Bobby D. Gerardo

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Programming and Problem Solving Using Java

• Bobby D. Gerardo
• Kunsan National University
• Summer 2004

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Chapter 4Control Structures Decision and Loops
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Review
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Object Oriented Programming

• Objects are key to understanding object-oriented
  technology. You can look around you now and see
  many examples of real-world objects your dog,
  your desk, your television set, your bicycle.
• These real-world objects share two
  characteristics They all have state and
  behavior. For example, dogs have state (name,
  color, breed, hungry) and behavior (barking,
  fetching, and wagging tail). Bicycles have state
  (current gear, current pedal cadence, two wheels,
  number of gears) and behavior (braking,
  accelerating, slowing down, changing gears).

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Object Oriented Programming (cont)

• Software objects are modeled after real-world
  objects in that they too have state and behavior.
  A software object maintains its state in one or
  more variables . A variable is an item of data
  named by an identifier. A software object
  implements its behavior with methods . A method
  is a function (subroutine) associated with an
  object.
• object-oriented design
• A software design method that models the
  characteristics of abstract or real objects using
  classes and objects.

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Encapsulation

• Because all data fields have private visibility
  (FoodItem Example), another class that is a
  client (user) of the class FoodItem cannot access
  the field directly.
• To change the data field value, the client must
  invoke one of the mutator methods.
• To retrieve a data field value, the client must
  invoke one of the accesor methods.
• This is the principle of encapsulation.

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Methods

• Constructor Methods always has the same name as
  the class. It should have the public visibility
  and does not have a result type because it does
  not return a value.
• Accessor Methods accessor methods are sometimes
  called getter methods and should begin with the
  word get.
• Mutator Methods Just as we need to access an
  objects private o protected data from another
  object, we sometimes need to store data in an
  objects private or protected fields. We use
  mutator or modifier methods to do this.

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Case Study Solution Format

1. Problem specify the problem requirement
2. Analysis analyze the problem an identify the
   classes that will be needed
3. Design design the classes to solve the problem.
   Locate the relevant classes in libraries. Modify
   existing classes if necessary. Design new classes
   where necessary
4. Implementation implement the new and modified
   classes.
5. Testing test and verify the completed program.

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Review Exercise
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Review Exercise

• What is an accessor?
• 2. What is a default constructor?
• 3. Explain the relationship between a browser and
  an applet? Which one support object?
• 4. The method paint() has an argument g of type
  Graphics. Explain what this object does in the
  paint() method?

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Answer Review Exercise

• What is an accessor?
• Accessor methods are sometimes called getter
  methods and should begin with the word get.
• 2. What is a default constructor?
• It should have the public visibility and does not
  have a result type because it does not return a
  value.
• 3. Explain the relationship between a browser and
  an applet? Which one support object?
• An applet is a Java class that has an associated
  drawing surface. It is intended as a support
  object in a larger context such as web browser
  like IE or Netscape.
• 4. The method paint() has an argument g of type
  Graphics. Explain what this object does in the
  paint() method?
• It represent an instance of the Graphics class
  (the graphics context or drawing surface passed
  by the browser or applet viewer) so that we may
  call the methods of the Graphics class.

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Chapter 4Control Structures Decision and Loops
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Control Structures and Statements

• Statements that control the flow of execution in
  a program or method.
• Study of boolean expressions.
• Decision making using if and switch statements.
• We will study also loops such as counting loops
  and state-controlled loops.
• Java loop control statements, while and for.

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4.1 Control Structures (1)

• Structured programming uses control structures to
  control the flow of execution in a program or
  method.
• Three kinds of control structure
• Sequence
• Selection
• Repetition

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4.1 Control Structures (2)

• A compound statement or block, written a group of
  statements bracketed by and, is used to specify
  sequential flow.
• Selection control structures enable the method
  to select or chose one step from several
  alternative algorithm steps. i.e. if and switch.
• Repetition control structures enable the
  programmer to specify that a block of statements
  should be repeated. i.e. loop body, while and for
  statements.

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4.1 Control Structures (3)

• Selection control structures enable the method
  to select or chose one step from several
  alternative algorithm steps. i.e. if and switch.
• Repetition control structures enable the
  programmer to specify that a block of statements
  should be repeated. i.e. loop body, while and for
  statements.

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4.2 boolean Expressions

• boolean expressions or, conditions, facilitate
  comparison.
• Two possible values true or false
• The simplest boolean expression is a boolean
  variable or boolean literal (true or false).
• Table 4.1 shows Java relational operators
• Example 4.2 and Table 4.2 shows some example
  conditions

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Java Relational Operators

• Operator Meaning
• lt Less than
• lt Less than or equal
• gt Greater than
• gt Greater than or equal
• Equal to
• ! Not equal

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boolean Operators

• We can form more complicated boolean expressions
  by using the three boolean operators
• (1) (and) (2) (or) (3) ! (not)
• Example
• a. (salaryltminimumSalary) (dependentsgt5)
• b. (temperaturegt90.0) (humidity gt 0.90)
• c. (salarygt 15000.00) (salary lt 2000.00)

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Case Study 1 Payroll Problem
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Case Study 1 Payroll Problem

• 1. Problem Your company pays its employees
  time and a half for all hours worked over 40
  hours a week. Employees who earns more than 100
  a week pay union dues of 25 per week. Write a
  program to compute an employees gross pay and
  net pay
• 2. Analysis analyze the problem an identify the
  classes that will be needed
• 3. Design design the classes to solve the
  problem. Locate the relevant classes in
  libraries. Modify existing classes if necessary.
  Design new classes where necessary
• 4. Implementation implement the new and
  modified classes.
• 5. Testing test and verify the completed
  program.

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Modified Case Study1

• Modify method computeNet to deduct union dues of
  10 for gross salary over 100 and 5 otherwise.
  Also deduct a 3 city wage tax for all employees.

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Summary of Programming Exercises and Projects
(Chap. 3)

• Exercises
• Figure 3.34 P. 159
• Modify Fig. 3.34 to draw Red and blue lines at
  position 240X340
• Figure 3.37 P. 162- Class House
• Modify Fig. 3.37 to use brown lines and red door.
• Figure 3.42 P. 166- Class HappyFace
• Modify Fig. 3.42 with eyes and nose filled with
  yellow color.
• Projects
• No.1 P. 177 Projectile Problem
• No. 7 P. 179 Bar Graph.

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Summary of Programming Exercises(Chap. 4)

1. Case Study 1 Payroll Problem
2. Modify method computeNet to deduct union dues of
   10 for gross salary over 100 and 5 otherwise.
   Also deduct a 3 city wage tax for all employees.

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boolean Operators (cont)

• Table 4.3 The (and) operator
• Table 4.4 The (or) operator
• Table 4.5 The ! (not) operator

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Table 4.6 Operator Precedence

• Operator Description
• !, (unary), -(unary) Logical not, uniray plus,
  uniray minus
• , /, Multiplication, Division, Remainder
• , - Addition, subtraction
• lt, lt, gt, gt Relational inequality
• , ! Equal, Not Equal
• Logical and
• Logical or
• Assignment

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Short-Circuit Evaluation of boolean Expressions

• When evaluating boolean expressions involving the
  operators and , Java employs a technique
  called Short-Circuit Evaluation .
• This means that Java stops evaluating a boolean
  expression as soon as its value can be
  determined.
• For example the value of flag is true, the
  expression
• flag (xgt0 y ! 7.5)

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Writing English Conditions in Java

• To solve programming problems, we write English
  conditions as boolean expressions. Often we want
  to test whether a variable has one of several
  possible values.
• For example if ch is type char, we may want to
  know if ch is the letter a or A.
• (cha)(chA) //true
• cha A //not valid
• (cha) (chA) //not valid

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Range of values

• (x gt min) (x lt max) //x is included
• (x gt min) (x lt max) //x is not included
• ! ((x gt min) (x lt max)) //negated
• (x lt min) (x gt max) //equivalent

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De Morgans Theorem

• Use simpler expressions using De Morgans
  theorem
• 1. !(expression1 expression2) equals !
  expression1!expression2
• ! ((x gt min) (x lt max) equals !(x gt min)
  ! (x lt max)
• 2. !(expression1 expression2) equals !
  Expression1 !expression2
• !((ch a) (ch A)) equals ! (ch
  a) ! (ch A)

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Character comparison

• The order of char comparison is based on the
  unicode position of each character (apprendix B).
• The first 128 characters of Unicode correspond to
  the ASCII code.

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Comparing String Equality

• We can also compare String objects
• Use methods in class String, not relational
  operators.
• Use method equals (not the equality operator )
• i.e. string1.equals(string2)

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Method equalsIgnoreCase()

• aces.equals(ACES) is false
• aces.equalsIgnoreCase(ACES) is true

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Lexicographic Comparison of Strings

• Java, provides a method compareTo(), that you can
  use to compare strings lexicographically. The
  lexicographic order of two strings is the order
  in which they would normally appear in a
  dictionary.
• i.e. string.compareTo(string2)
• Has a negative value if string1 is
  lexicographically less than string2.
• Has the value of 0 if string1 is
  lexicographically equal to string2.
• Has a positive value if string1 is
  lexicographically greater than string2.

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The if Statement

• In Java, the primary selection control structure
  is an if statement.
• If statement always contain a boolean expression.
• One consequence
• If (x ! 0.0) product product x
• Two consequence
• If (gross gt 100.00)
• net gross tax
• Else
• net gross

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Fig. 4.5 Flowchart of if statements
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If Statement forms

• One consequent
• if (condition)
• statement T
• Two consequent
• if (condition)
• statement T
• else
• statement F

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Decision Steps in Algorithms

• Algorithm steps that select from a choice of
  actions are called decision steps.
• For example, coded as if statement in Java
• if the first letter is a vowel or y
• dont translate
• else
• translate the word to pig Latin in normal way

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Program Style Choosing variable scope

• When choosing the scope of variables (where
  they are accessible), we follow these principles
• Keep variables as local as possible
• Avoid a calculation more than once
• Use variables sparingly

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Example, following principle 3

• Method computeNet() does not declare a local
  variable (for example, net) to store the method
  result (referring to the Fig. 4.13).
• public double computeNet(double gross)
• double net
• if (gross lt MAX_NO_DUES)
• net gross
• else
• net gross dues // deduct dues amount
• return net

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Multiple-Alternative Decisions Nested if and
switch

• In this section we use nested if statements (one
  if statement inside another) to code
  decisions with multiple alternatives.
• //increment numPos, numNeg, or numZero depending
  on x
• if (x gt 0)
• numPos numPos 1
• else
• if (x lt 0)
• numNeg numNeg 1
• else // x is zero
• numZero numZero 1

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Table 4.10 Trace of if Statement in Example 4.14
for x -7

• Statement Part Effect
• If (x gt 0) -7 gt 0 is false
• If (x lt 0) -7 lt 0 is true
• numNeg numNeg1 Add I to numNeg

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Comparison of Nested if and Sequence of ifs

• Beginning programmers often use a sequence of if
  statements rather than a single nested if
  statement. The nested if statement for Example
  4.1.4 is logically equivalent to the following
  sequence of if statements
• // In efficient sequence of if statements
• if (x gt 0)
• numPos numPos 1
• If (x lt 0)
• numNeg numNeg 1
• If (x 0)
• numZero numZero 1

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Java Rule for Matching else with if

• We use indention and braces to convey the
  logical structure of a nested if statement to
  the program reader.
• See Example 4.15

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Order of the Condition Matters

• When more than one condition in a
  multiple-alternative decision is true, only
  the task following the first true condition
  executes.

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switch Statements

• Java provides a convenient control
  structure called switch statement that can be
  used to implement a multiple- alternative
  decision.
• The break statement causes immediate exit from
  the switch statements.
• See Fig. 4.16

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Counting Loops, while and for Statements

• Counting Loops simplest kind of loop. A loop
  that executes a specified number of times is
  counting loop (or counter-controlled loop).

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The while Statement

• We first show how to implement a counting loop
  using a while statement, the most versatile loop
  control statement. A while statement begins with
  a word while and has the form
• while (repetitionCondition) //controlled by
  repetition condition
• statement //loop body, statement to be
  repeated

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Fig. 4.17 Counting Loop with a while statement
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Loop-Control Variable

• In fig. 4.17 the variable countEmp is called the
  loop-controlled variable because its value
  determines whether the loop body is repeated.
• Initialized set to an initial value
• Tested is tested before the start of loop
  repetition
• Update is updated during each iteration.

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Fig. 4.18 Flow Chart of a while Loop
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The for Statement

• Because counting loops are so common, most
  programming languages provide a special control
  statement for implementing them.
• In Java, this is the for statement.
• for (int countEmp 0 countEmp lt numberEmp
  countEmp countEmp 1)

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Fig. 4.19 Counting Loop with a for statement
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Case Study 2 A Program for Arithmetic Drill and
Practice
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Case Study 2 A Program for Arithmetic Drill and
Practice

• Problem Your little sister would like a
  program that gives her some practice in solving
  arithmetic problems. She would like program that
  can generate and solve arithmetic problems of
  varying difficulty and would like to be able to
  specify the problem type (addition,
  multiplication, and so on) and the difficulty
  level (easy, moderate, hard). Your program should
  generate a problem and solve it. It should also
  compare its solution to your sisters and let her
  know whether her solution is correct.

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Modified Case Study 2

• Modify Case Study 2 by displaying the output on a
  dialog window instead of displaying in the
  console window. Note that your program will use
  the javax.swing. package.

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Programming Projects

1. Problem 1, P. 243
2. Problem 4, p. 243

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? End of Chapter