1. CSS161: Fundamentals of Computing
Boolean Expressions
This slide set was compiled from the Absolute Java textbook slides (Walter Savitch) and the professor’s own class materials.
CSS161: Fundamentals of Computing

2. Simple Boolean Expressions
A Boolean expression is an expression that returns either true or false.
The simplest expressions are comparisons of two expressions:
time < limit
balance <= 0
If a Boolean expression is used in an if-else statement, it must be enclosed in parentheses.
if ( time < limit )
CSS161: Fundamentals of Computing

3. Java Comparison Operators
CSS161: Fundamentals of Computing

4. PITFALL 1: Using = in Place of ==
Comparison of two integers
Correct
int yourScore, myScore;
...; // yourScore, myScore initialized
if ( yourScore == myScore )
System.out.println( “A tie” );
Incorrect
if ( yourSocre = myScore)
Compilation error:
incompatible types
found : int
required: boolean
if ( yourScore = myScore ) ^
Comparison of two booleans
Correct
boolean passedCSS161 = false;
...; // passedCSS161 modified
if ( passedCSS161 == true )
System.out.println( “passed” );
Incorrect
if ( passedCSS161 = true )
No Compilation error! It’s always true.
CSS161: Fundamentals of Computing

5. PITFALL 2: Using == with Strings
Incorrect
String greetings;
Scanner keyboard = new Scanner( System.in );
greetings = keyboard.nextLine( );
if ( greetings = “How are you?” )
System.out.println( “Fine, thank you.” )
Output: no outputs
Correct
if ( greetings.equals( “How are you?” ) )
System.out.println( “Fine, thank you.” );
Output: Fine, thakn you.
== in Strings compare their references.
CSS161: Fundamentals of Computing

6. CSS161: Fundamentals of Computing
ASCII Code 32 56 8 80 P
104 h 33 ! 57 9 81 Q
105 iI 34 “ 58 : 82 R
106 j 35 # 59 ; 83 S
107 k 36 $ 60
< 84 T
108 l 37 % 61 = 85 U
109 m 38
& 62
> 86 V
110 n 39 ‘ 63 ? 87 W
111 o 40 ( 64 @ 88 X
112 p 41 ) 65 A 89 Y
113 q 42 * 66 B 90 Z
114 r 43 + 67 C 91 [
115 s 44 , 68 D 92 \
116 t 45 - 69 E 93 ]
117 u 46 . 70 F 94 ^
118 v 47 / 71 G 95 _
119 w 48 72 H 96 `
120 x 49 1 73 I 97 a
121 y 50 2 74 J 98 b
122 z 51 3 75 K 99 c
123 { 52 4 76 L
100 d
124 | 53 5 77 M
101 e
125 } 54 6 78 N
102 f
126 ~ 55 7 79 O
103 g
127
CSS161: Fundamentals of Computing

7. Alphabetical and Lexicographic Order
Alphabetical Order: A(a), B(b), C(c), …, Z(z)
Lexicographic Order: based on ASCII CODE
String has three methods:
string1.equals( string2 )
Returns true if string1 matches string2 exactly, otherwise false.
string1.compareTo( string2 )
Based on lexicographic order
Returns 0 if string1 matches string2 exactly, a positive integer if string1 comes before string2, otherwise a negative integer
string1.compareToIgnore( string2 )
Based on alphabetical order (in uppercase letters)
CSS161: Fundamentals of Computing

8. CSS161: Fundamentals of Computing
Example
String string1 = “abcd”;
String string2 = “abaf”;
System.out.println( string1.compareTo( string2 );
// ‘a’ – ‘a’ = 0; ‘b’ – ‘b’ = 0; ‘c’ – ‘d’ = 2; thus return 2.
// Note that ‘d’ – ‘f’ = -2 but this comparison is ignored.
String string3 = “abcd”;
String string4 = “ABCD”;
System.out.println( string3.compareTo( string4 );
// ‘a’- ‘A’ = -32; thus return -32.
System.out.println( string3.compareToIgnore( string4 );
// All letters are compared in uppercase. Thus return 0.
CSS161: Fundamentals of Computing

9. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on Textbook P111’s exercises 13 ~ 16.
CSS161: Fundamentals of Computing

10. Building Boolean Expressions
Two Boolean operators to combine Boolean expressions:
AND ( expression1 && expression2 ): returns true if both expressions are true.
( number > 2 ) && ( number < 7 )
// will be true if number is 3, 4, 5, 6
OR ( expression1 || expression2 ): returns true if either one of them is true.
( count < 3 ) || ( count > 12 )
// will be true if count is …, -2, -1, 0, 1, 2, 13, 14, 15, 16, …
Negation( ! ) used to negate a given expression:
!expression1: returns true if it is false.
! (savings < debt )
// true if savings >= debt
CSS161: Fundamentals of Computing

11. CSS161: Fundamentals of Computing
Truth Tables
CSS161: Fundamentals of Computing

12. Evaluating Boolean Expressions
!( ( count < 3 ) || ( count > 7 ) )
What if count is 8?
( number > 0 ) && !( number % 2 == 0 ) && !( number % 3 == )
What numbers make this expression true?
!( data < 0 ) && ( ( data / 100 < 1 ) || ( data % 100 == 0 ) )
What data make this expression true?
CSS161: Fundamentals of Computing

13. CSS161: Fundamentals of Computing
Self-Test Exercises
Work on Textbook p114’s exercises 17 ~ 18.
CSS161: Fundamentals of Computing

14. Short-Circuit or Lazy Evaluation
expression1 && expression2 && expression3
False && false && false returns false
False && false && true returns false
False && true && true returns false
Thus, if expression1 is false, no evaluation on expressions 2 and 3.
expression1 || expression2 || expression3
True || true || true returns true
True || true || false returns true
True || false || false returns true
Thus, if expression1 is true, no evaluation on expressions 2 and 3.
Useful to prevent a runtime error
if ( ( kids != 0 ) && ( ( pieces / kids ) >= 2 ) )
System.out.println( “Each child may have two pieces!” );
Complete evaluation
Use & and | instead of && and ||.
CSS161: Fundamentals of Computing

15. Precedence and Associativity Rules
CSS161: Fundamentals of Computing

16. CSS161: Fundamentals of Computing
Questions
Include parentheses in each of the following expressions to keep their operator precedence.
bonus + balance * rate / correctionFactor – penalty
number1 = number2 = number3 + 7 * factor
number + 1 > 2 || number + 5 < -3
expenses < income || expenses < savings || creditRating > 0
Recommended parentheses rule: include most parentheses, except where the intended meaning is obvious.
CSS161: Fundamentals of Computing

17. Side Effects and Evaluation Rule
Side Effects: when, in addition to returning a value, an expression changes something, such as the value of a variable
The assignment, increment, and decrement operators all produce side effects
Evaluation Rule:
Perform binding:
Determine the equivalent fully parenthesized expression using the precedence and associativity rules
Proceeding left to right, evaluate whatever subexpressions can be immediately evaluated
These subexpressions will be operands or method arguments, e.g., numeric constants or variables
Evaluate each outer operation and method invocation as soon as all of its operands (i.e., arguments) have been evaluated
CSS161: Fundamentals of Computing

18. CSS161: Fundamentals of Computing
Self-Test Exercises
( ( result = ( ++n ) + ( other = ( 2 * ( ++n ) ) ) )
What if n is 2 at first?
( ++n > 0 ) && ( 5 > n )
What if n is 0 at first?
Work on Textbook p125’s exercises 19 ~ 21.
CSS161: Fundamentals of Computing