3. S/E with Control Structures

• 3.1 Relational Operators and Expressions

• 3.2 If and if-else Statements

• 3.3 The Type Double

• 3.4 Program Design with the While Loop

• 3.5 Debugging

• 3.6 Getting Started with Objects and

Applets: Drawing Shapes

Objectives

• Learn the basic sequence, selection, and repetition statements

• Identify objects and their responsibilities• Simple debugging techniques• Handle floating-point numbers• Study applets, draw various shapes

3.1 Relational Operators and Expressions

• Relational operators include:

<, <=, >, >=, ==, and !=• Each relational operator takes two operands and

gives a boolean value: true or false• Be careful not to confuse == with =• Relational operators have precedence lower than

arithmetic operators but higher than assignment operators

Figure 3.2 Java relational and equality operators

is true292 != 377not equal to !=

is false 9 == 12 + 12equal to==

is false 9 >= 99greater than or equal to

>=

is false-98 > -12greater than>

is true464 <= 7213less than or equal to<=

is false31 < 25less than<

ExampleMeaningOperator Symbol

Control Flow

• The order Java executes the statements of a program is sequential if not told otherwise

int item1 = 25;

int item2 = 12;

item2 = item1 + 15;

• Conditional statement like the if and if-else statements allow us to make decision

• They change the default control flow

Figure 3.3 The sequence control flow

Entry

int item1 = 25;

int item2 = 12;

Item2 = item1+15;

Exit

3.2 The if and if-else statements• The syntax

if ( condition )if_true_statement;

[ elseif_false_statement; ]

• condition must be boolean: true or false• If condition is true, if_true_statement is executed;

if the condition is false, if_false_statement is executed

• One or the other will be executed, but not both

Figure 3.4 Control flow for the if statement

condition

False

if_true_statement

The if Statement: Example

hours = Integer.parseInt(input);

if (hours > 40)

System.out.println(“You worked overtime “ +

“this week”);

System.out.println(“You worked “ + hours +

“ hours”);

Figure 3.5 Control Flow for Example 3.2

hours = Integer.parselnt(input);

Hours>40

System.out.println(

“you worked ”

+ hours + “ hours”);

FalseSystem.out.println(“You “ + “worked overtime this week”)

True

Figure 3.6 Flow chart for the if-else statement

Condition

if_true_statement if_false_statement

True False

The if-else Statement: Example

if (hours <= 40)

wage = hourlyRate * hours;

else

wage = hourlyRate * 40 +

hourlyRate * 1.5 * (hours - 40);

Block Statements

• Several statements can be grouped together into a block statement

• A block is delimited by braces ( { … } )• A block statement can be used wherever a

statement is called for in the Java syntax• For example, in an if-else statement, the if-portion,

or the else-portion, or both, could be block statements

Figure 3.7 Flow chart for if statement with block, step 1

Z <= 10

True

Falsex = 2; y = 7;

Figure 3.8 Flow chart if statement with block, step 2

Z <= 10

x = 2;

True

False y = 7;

Block Statements• Use a consistent style for blocks

if (x < 10) { y = 5;

z = 8;}

else {

y = 9;

z = -2;}

if (x < 10){ y = 5; z = 8;}else{ y = 9; z = -2;}

3.3 The Type Double• Scientific (floating-point) notation for real

numbers: 123.45 and 0.0012345 can be written respectively as

0.12345E3

0.12345E-2• 0.12345 is the mantissa, 3 and -2 are the

exponents, with the implicit base of 10

• Scientific notation is good for numbers with very small or very large absolute value

• Java has type double that provides 16 decimal digits accurately.

public class Triangle{ private double side1, side2, side3; public Triangle(double a, double b, double c) { side1 = a; side2 = b; side3 = c; } public double circumference() { return side1 + side2 + side3; } public double area() { double s = circumference() / 2.0; return Math.sqrt(s*(s - side1)*(s - side2)*(s - side3)); }}

Double Value Output• When printed using print or println methods of

System.out, Java prints double values using the most convenient format

• Scientific format for numbers greater than 10,000,000 or less than -10,000,000, and for numbers between -.001 and .001.

• Java treats decimal literals as type double• Java does not print trailing zeroes• Type double is accurate to 16 significant digits• The 17th digit may be rounded

Input and Formatted Outputimport javax.swing.*;import java.text.*;…public static void main(String[] args){ NumberFormat nf = NumberFormat.getCurrentcyInstance(); String s = JOptionPane.showInputDialog(“Hours worked? “); double hours = Double.parseDouble(s); JOptionPane.showMessageDialog (null, “Your pay is “ + nf.format(hours * 5.5)); System.exit(0);}

Formatted Output• The NumberFormat class has static methods that

return formatter objectgetCurrencyInstance()getPercentInstance()

• Each formatter object has a method called format that returns a string with the specific information in the appropriate format

• The DecimalFormat class can be used to format a floating point value in generic way

• The DecimalFormat class takes a string that represents a pattern for the formatted number

Data Conversions• Sometimes it is convenient to convert data from one

type to another• Conversion must be handled carefully to avoid

losing information• Widening conversions are safest because they tend

to go from a small data type to a larger one (such as a short to an int)

• Narrowing conversions can lose information because they tend to go from a larger data type to a smaller one (such as an int to a short)

Data Conversions• In Java, data conversions can occur in three ways:

– assignment conversion– arithmetic promotion– casting

• Assignment conversion occurs when a value of one type is assigned to a variable of another type– Only widening conversion can happen via assignment

• Arithmetic promotion happens automatically when operators in expressions converts their operands

• Casting is the most powerful, and dangerous, technique for conversion

result = (float) total / count;

Figure 3.9 Conversion of mixed-mode expression

2.54

2.54

Original expression

After Conversion

+

+

361

361.0

3.4 The While Loop• Repetition statements allow us to execute a

statement multiple times repetitively• They are often referred to as loops• Like if-else statements, they are controlled by

boolean expressions’• Java has three kinds of repetition statements: the

while loop, the do loop, and the for loop• The while loop has this syntax

while ( condition )

while_true_statement;

The While Loop• If the condition is true, the statement is executed• Then the condition is evaluated again• The statement is executed repetitively until the

condition becomes false• If the condition is false initially, then the statement

is never executed• The body of a while loop must eventually make the

condition false• If not, it is an infinite loop, which is a common

logical error unless you are absolutely sure it is not

Figure 3.10 Flow chart for the while loop

Condition

while_true_statementFalse True

Problem Solving• The purpose of writing a program is to solve a

problem• The general steps in problem solving are:

– Understand the problem

– Dissect the problem

– Design a solution

– Consider alternatives to the solution and refine it

– Implement the solution

– Test the solution and fix and problems that exist

Problem Solving• Many software development project failed because

the developers didn’t understand the problems to be solved

• Avoid assumptions and clarify ambiguities• As problems and their solutions becomes larger,

we must organize our development into manageable pieces

• We will dissect our solutions into pieces called classes and objects, taking an object-oriented approach

Program Development• The creation of software involves four basic

activities:– establishing the requirements

– creating a design

– implementing the code

– testing the implementation

• Real development process is much more involved than this, but these four steps are a good starting point

Requirements• Requirements specify the tasks a program must

accomplish (what to do, not how to do it)• They often include description of the user interface• The initial requirements must be critiqued,

modified, and expanded• It is often difficult to establish detailed,

unambiguous, and complete requirements• Careful attention to the requirements can save

significant time and money in the overall project

Design• An algorithm is a step-by-step process for solving

a problem• A program follow one or more algorithms • The design of a program specifies the algorithms

and data needed• In object-oriented development, the design

establishes the classes, their data and methods • The details of a method may be expressed in

pseudocode, which is code-like, but does not necessarily follow any specific syntax

Implementation• Implementation is the process of translating a

design into source code• Most novice programmers think that writing code

is the heart of software development, but it is actually the least creative step

• Almost all important decisions are made during requirements analysis and design

• Implementation should focus on coding details, including guidelines, clarity, maintainability, expandability, and documentation

Testing• A program should be executed multiple times with

various input in an attempt to find errors• Debugging is the process of discovering the cause

of a problem and fix it• Debugging can only indicate the presence of errors

(bugs), but not the absence• Don’t ever think there is only one more bug to fix• tests should focus on design details as well as

overall requirements

Figure 3.11 Pseudocode for the sum of test scores problem

Read the quantity of scores;

while (count < quantity) {

Read the next score;

Add the score to the total so far;

Increment the count of scores;

}

Display the quantity and the total;

3.5 Debugging• Syntax error

– caught by the compiler– easy to fix

• Run-time error– reported at run-time by interpreter– often depending on input and environment– usually cause program to terminate immaturely

• Logical error– program runs but results are wrong– can be very difficult to find and/or fix

• Requirement error– disastrous

3.6 Getting Started with Applets• A java application is a stand-alone program with a

main method• An applet is a Java program that is intended to be

transported over the web and executed using a web browser

• An applet doesn’t have a main method• Instead, there are several special methods that

serve specific purposes• The paint method, for instance, is automatically

executed and is used to draw the applets contents

Drawing Shapes• The paint method accepts a parameter that is an

object of the Graphics class• A Graphics object defines a graphical context on

which we can draw shapes and text• The Graphics class has several methods for

drawing shapes• The class that defines the applet extends the

Applet class• This makes use of inheritance, an object-oriented

concept to be explored later

Figure 3.12 Drawing a line

(70,80)

(130,230)

Figure 3.13 Drawing a rectangle

(50,50) 200

100

Figure 3.14 An oval with its bounding rectangle

Figure 3.15 Degree measure around a circle

0º

270º

180º

90º

Figure 3.16 An arc from the oval of Figure 3.14

90º 45º

Figure 3.17 The structure of a rounded rectangle

Dimension d = getSize(); int w = d.width; int h = d.height; g.drawRect(2*w/3,0,w/3,h/3);

Figure 3.19 Drawing a rectangle relative to the applet's size