if Statement

The switch Statement

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The Random Class

• Uses the java.util package

• It provides methods – That performs complicated calculations

– These include:Random generator = new Random();

generator.nextInt();

generator.nextFloat();

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The Math Class

• Uses the java.lang package

• It provides methods

– That performs various mathematical functions

– These include:• absolute value • square root• exponentiation

Math.cos(90);

Math.sqrt(delta);

Math.pow (x,2);

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Formatting Output

• Uses the java.text package• It provides methods

– that allows you to format• These are:

–NumberFormat• formats values as currency or percentages

–DecimalFormat • formats values (decimal numbers) based on a pattern

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DecimalFormat method

• Creating a DecimalFormat instanceString pattern ="#.###";DecimalFormat fmt = new DecimalFormat (pattern);

• Print out the resultString areaFormat = fmt.format(area);System.out.println ("The circle's area: " + areaFormat);

orSystem.out.println ("The circle's circumference: " + fmt.format(circumference);

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NumberFormat• The java.text.Number format class

– Formats currencies according to a specific Locale (country)

• Creating a NumberFormat instanceLocale locale = new Locale("ee", "EE");

NumberFormat fmt = NumberFormat.getCurrencyInstance(locale);

• Print out the resultNumberFormat fmt= NumberFormat.getCurrencyInstance(locale);

String currency = fmt.format(balance);

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Number Format Pattern Syntax

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0 A digit - always displayed, even if number has less digits (then 0 is displayed)

# A digit, leading zeroes are omitted.. Marks decimal separator, Marks grouping separator (e.g. thousand separator)E Marks separation of mantissa and exponent for

exponential formats.; Separates formats- Marks the negative number prefix% Multiplies by 100 and shows number as percentage? Multiplies by 1000 and shows number as per mille¤ Currency sign - replaced by the currency sign for the

Locale. Also makes formatting use the monetary decimal separator instead of normal decimal separator. ¤¤ makes formatting use international monetary symbols.

X Marks a character to be used in number prefix or suffix' Marks a quote around special characters in prefix or suffix

of formatted number.

Pattern Number Formatted String

###.### 123.456 123.456###.# 123.456 123.5###,###.## 123456.78

9123,456.79

000.### 9.95 009.95##0.### 0.95 0.95

Example

Formatting Output

Classes: Scanner, Math.pow,

Math.PI and DecimalFormat

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Java program CircleStats

• Write code statements that – That calculates the area and circumference of a circle

given its radius.• Prompt for and read a integer value from the user

named radius• Calculate the area and circumference

area = Math.PI * Math.pow(radius, 2)circumference = 2 * Math.PI * radius;– Output the results to 3 decimal places. DecimalFormat fmt = new DecimalFormat (”#.###”)fmt.format(area)fmt.format(circumference)

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Java program FourthPower

• Write code statements that – Prompt for and read a double value from the user – Then print the result of raising that value to the

fourth power. • Math.pow (x,4)

– Output the results to 3 decimal places. DecimalFormat fmt = new DecimalFormat (”#.###”)fmt.format()

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Java program Calculator

• Write code statements that – Prompt for and read two double values from the

user • Then performs

– 5 arithmetic operations (+,-,*,/ and %). – writes the results on the screen. – Output the results to 3 decimal places.

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Review - Lesson 3Class Libraries

The Random and Math Classes

Formatting Output


The if Statement


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• A conditional statement – Or selection statements

• lets us choose which statement will be executed next– give us the power to make basic decisions

• The Java conditional statements are the:– If; – if-else statement; and – switch statement;

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Logic of an if statement

conditionevaluated

statement

truefalse

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The if Statement• Let's now look at the if statement in more detail

– The if statement has the following syntax:

if ( condition ) statement;

if is a Javareserved word

The condition must be aboolean expression. It mustevaluate to either true or false.

If the condition is true, the statement is executed.If not if it is false, the statement is skipped.

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Logic of an if-else statement

• If the condition is true,

– statement1 is executed;

– if the condition is false, statement2 is executed

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conditionevaluated

statement1

true false

statement2

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Logic of an if-else statement

conditionevaluated

statement1

true false

statement2

Condition 2evaluated

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Block Statements

• Several statements can be grouped together

– into a block statement delimited by braces

if (total > MAX){ System.out.println ("Error!!"); errorCount++;}

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if (total > MAX){ System.out.println ("Error!!"); errorCount++;}else{ System.out.println ("Total: " + total); current = total*2;}

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Nested if Statements

• A statement executed as a result of an if or else clause

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if (num1 < num2) if (num1 < num3) min = num1; else min = num3; else if (num2 < num3) min = num2; else min = num3;

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Indentation

• Remember – Indentation helps to better read your program,

• Indentation is ignored by the compiler

if (depth >= UPPER_LIMIT) delta = 100;else System.out.println("Reseting

Delta"); delta = 0;

Despite what the indentation implies, delta will be set to 0 no matter what

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Arithmetic or math expressions

AdditionSubtractionMultiplicationDivisionRemainder

+-*/%

Increment and Decrement

Increment operatorcount++; or count = count + 1;Decrement operatorcount--; or count = count – 1;

Equal toNot equal toGreater thanGreater tan or equal toLess thanLess than or equal to

==!=>>=<<=

Conditional operators Boolean operators

Conditional-ANDConditional-ORTernary(shorthand for if-then-else statement

&&||?:

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The switch Statement• The switch statement evaluates an expression,

– then attempts to match the result to one of several possible cases

switch ( expression ){ case value1 : statement-list1 case value2 : statement-list2 case value3 : statement-list3 case ...

}

switchandcaseare

reservedwords

If expressionmatches value2,control jumpsto here

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• An example of a switch statement:switch (option){ case 'A': aCount++; break; case 'B': bCount++; break; case 'C': cCount++; break;}

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Outline

Conditional Statements

Comparing Data

Encapsulation

Anatomy of a Method

Anatomy of a Class

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Comparing Data

• When comparing data – Using boolean expressions,

• it's important to understand the nuances of certain data types

• Let's examine some key situations:

– Comparing floating point values for equality– Comparing characters– Comparing strings (alphabetical order)

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Comparing Float Values

• You should rarely use – the equality operator (==)

• To compare two floating point values – float or double

• As two floating point values – are equal only

• if their underlying binary representations match exactly

• In many situations, you might consider – two floating point numbers to be "close enough"

• even if they aren't exactly equal

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Comparing Characters or Strings

• We cannot use the relational operators to compare strings– Two ways to compare strings and characters

are• Based on a character set,

– it is called a lexicographic ordering

• Or using the equals method – to determine if two strings contain exactly the same

characters in the same order

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Comparing Strings

• Remember that in Java – A string is an object

• The String class contains – the compareTo method

• for determining if one string comes before another– The equals method

• for determining if one string contains the same characters as another

• It returns a Boolean result

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Comparing Characters

• Java character data is based on – the Unicode character set– Unicode establishes a particular numeric

value for • each character, and an ordering

– For example, • the character '+' is less than the character 'J'

– because it comes before it in the Unicode character set

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Comparing Characters

• In Unicode, the digit characters (0-9) are contiguous and in order

• Likewise, the uppercase letters (A-Z) and lowercase letters (a-z) are contiguous and in order

Characters Unicode Values0 – 9 48 through 57A – Z 65 through 90a – z 97 through 122

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Lexicographic Ordering

• Lexicographic ordering is not strictly alphabetical – When uppercase and lowercase characters are mixed

• For example, – the string "Great" comes before – the string "fantastic" – because all of the uppercase letters

• come before all of the lowercase letters in Unicode

• Also, short strings come before longer strings – with the same prefix (lexicographically)

• Therefore "book" comes before "bookcase"

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Comparing Data

Comparing float values and String

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Java program FloatCompare

• Write a code statements that compares Float Values– Prompt for and read 2 float value from the user

– Compares them and returns if ( Math.abs(f1 - f2) < TOLERANCE) {… }

• This means If – the difference between the two floating point values is less than the tolerance they are

considered to be equal

• The tolerance could be set to any appropriate level, – such as 0.000001

– Then print the result • The float numbers are ("Essentially equal")

• The float numbers are (”Not equal")

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Java program StringCompare1

• Write a code statement – That asks the user for 2 Strings names– Compares them and returns

• ”Same name”• ”Not the same names”

• Use the equals method – It returns a Boolean result (true or false)

if (name1.equals(name2)){ System.out.println ("Same name");

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Java program StringCompare2 • Write a code statements that

– asks the user for 2 String names– Then compare them using the compareTo method

• The comparison is based on the Unicode value of – this String object is compared lexicographically

• The result is a – negative integer if this String object lexicographically precedes the argument string.

» if name1 is less than name2» zero if name1 and name2 are equal contain the same characters» returns a positive value if name1 is greater than name2

int result = name1.compareTo(name2);– Output the results

• Name1 "comes first”• "Same name”• Name2 "comes first”

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Outline


Comparing Data

Encapsulation

Anatomy of a Method

Anatomy of a Class

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Writing Classes

• The programs we’ve written used – Classes defined in the Java standard class library

• Now we will begin to design programs that – Rely on classes that we write ourselves

• The class that contains – the main method is just the starting point of a program

• True object-oriented programming is based on defining classes that represent objects with well-defined characteristics and functionality

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Encapsulation• But first you need to understand

– The principle of object oriented language concept depended on a few concepts

• One is Encapsulation

• It means – Packaging complex functionality into small pieces of

functions• You can see it as your application functions wrapped in a box

• Why – Hiding it complexity make it easier to use

• Easier o debug• Easier to add more code

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Encapsulation

• We’ve learn how to create your program by putting all your code in the main method – But when your application get larger It becomes

very difficult to manage – So instead you need to break your code into

individual classes• Grouping it functionality for your application

– When you do this you can for instance • Restrict access to one part of the application

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Java application

• Consisted of more than one class– The starting class has the main method– Then you have all sort of supporting classes

• Those are called customize methods– either encapsulate data or functionality, or both

• We can take one of two views of an object:– internal

• the details of the variables and methods of the class that defines it

– external • the services that an object provides and how the object interacts

with the rest of the system

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Encapsulation• From the external view, an object is

– an encapsulated entity, providing a set of specific services

• These services define the interface to the object• For instance

– One object (called the client) may use another object for the services it provides– The client of an object may request its services (call its methods), but it should

not have to be aware of how those services are accomplished – Any changes to the object's state (its variables) should be made by that object's

methods

• We should make it difficult, if not impossible, for a client to access an object’s variables directly

• That is, an object should be self-governing

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Encapsulation

• An encapsulated object can be thought of as a black box -- its inner workings are hidden from the client

• The client invokes the interface methods and they manage the instance data

Methods

Data

Client

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Visibility Modifiers

• In Java, we accomplish encapsulation through the appropriate use of visibility modifiers– A modifier is a Java reserved word that

specifies particular characteristics of a method or data

• Java has three visibility modifiers: – public, protected, and private

– But first we need to understand the anatomy of the method

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Outline


Comparing Data

Encapsulation

Anatomy of a Method

Anatomy of a Class

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Method Declarations

• Let’s now examine methods in more detail– A method declaration Specifies

• the code that will be executed – when the method is invoked (called)

• In object oriented vocabulary– A method, as you know is a function or

• A piece of code with a given name – that can be called (invoked) in parts of an application.

• A function in Java is a member of a class.

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Method Control Flow

• When you run an java application– First thing it looks is for

• The main method public static void main (String[] args)

– But you can define any method you wont• To define your own method.

– you need to declare a method within you application.

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Method Declarations

• When a method is invoked (called), – The flow control jumps to the called method and

starts executing its code

• When complete, – The flow returns to the place where the method

was called and continues

• The invocation may or may not return a value, – depending on how the method is defined

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myMethod();

myMethodcompute

Method Control Flow• Method called in the same class

– You only need the method name

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doIt

helpMe

helpMe();

obj.doIt();

main

Method Control Flow• Method called as part of another class or object

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Method Header

private static void calc (int num1, int num2)

methodnamereturn

typeparameter list

The parameter list specifies the type and name of each parameter

The name of a parameter in the method declaration is called a formal parameter


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Method Bodyprivate static int calc (int num1, int num2)

{ int sum = num1 + num2; char result = sum;

return result;}

The return expressionmust be consistent withthe return type

sum and resultare local data

They are created each time the method is called, and are destroyed when it finishes executing

Input parameters or arguments

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Method Header • Visibility Modifiers: Options available • Members declared with

– Public visibility • Is available in the entire application. You can call it everywhere. Or • In Object oriented words

– can be referenced anywhere

– Private visibility • The opposite of public. Is only available in that class.

– can be referenced only within that class

– Without a visibility modifier • have default visibility and • Can be referenced by any class in the same package

– Protected visibility • Is an inherence. • Is available to current class and to any subclasses• But, we will not we will not cover in this course

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• Public variables violate encapsulation – because they allow the client to modify the values directly

• Public constants do not violate encapsulation because, – although the client can access it,– its value cannot be changed

• Therefore instance variables should not be declared with public visibility– It is acceptable to give a constant public visibility, which

allows it to be used outside of the class

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• Methods that provide the object's services are declared – with public visibility so that they can be invoked by clients

• Public methods are also called service methods• A method created simply to assist a service method

is called a support method• Since a support method is not intended to be called

by a client, it should not be declared with public visibility

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Visibility Modifierspublic private

Variables

Methods Provide servicesto clients

Support othermethods in the

class

Enforceencapsulation

Violateencapsulation

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• In Java, we accomplish encapsulation through the appropriate use of visibility modifiers

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Visibility Modifiers• In Java, we accomplish encapsulation through the

appropriate use of visibility modifiers

• A modifier is a Java reserved word that specifies particular characteristics of a method or data

• We've used the final modifier to define constants

• Java has three visibility modifiers: public, protected, and private

• The protected modifier involves inheritance, which we will not cover in this course

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Method Header

• Next characteristics of a method is – if it is static or not.

• When you wont method to be static. – This means that

• The method is a class method or an instance of that class.

– It can be called direct from the class definition• If you don't know if to add static or not just

– try not to add static and see if you call it in the class or not.

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Method Header• The return type.

• A return statement specifies the value that will be returned– return expression return result;

!!! The expression must conform to the return type

• The return type of a method

– indicates the type of value that the method sends back to the calling location

• void: means the method is not returning anything• int: means the method is returning an integer• char: means the method is returning a character

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Method Header

• In end of the method Header you should – open and close parenthesis ()

private static void getInput(){ method body }

– To name your method • use same rules assigned to variables

– In between parenthesis you add the received parameters or values

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Declaring Methods with arguments

• When you declare your own custom method– You can receive parameters or values by declaring

them in the method header– You can also return parameters or values as well

• Let’s design two custom methods to better understand how it works– Called executeFormula, scanning and calc

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Anatomy of a Method

Creating reusable code with Methods

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Java program Wages

• Look for your wages.java program– Customize two private methods

• Named scanCondition and payCondition– scanCondition execute scanner commands – payCondition execute if/else commands

– Call the new customized methods in the main method

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Anatomy of a Method

Declaring Methods with arguments

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Passing parameters• In java variable are always passed by copy

– When a method is called, the actual parameters in the invocation are copied into the formal parameters in the method header

Int calc (int s1, int s2){ int sum = num1 + num2; return sum;}

The sum is = calc (s1, s2);

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Java program Aritemetic

• Write a code statement that add two values– Customize it in a method called AddValues – This method should receive two valuesprivate static int addvalues(int int1, int int2)

– The assigned two values are 10 and 12int value1=10;int value2=12;

– Call the method in the main method and print out the resultsint results= addvalues(value1, value2);

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Modifying AddValues

• In Java you can use the same method name more than once – You just need to change it signature.

• Try it, use the method called addValues– Copy/paste it and add one more parameter to the new

oneprivate static int addvalues(int int1, int int2, int int3)int sum = int1+int2+int3;

– Assigning one more value (value3) and print the results and print the resultint value3=10;

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Java program (name it testMethod)

• That reads one integers from the user. Then– Create a method called executeFormula

• This method receives an int xstatic int executeFormula(int x)

• The value that will be returned is return result

• It verifies if x is positive– If so multiply it by 1000 – Else multiply by 500 if it is negative

– Call executeFormula and Write the result

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Anatomy of a Method

Declaring a Method with arguments

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Java program Calculator2

• Reuse your previous Calculator program – That Prompt for and read two double values from the user and then performs 5

arithmetic operations (+,-,*,/ and %). writes the results on the screen. And output the results to 3 decimal places.

• Step1: Customize a method called scanning– This method read a number from the user

• returns an int X• Step 2: Then… Customize a method called calcSum

– So it receives two int parameters and calculates the sumstatic int calc (int s1, int s2)return sum;

• Call it in the main method• Ask the user two numbers (by calling scanning)

int s1= scanning();int s2= scanning();

• Write the result (by calling calcSum).

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Java program Calculator2

• Customize 3 more methods– called calcSub– called calcDiv– called calcMult

• Do that by asking the user what operation to do– 1= add, 2= substract; 3=divide, 4=multiply

• Use a switch statement to see – which case the user choose; and– Adjust your method to calculate the right values

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Sum up• We’ve learn how to build your own method • Learn how to

– encapsulate functionality inside methods that are part of classes

• We‘ve seen, that– local variables can be declared inside a method

• Keep in mind, that – when the method finishes,

• all local variables are destroyed (including the formal parameters)– instance variables, are declared at the class level,– exists as long as the object exists

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Education

Ifi7184 lesson4