Decisions

1
Chapter 5

• Decisions

2
Chapter Goals

• To be able to implement decisions using if
  statements
• To understand how to group statements into blocks
  
• To learn how to compare integers, floating-point
  numbers, strings, and objects
• To recognize the correct ordering of decisions in
  multiple branches
• To program conditions using Boolean operators and
  variables

3
if Statement

• if (amount lt balance) balance balance -
  amount

4
if/else Statement

• if (amount lt balance) balance balance -
  
• amount
• else balance balance - OVERDRAFT_PENALTY

5
Block Statement

• if (amount lt balance)
• double newBalance balance - amount
  balance newBalance

6
Syntax 5.1. The if Statement

• if(condition)statement
• if (condition)statement
• elsestatement
• Example
• if (amount lt balance) balance balance -
  amount

7

• if (amount lt balance) balance balance -
  amountelse balance balance -
  OVERDRAFT_PENALTY
• Purpose
• To execute a statement when a condition is true
  or false

8
Syntax 5.2. Block Statement

• statement. . .
• Example
• double newBalance balance - amount
• balance newBalance
• Purpose
• To group several statements together to form a
  single statement

9
Statement Types

• Simple statementbalance balance - amount
• Compound statementif (balance gt amount) balance
  balance - amountAlso while, for, etc. (see
  chapter 6)
• Block statementdouble newBalance balance -
  amountbalance newBalance

10
Relational Operators
11
Equality Testing vs. Assignment

• The operator tests for equalityif (x 0)
  . . // if x equals zero
• The operator assigns a value to a variablex
  0 // assign 0 to x
• Don't confuse them.
• Java is the same as mathematical

12
Comparing Floating-Point Numbers

• Consider this code
• double r Math.sqrt(2) double d r r -2
  if (d 0) System.out.println(
  "sqrt(2)squared minus 2 is 0") else
  System.out.println( "sqrt(2)squared minus 2
  is not 0 but " d)

13

• It printssqrt(2)squared minus 2 is not 0 but
  4.440892098500626E-16
• Don't use to compare floating-point numbers

14
Comparing Floating-Point Numbers

• Two numbers are close to another if x - y lt e
• e is a small number such as 10-14
• Not good enough if x, y very large or very small.
  Then usex - y / max(x, y) lt e
• But if one of the numbers is zero, don't divide
  by max(x , y)

15
String Comparison

• Don't use for strings!if (input "Y") //
  WRONG!!!
• Use equals methodif (input.equals("Y"))
• tests identity, equals tests equal contents
• Case insensitive test ("Y" or "y")if
  (input.equalsIgnoreCase("Y"))

16
Lexicographic Comparison

• s.compareTo(t) lt 0 means s comes before t in
  the dictionary
• "car"comes before "cargo" "cargo" comes before
  "cathode"
• All uppercase letters come before lowercase
  "Hello" comes before "car"

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Lexicographic Comparison
18
Object Comparison

• tests for identity, equals for identical
  content
• Rectangle cerealBox new Rectangle(5, 10, 20,
  30)Rectangle oatmealBox new Rectangle(5, 10,
  20, 30)Rectangle r cerealBox
• cerealBox ! oatmealBox, but cerealBox.equals(oat
  mealBox)
• cerealBox r
• Caveat equals must be defined for the class (see
  chapter 11)

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Object Comparison
20
The null Reference

• null reference refers to no object at all
• Can be used in testsif (account null) . . .
• Use , not equals, to test for null
• showInputDialog returns null if the user hit the
  cancel buttonString input JOptionPane.showInpu
  tDialog("...")if (input ! null) ...
• null is not the same as the empty string ""

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Multiple Alternatives

• if (condition1)statement1else if
  (condition2)statement2else if
  (condition3)statement3elsestatement4
• The first matching condition is executed.
• Order matters.

22
File Earthquake.java

• 1 /
• 2 A class that describes the effects of an
  earthquake.
• 3 /
• 4 public class Earthquake
• 5
• 6 /
• 7 Constructs an Earthquake object.
• 8 _at_param magnitude the magnitude on the Richter
  scale
• 9 /

23

• 10 public Earthquake(double magnitude)
• 11
• 12 richter magnitude
• 13
• 14
• 15 /
• 16 Gets a description of the effect of the
  earthquake.
• 17 _at_return the description of the effect
• 18 /
• 19 public String getDescription()
• 20
• 21 String r

24

• 22 if (richter gt 8.0)
• 23 r "Most structures fall"
• 24 else if (richter gt 7.0)
• 25 r "Many buildings destroyed"
• 26 else if (richter gt 6.0)
• 27 r "Many buildings considerably damaged, some
  collapse"
• 28 else if (richter gt 4.5)
• 29 r "Damage to poorly constructed buildings"
• 30 else if (richter gt 3.5)
• 31 r "Felt by many people, no destruction"

25

• 32 else if (richter gt 0)
• 33 r "Generally not felt by people"
• 34 else
• 35 r "Negative numbers are not valid"
• 36 return r
• 37
• 38
• 39 private double richter
• 40

26
File EarthquakeTest.java

• 1 import javax.swing.JOptionPane
• 2
• 3 /
• 4 A class to test the Earthquake class.
• 5 /
• 6 public class EarthquakeTest
• 7

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• 8 public static void main(String args)
• 9
• 10 String input JOptionPane.showInputDialog(
• 11 "Enter a magnitude on the Richter scale")
• 12 double magnitude Double.parseDouble(input)
• 13 Earthquake quake new Earthquake(magnitude)
• 14 System.out.println(quake.getDescription())
• 15 System.exit(0)
• 16
• 17

28
Multiple Alternatives

• Order mattersif (richter gt 0) // always
  passesr "Generally not felt by people"else
  if (richter gt 3.5) // not testedr "Felt by
  many people, no destruction". . .
• Don't omit elseif (richter gt 8.0)r "Most
  structures fall"if (richter gt 7.0) // omitted
  else--ERRORr "Many buildings destroyed

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Nested Branches

• Branch inside another branch
• if (condition1) if (condition1a)
  statement1a else statement1belse
  statement2

30
Tax Return

• If your filing status is Single

31

• If your filing status is Married

32
Tax Return
33
File TaxReturn.java

• 1 /
• 2 A tax return of a taxpayer in 1992.
• 3 /
• 4 class TaxReturn
• 5
• 6 /
• 7 Constructs a TaxReturn object for a given
  income and
• 8 marital status.
• 9 _at_param anIncome the taxpayer income
• 10 _at_param aStatus either SINGLE or MARRIED
• 11 /

34

• 12 public TaxReturn(double anIncome, int aStatus)
  
• 13
• 14 income anIncome
• 15 status aStatus
• 16
• 17
• 18 public double getTax()
• 19
• 20 double tax 0
• 21

35

• 22 if (status SINGLE)
• 23
• 24 if (income lt SINGLE_CUTOFF1)
• 25 tax RATE1 income
• 26 else if (income lt SINGLE_CUTOFF2)
• 27 tax SINGLE_BASE2
• 28 RATE2 (income - SINGLE_CUTOFF1)
• 29 else
• 30 tax SINGLE_BASE3
• 31 RATE3 (income - SINGLE_CUTOFF2)
• 32

36

• 33 else
• 34
• 35 if (income lt MARRIED_CUTOFF1)
• 36 tax RATE1 income
• 37 else if (income lt MARRIED_CUTOFF2)
• 38 tax MARRIED_BASE2
• 39 RATE2 (income - MARRIED_CUTOFF1)
• 40 else
• 41 tax MARRIED_BASE3
• 42 RATE3 (income - MARRIED_CUTOFF2)
• 43

37

• 44
• 45 return tax
• 46
• 47
• 48 public static final int SINGLE 1
• 49 public static final int MARRIED 2
• 50
• 51 private static final double RATE1 0.15
• 52 private static final double RATE2 0.28
• 53 private static final double RATE3 0.31
• 54

38

• 55 private static final double SINGLE_CUTOFF1
  21450
• 56 private static final double SINGLE_CUTOFF2
  51900
• 57
• 58 private static final double SINGLE_BASE2
  3217.50
• 59 private static final double SINGLE_BASE3
  11743.50
• 60 private static final double MARRIED_CUTOFF1
  35800
• 61 private static final double MARRIED_CUTOFF2
  86500

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• 62
• 63 private static final double MARRIED_BASE2
  5370
• 64 private static final double MARRIED_BASE3
  19566
• 65
• 66 private double income
• 67 private int status
• 68

40
File TaxReturnTest.java

• 1 import javax.swing.JOptionPane
• 2
• 3 /
• 4 A class to test the TaxReturn class.
• 5 /
• 6 public class TaxReturnTest
• 7
• 8 public static void main(String args)
• 9

41

• 10 String input JOptionPane.showInputDialog(
• 11 "Please enter your income")
• 12 double income Double.parseDouble(input)
• 13
• 14 input JOptionPane.showInputDialog(
• 15 "Please enter S (single) or M (married)")
• 16 int status 0
• 17
• 18 if (input.equalsIgnoreCase("S"))
• 19 status TaxReturn.SINGLE
• 20 else if (input.equalsIgnoreCase("M"))
• 21 status TaxReturn.MARRIED

42

• 22 else
• 23
• 24 System.out.println("Bad input.")
• 25 System.exit(0)
• 26
• 27
• 28 TaxReturn aTaxReturn new TaxReturn(income,
  status)
• 29
• 30 System.out.println("The tax is "
• 31 aTaxReturn.getTax())

43

• 32
• 33 System.exit(0)
• 34
• 35

44
The boolean Type

• George Boole (1815-1864) pioneer in the study of
  logic
• value of expression amount lt 1000 is true or
  false.
• boolean type one of these 2 truth values

45
Predicate Method

• return type boolean
• Examplepublic boolean isOverdrawn() return
  balance lt 0
• Use in conditionsif (harrysChecking.isOverdrawn()
  ) ...
• Useful predicate methods in Character
  classisDigitisLetterisUpperCaseisLowerCase
• if (Character.isUpperCase(ch)) ...

46
Boolean Operators

• and
• or
• ! not
• if (0 lt amount amount lt 1000) ...
• if (input.equals("S") input.equals("M")) ...

47
and Operators
48
Truth Tables
49
(No Transcript)
50
De Morgan's Law

• Negating a complex condition can be confusingif
  (!(0 lt amount amount lt 1000))
• De Morgan's law states that!(A B) is the same
  as !A !B!(A B) is the same as !A !B
• Note that the and flip when moving the !
  inwards
• (!(0 lt amount amount lt 1000)) is!(0 lt amount)
  !(amount lt 1000) , that is0 gt amount
  amount gt 1000
• Note that the opposite of lt is gt

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Boolean Variables

• private boolean married
• Set to truth valuemarried input.equals("M")
• Use in conditionsif (married) ... else ...if
  (!married) ...
• Also called flag
• Don't test Boolean variables against truth
  values--sign of cluelessnessif (married
  true) // DON'Tif (married false) // DON'Tif
  (married ! false) // NO!!!