Chapter 5 Decisions

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Chapter 5 Decisions

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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

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if Statement

if (amount <= balance)

balance = balance - amount;

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if/else Statement

else

balance = balance - OVERDRAFT_PENA LTY;

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Block Statement

if (amount <= balance) {

double newBalance = balance - amount;

balance = newBalance;

}

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Syntax 5.1. The if Statement

if(condition) statement

if (condition) statement else

statement

Example:

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else

balance = balance - OVERDRAFT_PENALTY;

– Purpose:

To execute a statement when a condition is true or false

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Syntax 5.2. Block Statement

{

statement . . .

}

Example:

{

double newBalance = balance - amount;

balance = newBalance;

}

Purpose:

To group several statements together to form a single

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Statement Types

 Simple statement

 Compound statement

if (balance >= amount) balance = balance - amount;

Also while, for, etc. (see chapter 6)

 Block statement {

double newBalance = balance - amount;

balance = newBalance;

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Relational Operators

Java Math Notation Description

> > Greater than

>= ≥

Greater than or equal

< < Less than

<= ≤

Less than or equal

== = Equal

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Equality Testing vs. Assignment

 The = = operator tests for equality:

if (x = = 0) . . // if x equals zero

 The = operator assigns a value to a variable:

x = 0; // assign 0 to x

 Don't confuse them.

 Java = = is the same as mathematical =

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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);

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 It prints:

sqrt(2)squared minus 2 is not 0 but 4.440892098500626E-16

 Don't use == to compare floating-point numbers

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Comparing Floating-Point Numbers

 Two numbers are close to another if

|x - y| <= ε

 ε is a small number such as 10^-14

 Not good enough if x, y very large or very small.

Then use

|x - y| / max(|x|, |y|) <= ε

 But if one of the numbers is zero, don't divide by max(|x |, |y|)

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String Comparison

 Don't use = = for strings!

if (input = = "Y") // WRONG!!!

 Use equals method:

if (input.equals("Y"))

 = = tests identity, equals tests equal contents

 Case insensitive test ("Y" or "y") if (input.equalsIgnoreCase("Y"))

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Lexicographic Comparison

 s.compareTo(t) < 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

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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(oatmealBox)

 cerealBox == r

 Caveat: equals must be defined for the class (see

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Object Comparison

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The null Reference

null reference refers to no object at all

Can be used in tests:

if (account == null) . . .

Use ==, not equals, to test for null

showInputDialog returns null if the user hit the cancel button:

String input =

JOptionPane.showInputDialog("...");

if (input != null) { ... }

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Multiple Alternatives

• if (condition1) statement1;

else if (condition2) statement2;

else if (condition3) statement3;

else

statement4;

• The first matching condition is executed.

• Order matters.

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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 @param magnitude the magnitude on the Richter scale

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10 public Earthquake(double magnitude) 11 {

12 richter = magnitude;

13 } 14

15 /**

16 Gets a description of the effect of the earthquake.

17 @return the description of the effect 18 */

19 public String getDescription() 20 {

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22 if (richter >= 8.0)

23 r = "Most structures fall";

24 else if (richter >= 7.0)

25 r = "Many buildings destroyed";

27 r = "Many buildings considerably damaged, some collapse";

29 r = "Damage to poorly constructed buildings";

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32 else if (richter >= 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 }

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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 }

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Multiple Alternatives

 Order matters:

if (richter >= 0) // always passes r = "Generally not felt by people";

else if (richter >= 3.5) // not tested

r = "Felt by many people, no destruction";

. . .

 Don't omit else if (richter >= 8.0)

r = "Most structures fall";

if (richter >= 7.0) // omitted else--ERROR

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Nested Branches

• Branch inside another branch if (condition1)

{

if (condition1a) statement1a;

else

statement1b;

} else

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Tax Return

• If your filing status is Single

If the taxable income is over

But not over

The tax is Of the

amount over

$0 $21,450 15% $0

$21,450 $51,900 $3,217.50 + 28%

$21,450

$51,900 $11,743.50

+ 31%

$51,900

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• If your filing status is Married

If the taxable income is over

But not over

The tax is Of the amount over

$0 $35,800 15% $0

$35,800 $86,500 $5,370.00 + 28%

$35,800

$86,500 $19,566.0

0 + 31%

$86,500

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Tax Return

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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 @param anIncome the taxpayer income

10 @param aStatus either SINGLE or MARRIED 11 */

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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

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22 if (status == SINGLE) 23 {

24 if (income <= SINGLE_CUTOFF1) 25 tax = RATE1 * income;

26 else if (income <= SINGLE_CUTOFF2) 27 tax = SINGLE_BASE2

28 + RATE2 * (income - SINGLE_CUTOFF1);

29 else

30 tax = SINGLE_BASE3

31 + RATE3 * (income - SINGLE_CUTOFF2);

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33 else 34 {

35 if (income <= MARRIED_CUTOFF1) 36 tax = RATE1 * income;

37 else if (income <= MARRIED_CUTOFF2) 38 tax = MARRIED_BASE2

39 + RATE2 * (income - MARRIED_CUTOFF1);

40 else

41 tax = MARRIED_BASE3

42 + RATE3 * (income - MARRIED_CUTOFF2);

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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;

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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 }

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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)

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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;

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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());

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32

33 System.exit(0);

34 } 35 }

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The boolean Type

 George Boole (1815-1864): pioneer in the study of logic

 value of expression amount < 1000 is true or false.

 boolean type: one of these 2 truth values

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Predicate Method

 return type boolean

 Example:

public boolean isOverdrawn() { return balance < 0; }

 Use in conditions

if (harrysChecking.isOverdrawn()) ...

 Useful predicate methods in Character class:

isDigit isLetter

isUpperCase isLowerCase

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Boolean Operators

 && and

 || or

 ! not

 if (0 < amount && amount < 1000) ...

 if (input.equals("S") || input.equals("M")) ...

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&& and || Operators

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Truth Tables

A B A && B

true true true

true false false

false Any false

A B A || B

true Any true

false true true

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A ! A

true false

false true

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De Morgan's Law

 Negating a complex condition can be confusing:

if (!(0 < amount && amount < 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 < amount && amount < 1000)) is

!(0 < amount) || !(amount < 1000) , that is 0 >= amount || amount >= 1000

 Note that the opposite of < is >=

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Boolean Variables

 private boolean married;

 Set to truth value:

married = input.equals("M");

 Use in conditions:

if (married) ... else ...

if (!married) ...

 Also called flag

 Don't test Boolean variables against truth values--sign of cluelessness:

if (married == true) // DON'T if (married == false) // DON'T

Chapter 5 Decisions

Chapter 5

Decisions

Chapter Goals

if Statement

if/else Statement

Block Statement

if (amount <= balance) {

double newBalance = balance - amount;

balance = newBalance;

}

Syntax 5.1. The if Statement

Syntax 5.2. Block Statement

Statement Types

Relational Operators

Java Math Notation Description

> > Greater than

>= &ge;

Greater than or equal

< < Less than

<= &le;

Less than or equal

== = Equal

Equality Testing vs. Assignment

Comparing Floating-Point Numbers

Comparing Floating-Point Numbers

String Comparison

Lexicographic Comparison

Lexicographic Comparison

Object Comparison

Object Comparison

The null Reference

null reference refers to no object at all

Can be used in tests:

if (account == null) . . .

Use ==, not equals, to test for null

showInputDialog returns null if the user hit the cancel button:

String input =

JOptionPane.showInputDialog("...");

if (input != null) { ... }

Multiple Alternatives

File Earthquake.java

File EarthquakeTest.java

Multiple Alternatives

Nested Branches

Tax Return

• If your filing status is Single

• If your filing status is Married

Tax Return

File TaxReturn.java

File TaxReturnTest.java

The boolean Type

Predicate Method

Boolean Operators

&& and || Operators

Truth Tables

De Morgan's Law

Boolean Variables

>= ≥

<= ≤