Java Programmin: Selections

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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?

3
Objectives
To declare boolean type and write Boolean expressions using comparison operators .
To program Addition Quiz using Boolean expressions .
To implement selection control using one-way if statements
To program the Guess Birthday game using one-way if statements .
To implement selection control using two-way if statements.
To implement selection control using nested if statements.
To avoid common errors in if statements.
To generate random numbers using the Math .random() method.
To combine conditions using logical operators (, ||, and !).
To program using selection statements with combined conditions (Leap Year, Lottery) .
To implement selection control using switch statements .
To write expressions using the conditional operator .
To format output using the System.out.printf method and to format strings using the String.format
method.
To examine the rules governing operator precedence and associativity.
(GUI) To get user confirmation using confirmation dialogs.

4
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 ccoommppaarriissoonn ooppeerraattoorrss (also known
as relational operators) that can be used to
compare two values. The result of the
comparison is a BBoooolleeaann vvaalluuee:: ttrruuee oorr ffaallssee.
boolean b = (1 2);

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Comparison Operators
Operator Name
less than
= less than or equal to
greater than
= greater than or equal to
== equal to
!= not equal to

if (radius = 0) {
area = radius * radius * PI;
System.out.println(The area
+ for the circle of radius
+ radius + is + area);
}
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One-way if Statements
if (boolean-expression) {
statement(s);
}
Boolean
Expression
true
Statement(s)
false
(radius = 0)
true
false
area = radius * radius * PI;
System.out.println(The area for the circle of +
radius + radius + is + area);
(A) (B)

System.out.println(i is positive);
7
Note
if i 0 {
}
if (i 0) {
}
(a) Wrong (b) Correct
if (i 0) {
}
(a)
Equivalent
(b)
if (i 0)

8
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

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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);
}

Multiple Alternative if Statements-m
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if (score = 90.0)
grade = 'A';
else
if (score = 80.0)
grade = 'B';
else
if (score = 70.0)
grade = 'C';
else
if (score = 60.0)
grade = 'D';
else
grade = 'F';
Equivalent
if (score = 90.0)
grade = 'A';
else if (score = 80.0)
grade = 'B';
grade = 'C';
grade = 'D';
else
grade = 'F';

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Trace if-else statement
Suppose score is 70.0 The condition is false
if (score = 90.0)
grade = 'A';
grade = 'B';
grade = 'C';
grade = 'D';
else
grade = 'F';

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Suppose score is 70.0 The condition is false
if (score = 90.0)
grade = 'A';
grade = 'B';
grade = 'C';
grade = 'D';
else
grade = 'F';

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Suppose score is 70.0 The condition is true
if (score = 90.0)
grade = 'A';
grade = 'B';
grade = 'C';
grade = 'D';
else
grade = 'F';

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Suppose score is 70.0 grade is C
if (score = 90.0)
grade = 'A';
grade = 'B';
grade = 'C';
grade = 'D';
else
grade = 'F';

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Suppose score is 70.0 Exit the if statement
if (score = 90.0)
grade = 'A';
grade = 'B';
grade = 'C';
grade = 'D';
else
grade = 'F';

16
Note
The else clause matches the most recent if clause in the
same block.
int i = 1;
int j = 2;
int k = 3;
if (i j)
if (i k)
System.out.println(A);
else
System.out.println(B);
(a)
Equivalent
(b)
int i = 1;
int j = 2;
int k = 3;
if (i j)
if (i k)
else

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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)
}
else
This statement prints B.

18
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.

19
TIP
if (number % 2 == 0)
even = true;
else
even = false;
(a)
Equivalent boolean even
= number % 2 == 0;
(b)

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

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Logical Operators (evening)
Operator Name
! not
and
|| or
^ exclusive or

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Truth Table for Operator !
p !p
true false
false true
Example (assume age = 24, gender = 'M')
!(age 18) is false, because (age 18) is true.
!(gender != 'F') is true, because (grade != 'F') is false.

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Truth Table for Operator
p1 p2 p1 p2
false false false
false true false
true false false
true true true
Example (assume age = 24, gender = 'F')
(age 18) (gender == 'F') is true, because (age
18) and (gender == 'F') are both true.
(age 18) (gender != 'F') is false, because
(gender != 'F') is false.

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Truth Table for Operator ||
p1 p2 p1 || p2
false false false
false true true
true false true
true true true
(age 34) || (gender == 'F') is true, because (gender
== 'F') is true.
(age 34) || (gender == 'M') is false, because (age
34) and (gender == 'M') are both false.

25
Truth Table for Operator ^
p1 p2 p1 ^ p2
false false false
false true true
true false true
true true false
(age 34) ^ (gender == 'F') is true, because (age
34) is false but (gender == 'F') is true.
(age 34) || (gender == 'M') is false, because (age
34) and (gender == 'M') are both false.

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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:
Note: Look practical part

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)
A leap year, occurring once every four years, that has
366 days including February 29 as an intercalary day.
27

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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(0);
}

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switch Statement Flow Chart
status is 0
Compute tax for single filers break
Compute tax for married file jointly break
status is 1
Compute tax for married file separatly break
status is 2
Compute tax for head of household break
status is 3
Default actions
default
Next Statement

30
switch Statement Rules
switch (switch-expression) {
case value1: statement(s)1;
break;
break;
…
case valueN: statement(s)N;
break;
default: statement(s)-for-default;
}
The switch-expression
must yield a value of char,
byte, short, or int type
and must always be
enclosed in parentheses.
-The value1, ..., and valueN must
have the same data type as the value
of the switch-expression.
-The resulting statements in the case
statement are executed when the
value in the case statement 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.

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switch Statement Rules
The keyword break is optional,
but it should be used at the end of
each case in order to terminate the
remainder of the switch statement.
If the break statement is not
present, the next case statement
will be executed.
switch (switch-expression) {
break;
break;
…
case valueN: statement(s)N;
break;
The default case, which is
optional, can be used to perform
actions when none of the
specified cases matches the
switch-expression. The case statements are executed in sequential
default: statement(s)-for-default;
}
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.

32
import java.util.Scanner;
public class SwitchCase {
public static void main(String[] args) {
Scanner In=new Scanner(System.in);
System.out.print(please inter the number);
int number=In.nextInt();
switch ( number) {
case 1: System.out.println(“the number is “ + number);
break;
break;
break;
default: System.out.println(you inserted unknown number);
}
}
}

33
Trace switch statement
switch (ch) {
case 'a': System.out.println(ch);
case 'b': System.out.println(ch);
case 'c': System.out.println(ch);
}
Suppose ch is 'a':

34
switch (ch) {
}
ch is 'a':

35
switch (ch) {
}
Execute this line

36
switch (ch) {
}
Execute this line

37
switch (ch) {
}
Execute this line

38
switch (ch) {
}
Next statement;
Execute next statement

39
switch (ch) {
break;
break;
}
Suppose ch is 'a':

40
switch (ch) {
break;
break;
}
ch is 'a':

41
switch (ch) {
break;
break;
}
Execute this line

42
switch (ch) {
break;
break;
}
Execute this line

43
switch (ch) {
break;
break;
}
Next statement;
Execute next statement

44
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

45
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”);

46
Conditional Operator, cont.
(boolean-expression) ? exp1 : exp2

47
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.
int count = 5;
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
items

48
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;
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
items

49
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)

50
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))

51
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
3 + 4 * 4 5 * 7 – 1
3 + 16 5 * 7 – 1
3 + 16 35 – 1
19 35 – 1
19 34
false
(1) inside parentheses first
(2) multiplication
(3) multiplication
(4) addition
(5) subtraction
(6) greater than

52
(GUI) Confirmation Dialogs
int option = JOptionPane.showConfirmDialog
(null, Continue);

Java Programmin: Selections

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