JavaYDL3

Chapter 3 Selections

Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All rights reserved.
1

Motivations
If you assigned a negative value for radius in
Listing 2.1, ComputeArea.java, the program would
print an invalid result. If the radius is negative, you
don't want the program to compute the area. How
can you deal with this situation?

Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All
rights reserved.
2

Objectives
 To declare boolean variables and write Boolean expressions using comparison operators (§3.2).
 To implement selection control using one-way if statements (§3.3).
 To program using one-way if statements (GuessBirthday) (§3.4).
 To implement selection control using two-way if-else statements (§3.5).
 To implement selection control using nested if and multi-way if statements (§3.6).
 To avoid common errors in if statements (§3.7).
 To generate random numbers using the Math.random() method (§3.8).
 To program using selection statements for a variety of examples (SubtractionQuiz, BMI,
ComputeTax) (§§3.8–3.10).
 To combine conditions using logical operators (&&, ||, and !) (§3.11).
 To program using selection statements with combined conditions (LeapYear, Lottery) (§§3.12–
3.13).
 To implement selection control using switch statements (§3.14).
 To write expressions using the conditional operator (§3.15).
 To format output using the System.out.printf method (§3.16).
 To examine the rules governing operator precedence and associativity (§3.17).
 To get user confirmation using confirmation dialogs (§3.18).
 To apply common techniques to debug errors (§3.19).

Liang, Introduction to Java Programming, Ninth Edition, (c) 2013 Pearson Education, Inc. All rights reserved.
3

The boolean Type and Operators
Often in a program you need to compare two
values, such as whether i is greater than j. Java
provides six comparison operators (also known
as relational operators) that can be used to
compare two values. The result of the
comparison is a Boolean value: true or false.

boolean b = (1 > 2);

rights reserved.
4

Comparison Operators
Java Mathematics Name Example Result
Operator Symbol (radius is 5)

< < less than radius < 0 false
<= ≤ less than or equal to radius <= 0 false
> > greater than radius > 0 true
>= ≥ greater than or equal to radius >= 0 true
== = equal to radius == 0 false
!= ≠ not equal to radius != 0 true

rights reserved.
5

Problem: A Simple Math Learning Tool
This example creates a program to let a first grader
practice additions. The program randomly
generates two single-digit integers number1 and
number2 and displays a question such as “What is
7 + 9?” to the student. After the student types the
answer, the program displays a message to indicate
whether the answer is true or false.

AdditionQuiz Run

rights reserved.
6

One-way if Statements
if (radius >= 0) {
area = radius * radius * PI;
if (boolean-expression) { System.out.println("The area"
statement(s); + " for the circle of radius "
}
+ radius + " is " + area);
}

false false
Boolean (radius >= 0)
Expression

true true

Statement(s) area = radius * radius * PI;
System.out.println("The area for the circle of " +
"radius " + radius + " is " + area);

(A) (B)
rights reserved.
7

Note
if i > 0 { if (i > 0) {
System.out.println("i is positive"); System.out.println("i is positive");
} }
(a) Wrong (b) Correct

if (i > 0) { if (i > 0)
System.out.println("i is positive"); Equivalent System.out.println("i is positive");
}

(a) (b)

rights reserved.
8

Simple if Demo

Write a program that prompts the user to enter an integer. If the
number is a multiple of 5, print HiFive. If the number is divisible
by 2, print HiEven.

SimpleIfDemo Run

rights reserved.
9

Problem: Guessing Birthday
The program can guess your birth date. Run
to see how it works.
= 19

+

1 3 5 7 2 3 6 7 4 5 6 7 8 9 10 11 16 17 18 19
9 11 13 15 10 11 14 15 12 13 14 15 12 13 14 15 20 21 22 23
17 19 21 23 18 19 22 23 20 21 22 23 24 25 26 27 24 25 26 27
25 27 29 31 26 27 30 31 28 29 30 31 28 29 30 31 28 29 30 31
Set1 Set2 Set3 Set4 Set5

GuessBirthday Run
rights reserved.
10

Mathematics Basis for the Game
19 is 10011 in binary. 7 is 111 in binary. 23 is 11101 in binary
10000
10000 00110 1000
10 10 100
+ 1 + 1 + 1
10011 00111 11101

19 7 23

= 19

+

1 3 5 7 2 3 6 7 4 5 6 7 8 9 10 11 16 17 18 19
9 11 13 15 10 11 14 15 12 13 14 15 12 13 14 15 20 21 22 23
17 19 21 23 18 19 22 23 20 21 22 23 24 25 26 27 24 25 26 27
25 27 29 31 26 27 30 31 28 29 30 31 28 29 30 31 28 29 30 31
rights reserved.
11

The Two-way if Statement
if (boolean-expression) {
statement(s)-for-the-true-case;
}
else {
statement(s)-for-the-false-case;
}

true false
Boolean
Expression

Statement(s) for the true case Statement(s) for the false case

rights reserved.
12

if-else Example
if (radius >= 0) {
area = radius * radius * 3.14159;

System.out.println("The area for the “
+ “circle of radius " + radius +
" is " + area);
}
else {
System.out.println("Negative input");
}

rights reserved.
13

Multiple Alternative if Statements

if (score >= 90.0) if (score >= 90.0)
grade = 'A'; grade = 'A';
else else if (score >= 80.0)
if (score >= 80.0) Equivalent grade = 'B';
grade = 'B'; else if (score >= 70.0)
else grade = 'C';
if (score >= 70.0) else if (score >= 60.0)
grade = 'C'; grade = 'D';
else else
if (score >= 60.0) grade = 'F';
grade = 'D';
else
grade = 'F';

rights reserved.
14

Multi-Way if-else Statements

false
score >= 90
false
true score >= 80
false
grade = 'A' true score >= 70
false
grade = 'B' rue score >= 60

grade = 'C' true

grade = 'D'

grade = 'F'

rights reserved.
15

animation
Trace if-else statement
Suppose score is 70.0 The condition is false

if (score >= 90.0)
grade = 'A';
else if (score >= 80.0)
grade = 'B';
grade = 'C';
grade = 'D';
else
grade = 'F';

rights reserved.
16

animation
Suppose score is 70.0 The condition is false

if (score >= 90.0)
grade = 'A';
grade = 'B';
grade = 'C';
grade = 'D';
else
grade = 'F';

rights reserved.
17

animation
Suppose score is 70.0 The condition is true

if (score >= 90.0)
grade = 'A';
grade = 'B';
grade = 'C';
grade = 'D';
else
grade = 'F';

rights reserved.
18

animation
Suppose score is 70.0 grade is C

if (score >= 90.0)
grade = 'A';
grade = 'B';
grade = 'C';
grade = 'D';
else
grade = 'F';

rights reserved.
19

animation
Suppose score is 70.0 Exit the if statement

if (score >= 90.0)
grade = 'A';
grade = 'B';
grade = 'C';
grade = 'D';
else
grade = 'F';

rights reserved.
20

Note
The else clause matches the most recent if clause in the
same block.
int i = 1; int i = 1;
int j = 2; int j = 2;
int k = 3; int k = 3;
Equivalent
if (i > j) if (i > j)
if (i > k) if (i > k)
System.out.println("A"); System.out.println("A");
else else
System.out.println("B"); System.out.println("B");
(a) (b)

rights reserved.
21

Note, cont.
Nothing is printed from the preceding statement. To
force the else clause to match the first if clause, you must
add a pair of braces:
int i = 1;
int j = 2;
int k = 3;
if (i > j) {
if (i > k)
System.out.println("A");
}
else
System.out.println("B");
This statement prints B.
rights reserved.
22

Common Errors
Adding a semicolon at the end of an if clause is a common
mistake.
if (radius >= 0); Wrong
{
area = radius*radius*PI;
System.out.println(
"The area for the circle of radius " +
radius + " is " + area);
}
This mistake is hard to find, because it is not a compilation error or
a runtime error, it is a logic error.
This error often occurs when you use the next-line block style.
rights reserved.
23

TIP
if (number % 2 == 0) Equivalent
even = true; boolean even
else = number % 2 == 0;
even = false;
(a) (b)

rights reserved.
24

CAUTION
if (even == true) Equivalent if (even)
System.out.println( System.out.println(
"It is even."); "It is even.");
(a) (b)

rights reserved.
25

Problem: An Improved Math Learning Tool
This example creates a program to teach a
first grade child how to learn subtractions.
The program randomly generates two single-
digit integers number1 and number2 with
number1 > number2 and displays a question
such as “What is 9 – 2?” to the student. After
the student types the answer in the input
dialog box, the program displays a message
dialog box to indicate whether the answer is
correct.
SubtractionQuiz Run
rights reserved.
26

Problem: Body Mass Index
Body Mass Index (BMI) is a measure of health on
weight. It can be calculated by taking your weight
in kilograms and dividing by the square of your
height in meters. The interpretation of BMI for
people 16 years or older is as follows:
BMI Interpretation

Below 18.5 Underweight
18.5-24.9 Normal
25.0-29.9 Overweight
Above 30.0 Obese

ComputeAndInterpretBMI Run
rights reserved.
27

Problem: Computing Taxes
The US federal personal income tax is calculated
based on the filing status and taxable income.
There are four filing statuses: single filers, married
filing jointly, married filing separately, and head of
household. The tax rates for 2009 are shown below.
Married Filing
Marginal Married Filing
Single Jointly or Qualifying Head of Household
Tax Rate Separately
Widow(er)
10% $0 – $8,350 $0 – $16,700 $0 – $8,350 $0 – $11,950

15% $8,351– $33,950 $16,701 – $67,900 $8,351 – $33,950 $11,951 – $45,500

25% $33,951 – $82,250 $67,901 – $137,050 $33,951 – $68,525 $45,501 – $117,450

28% $82,251 – $171,550 $137,051 – $208,850 $68,525 – $104,425 $117,451 – $190,200

33% $171,551 – $372,950 $208,851 – $372,950 $104,426 – $186,475 $190,201 - $372,950

35% $372,951+ $372,951+ $186,476+ $372,951+
rights reserved.
28

Problem: Computing Taxes, cont.
if (status == 0) {
// Compute tax for single filers
}
else if (status == 1) {
// Compute tax for married file jointly
// or qualifying widow(er)
}
// Compute tax for married file separately
}
// Compute tax for head of household
}
else {
// Display wrong status
}
ComputeTax Run
rights reserved.
29

Logical Operators
Operator Name
! not
&& and
|| or
^ exclusive or

rights reserved.
30

Truth Table for Operator !
p !p Example (assume age = 24, gender = 'M')

true false !(age > 18) is false, because (age > 18) is true.
false true !(gender != 'M') is true, because (grade != 'M') is false.

rights reserved.
31

Truth Table for Operator &&
p1 p2 p1 && p2 Example (assume age = 24, gender = 'F')

false false false (age > 18) && (gender == 'F') is true, because (age
false true false > 18) and (gender == 'F') are both true.

true false false (age > 18) && (gender != 'F') is false, because
(gender != 'F') is false.
true true true

rights reserved.
32

Truth Table for Operator ||
p1 p2 p1 || p2 Example (assume age = 24, gender = 'F')

false false false (age > 34) || (gender == 'F') is true, because (gender
false true true == 'F') is true.

true false true (age > 34) || (gender == 'M') is false, because (age >
true true true 34) and (gender == 'M') are both false.

rights reserved.
33

Truth Table for Operator ^
p1 p2 p1 ^ p2 Example (assume age = 24, gender = 'F')

false false false (age > 34) ^ (gender == 'F') is true, because (age
false true true > 34) is false but (gender == 'F') is true.

true false true (age > 34) || (gender == 'M') is false, because (age
> 34) and (gender == 'M') are both false.
true true false

rights reserved.
34

Examples
Here is a program that checks whether a number is divisible by 2
and 3, whether a number is divisible by 2 or 3, and whether a
number is divisible by 2 or 3 but not both:

TestBooleanOperators Run

rights reserved.
35

Examples
System.out.println("Is " + number + " divisible by 2 and 3? " +
((number % 2 == 0) && (number % 3 == 0)));

System.out.println("Is " + number + " divisible by 2 or 3? " +
((number % 2 == 0) || (number % 3 == 0)));

System.out.println("Is " + number +
TestBooleanOperators
" divisible by 2 or 3, but not both? " +
((number % 2 == 0) ^ (number % 3 == 0))); Run

rights reserved.
36

Companion
Website
The & and | Operators

Supplement III.B, “The & and | Operators”

rights reserved.
37

Companion
Website
The & and | Operators
If x is 1, what is x after this
expression?
(x > 1) & (x++ < 10)

If x is 1, what is x after this
expression?
(1 > x) && ( 1 > x++)

How about (1 == x) | (10 > x++)?
(1 == x) || (10 > x++)?
rights reserved.
38

Problem: Determining Leap Year?
This program first prompts the user to enter a year as
an int value and checks if it is a leap year.
A year is a leap year if it is divisible by 4 but not by
100, or it is divisible by 400.
(year % 4 == 0 && year % 100 != 0) || (year % 400
== 0)

LeapYear Run
rights reserved.
39

Problem: Lottery
Write a program that randomly generates a lottery of a two-
digit number, prompts the user to enter a two-digit number,
and determines whether the user wins according to the
following rule:
• If the user input matches the lottery in exact order, the
award is $10,000.
• If the user input matches the lottery, the award is
$3,000.
• If one digit in the user input matches a digit in the
lottery, the award is $1,000.

Lottery Run
rights reserved.
40

switch Statements
switch (status) {
case 0: compute taxes for single filers;
break;
case 1: compute taxes for married file jointly;
break;
case 2: compute taxes for married file separately;
break;
case 3: compute taxes for head of household;
break;
default: System.out.println("Errors: invalid status");
System.exit(1);
}

rights reserved.
41

switch Statement Flow Chart

status is 0
Compute tax for single filers break

status is 1
Compute tax for married jointly or qualifying widow(er) break

status is 2
Compute tax for married filing separately break

status is 3
Compute tax for head of household break

default
Default actions

rights reserved.
42

switch Statement Rules
The switch-expression
must yield a value of char, switch (switch-expression) {
byte, short, or int type and
must always be enclosed in case value1: statement(s)1;
parentheses. break;
case value2: statement(s)2;
The value1, ..., and valueN must break;
have the same data type as the …
value of the switch-expression.
The resulting statements in the case valueN: statement(s)N;
case statement are executed when break;
the value in the case statement default: statement(s)-for-default;
matches the value of the switch-
}
expression. Note that value1, ...,
and valueN are constant
expressions, meaning that they
cannot contain variables in the
expression, such as 1 + x.

rights reserved.
43

switch Statement Rules
The keyword break is optional, switch (switch-expression) {
but it should be used at the end of
case value1: statement(s)1;
each case in order to terminate the
remainder of the switch break;
statement. If the break statement case value2: statement(s)2;
is not present, the next case
statement will be executed. break;
…
case valueN: statement(s)N;
The default case, which is break;
optional, can be used to perform default: statement(s)-for-default;
actions when none of the
specified cases matches the
}
switch-expression. The case statements are executed in sequential
order, but the order of the cases (including the
default case) does not matter. However, it is good
programming style to follow the logical sequence
of the cases and place the default case at the end.

rights reserved.
44

animation

Trace switch statement
Suppose ch is 'a':

switch (ch) {
case 'a': System.out.println(ch);
case 'b': System.out.println(ch);
case 'c': System.out.println(ch);
}

rights reserved.
45

animation

ch is 'a':

switch (ch) {
}

rights reserved.
46

animation

Execute this line

switch (ch) {
}

rights reserved.
47

animation

Execute this line

switch (ch) {
}

rights reserved.
48

animation

Execute this line

switch (ch) {
}

rights reserved.
49

animation

Execute next statement

switch (ch) {
}

Next statement;

rights reserved.
50

animation

Suppose ch is 'a':

switch (ch) {
break;
break;
}

rights reserved.
51

animation

ch is 'a':

switch (ch) {
break;
break;
}

rights reserved.
52

animation

Execute this line

switch (ch) {
break;
break;
}

rights reserved.
53

animation

Execute this line

switch (ch) {
break;
break;
}

rights reserved.
54

animation

Execute next statement

switch (ch) {
break;
break;
}

Next statement;

rights reserved.
55

Problem: Chinese Zodiac
Write a program that prompts the user to enter a year
and displays the animal for the year.

pig
0: monkey
rat
1: rooster
dog ox 2: dog
3: pig
rooster tiger 4: rat
year % 12 = 5: ox
monkey rabbit 6: tiger
7: rabbit
8: dragon
sheep dragon
9: snake
horse snake 10: horse
11: sheep

ChineseZodiac Run
rights reserved.
56

Conditional Operator
if (x > 0)
y=1
else
y = -1;

is equivalent to

y = (x > 0) ? 1 : -1;
(boolean-expression) ? expression1 : expression2

Ternary operator
Binary operator
Unary operator
rights reserved.
57

Conditional Operator
if (num % 2 == 0)
System.out.println(num + “is even”);
else
System.out.println(num + “is odd”);

System.out.println(
(num % 2 == 0)? num + “is even” :
num + “is odd”);

rights reserved.
58

Conditional Operator, cont.
(boolean-expression) ? exp1 : exp2

rights reserved.
59

Formatting Output
Use the printf statement.
System.out.printf(format, items);
Where format is a string that may consist of substrings and
format specifiers. A format specifier specifies how an item
should be displayed. An item may be a numeric value,
character, boolean value, or a string. Each specifier begins
with a percent sign.

rights reserved.
60

Frequently-Used Specifiers
Specifier Output Example
%b a boolean value true or false
%c a character 'a'
%d a decimal integer 200
%f a floating-point number 45.460000
%e a number in standard scientific notation 4.556000e+01
%s a string "Java is cool"

int count = 5;
items
double amount = 45.56;
System.out.printf("count is %d and amount is %f", count, amount);

display count is 5 and amount is 45.560000
rights reserved.
61

Operator Precedence
 var++, var--
 +, - (Unary plus and minus), ++var,--var
 (type) Casting
 ! (Not)
 *, /, % (Multiplication, division, and remainder)
 +, - (Binary addition and subtraction)
 <, <=, >, >= (Comparison)
 ==, !=; (Equality)
 ^ (Exclusive OR)
 && (Conditional AND) Short-circuit AND
 || (Conditional OR) Short-circuit OR
 =, +=, -=, *=, /=, %= (Assignment operator)

rights reserved.
62

Operator Precedence and Associativity
The expression in the parentheses is evaluated first.
(Parentheses can be nested, in which case the expression
in the inner parentheses is executed first.) When
evaluating an expression without parentheses, the
operators are applied according to the precedence rule and
the associativity rule.

If operators with the same precedence are next to each
other, their associativity determines the order of
evaluation. All binary operators except assignment
operators are left-associative.

rights reserved.
63

Operator Associativity
When two operators with the same precedence
are evaluated, the associativity of the operators
determines the order of evaluation. All binary
operators except assignment operators are left-
associative.
a – b + c – d is equivalent to ((a – b) + c) – d
Assignment operators are right-associative.
Therefore, the expression
a = b += c = 5 is equivalent to a = (b += (c = 5))

rights reserved.
64

Example
Applying the operator precedence and associativity rule,
the expression 3 + 4 * 4 > 5 * (4 + 3) - 1 is evaluated as
follows:

3 + 4 * 4 > 5 * (4 + 3) - 1
(1) inside parentheses first
3 + 4 * 4 > 5 * 7 – 1
(2) multiplication
3 + 16 > 5 * 7 – 1
(3) multiplication
3 + 16 > 35 – 1
(4) addition
19 > 35 – 1
(5) subtraction
19 > 34
(6) greater than
false
rights reserved.
65

Companion
Website
Operand Evaluation Order
Supplement III.A, “Advanced discussions on
how an expression is evaluated in the JVM.”

rights reserved.
66

(GUI) Confirmation Dialogs

int option = JOptionPane.showConfirmDialog
(null, "Continue");

rights reserved.
67

Problem: Guessing Birth Date
The program can guess your birth date. Run
to see how it works.
= 19

+

1 3 5 7 2 3 6 7 4 5 6 7 8 9 10 11 16 17 18 19
9 11 13 15 10 11 14 15 12 13 14 15 12 13 14 15 20 21 22 23
17 19 21 23 18 19 22 23 20 21 22 23 24 25 26 27 24 25 26 27
25 27 29 31 26 27 30 31 28 29 30 31 28 29 30 31 28 29 30 31

GuessBirthdayUsingConfirmationDialog Run
rights reserved.
68

JavaYDL3

More Related Content

Viewers also liked (20)

Similar to JavaYDL3 (20)

More from Terry Yoast (20)

Recently uploaded (20)

JavaYDL3