1 decisions, decisions, decisions chapter 5 fall 2006 cs 101 aaron bloomfield
TRANSCRIPT
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Background Our problem-solving solutions so far have the straight-line
property They execute the same statements for every run of the
program
public class DisplayForecast // main(): application entry point
public static void main(String[] args) { System.out.print("I think there is a world"); System.out.print(" market for maybe five "); System.out.println("computers. “); System.out.print(" Thomas Watson, IBM, “); System.out.println("1943.“);
}}
3
Background For general problem solving we need more capabilities
The ability to control which statements are executed The ability to control how often a statement is executed
We will concentrate first on controlling which statements are executed
Java provides the if and switch conditional constructs to control whether a statement list is executed The if constructs use logical expressions to determine
their course of action
We will start with logical expressions
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Logical expressions The branch of mathematics dealing with logical expressions
is Boolean algebra Developed by the British mathematician George Boole
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Logical expressions A logical expression has either the value logical true or
logical false
Some expressions whose values are logical true The year 2004 is a leap year A meter equals 100 centimeters
Some expressions whose values are logical false A triangle has four sides The area of square is always equal to twice its
perimeter
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Logical expressions There are three primary logical operators for manipulating
logical values Logical and Logical or Logical not
The operators work as most of us would expect
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Truth tables We use truth tables to give formal specifications of the
operators “It works as most of us would expect” allows for
ambiguity of interpretation Jim is smiling or Patty is smiling
Can both Jim and Patty both be smiling? Truth tables
Lists all combinations of operand values and the result of the operation for each combination
p q p and q
False False FalseFalse True FalseTrue False FalseTrue True True
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Or and not truth tables
p q p or q
False False FalseFalse True TrueTrue False TrueTrue True True p not p
False TrueTrue False
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Boolean algebra Can create complex logical expressions by combining simple
logical expressions
not (p and q)
p q p and q not (p and q)
False False False TrueFalse True False TrueTrue False False TrueTrue True True False
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DeMorgan’s laws not (p and q) equals (not p) or (not q)
(not p) or p q p and q not (p and q) ( not p) (not q) (not q)
False False False True True True TrueFalse True False True True False TrueTrue False False True False True TrueTrue True True False False False False
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DeMorgan’s laws not (p or q) equals (not p) and (not q)
(not p) and p q p or q not (p or q) ( not p) (not q) (not q)
False False False True True True TrueFalse True True False True False FalseTrue False True False False True FalseTrue True True False False False False
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DeMorgan’s laws If you remember nothing else about the Boolean operators,
remember that:
not (a and b) == (not a) or (not b) not (a or b) == (not a) and (not b)
1414
Sidewalk chalk guySidewalk chalk guy
Source: http://www.gprime.net/images/sidewalkchalkguy/Source: http://www.gprime.net/images/sidewalkchalkguy/
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A boolean type Java has the logical type boolean
Type boolean has two literal constants true false
Operators The and operator is &&
Don’t use & The or operator is ||
Don’t use | The not operator is !
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Defining boolean variables Local boolean variables are uninitialized by default
boolean isWhitespace;boolean receivedAcknowledgement;boolean haveFoundMissingLink;
-isWhitespace
-receivedAcknowledgement
-haveFoundMissingLink
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Defining boolean variables Local boolean variables with initialization
boolean canProceed = true;boolean preferCyan = false;boolean completedSecretMission = true;
truecanProceed
falsepreferCyan
truecompletedSecretMission
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Assignment vs. comparison = is the assignment operator
It copies the value on the right to the location on the left Consider:
int x;x = 5;
The value 5 is copied to the spot x in memory
== is the comparison operator Returns a boolean (true or false) if the two sides are
equal Consider:
int x = 5;System.out.println (x == 5);System.out.println (x == 6);
Prints out true, false
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Other operators Equality operators == and !=
Operator == Returns true if the operands have the same value;
otherwise, returns false This is not the assignment operator!
Operator != Returns true if the operands have different values;
otherwise, returns false
The operators work with all types of values
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Evaluating boolean expressions Suppose
boolean p = true;boolean q = false;boolean r = true;boolean s = false;
What is the value ofp p && s !s p == q q q != r p && r r == s q || s q != s
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Evaluating boolean expressions Suppose
int i = 1;int j = 2;int k = 2;char c = '#';char d = '%';char e = '#';
What is the value ofj == k i != ki == j j != kc == e d != ec == d c != e
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Translating English to logical expressions English doesn’t always translate cleanly into logical expressions To see this, we need to examine the NOR operator
It doesn’t exist in Java, but we can fake it p NOR q == NOT (p OR q) NOR is represented by a downward arrow:
In Java, given variables p and q NOR is done by: !(p||q)
p q p or q not (p or q) p nor q
False False False True TrueFalse True True False FalseTrue False True False FalseTrue True True False False
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Translation Example “I have neither given nor received help on this exam”
Rephrased: “I have not given nor received …” Let p = “I have given help on this exam” Let q = “I have received help on this exam”
Translation is: pq Remember the precedence: NOT is done first!
p q p pq
T T F F
T F F T
F T T F
F F T F
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Translation example, take 2 “I have neither given nor received help on this exam”
Rephrased: “I have not (given nor received …)” Let p = “I have given help on this exam” Let q = “I have received help on this exam”
Another translation is: (pq) = p || q
p q p || q
T T T
T F T
F T T
F F F
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Translation example, rephrased What is meant is “I have not given and I have not received help
on this exam” Or “I have not (given or received) help on this exam” This is a DeMorgaization of the one above it
The problem: has a higher precedence than in Boolean logic, but not always in English
Also, “neither” is vague
p q pq (p||q)
T T F F
T F F F
F T F F
F F T T
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Disclaimer The preceding example was showing that when English can
be clear, it does not always translate clearly into Boolean logic The English meaning is exact The Boolean translation is not
This does not mean that you can cheat, and say that I said it was allowed by the honor code
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Floating point precission What gets printed?
class FloatTest {public static void main (String args[]) {
double y = 0.1 + 0.1 + 0.1 + 0.1 + 0.1 +
0.1 + 0.1 + 0.1 + 0.1 + 0.1;
System.out.println (y);}
} There are 10 0.1’s
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Take care with floating-point values Consider
double a = 1;double b = 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1 + 0.1double c = .9999999999999999;
Two true expressions! c == b b != a
Two false expressions!a == b b != c
Problem lies with the finite precision of the floating-point types
Instead with the ordering operators for closeness
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How to solve this Don’t compare floating-point values if you can help it!
Both doubles and floats
Need to test if the two doubles are “close” in value
final double EPSILON = 0.000001;boolean foo = Math.abs (a-b) < EPSILON;
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Ordering operators Java provides ordering operators for the primitive types
Four ordering operators, <, >, <=, and >= They correspond to mathematical operators of <, >, ≤,
and ≥
Together the equality and ordering operators are known as the relational operators
False is less than true
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Evaluation boolean expressions Suppose
int i = 1;int j = 2;int k = 2;
What is the value ofi < jj < ki <= kj >= ki >= k
39
Unicode values Character comparisons are based on their Unicode values
Characters ‘0’, ‘1’, … ‘9’ have expected order Character ‘0’ has the encoding 48 Character ‘1’ has the encoding 49, and so on.
Upper case Latin letters ‘A’, ‘B’, … ‘Z’ have expected order Character ‘A’ has the encoding 65, character ‘B’ has the
encoding 66, and so on.
Lower case Latin letters ‘a’, ‘b’, … ‘z’ have expected order Character ‘a’ has the encoding 97 Character ‘b’ has the encoding 98, and so on.
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Evaluation boolean expressions Suppose
char c = '2';char d = '3';char e = '2';
What is the value ofc < dc < ec <= ed >= ec >= e
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Operator precedence revisited Highest to lowest
Parentheses Unary operators Multiplicative operators Additive operators Relational ordering Relational equality Logical and Logical or Assignment
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Expressions vs. statements A statement is a single command for Java to do, and always
ends in a semi-colon (for now, at least) System.out.println (“hello world”); int x = 4; ++x;
An expression returns a value, and does not have a semi-colon 5 circle.getRadius() x
Note the difference between the following: ++i is an expression ++i; is a statement
45
Conditional constructs Provide
Ability to control whether a statement list is executed
Two constructs
If statement if if-else if-else-if
Switch statement
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Basic if statement Syntax
if (Expression) Action
If the Expression is true then execute Action
Action is either a single statement or a group of statements within braces
For us, it will always be a group of statements within braces
Expression
Action
true false
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Example
if (value < 0) { value = -value;}
Is our numbernegative?
If Value is notless than zero
then our numberis fine as is
If Value is less thanzero then we needto update its value
to that of its additiveinverse
false
Value < 0
Value = -Value
true
Our number is nowdefinitely
nonnegative
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Sorting two valuesSystem.out.print("Enter an integer number: ");int value1 = stdin.nextInt();System.out.print("Enter another integer number: ");int value2 = stdin.nextInt();
// rearrange numbers if necessaryif (value2 < value1) {
// values are not in sorted orderint rememberValue1 = value1;value1 = value2;value2 = rememberValue1;
}
// display valuesSystem.out.println("The numbers in sorted order are "
+ value1 + " and then " + value2);
What happens if the user enters 11 and 28?
What happens if the user enters 11 and 4?
If semanticsAre the numbers outof order
Rearrange value1 andvalue2 to put their
values in the properorder
value2 < value1
int rememberValue1 = value1 value1 = value2 value2 = rememberValue1
true false
The numbers were initially inorder
The numbers were rearranged intothe proper order
The numbers are in order
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What an if statement executes An if statement executes the next block of code
A block is either:
A single statement without curly brackets:
if (a == b)System.out.println (“a==b!!!”);
A number of statements enclosed by curly brackets:
if (a == b) {System.out.print (“a”);System.out.print (“==”);System.out.print (“b”);System.out.println (“!!!”);
}
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Why we always use braces What is the output?
int m = 5;int n = 10;
if (m < n) ++m;++n;
System.out.println(" m = " + m + " n = “ + n);
5252
Warn your grandparents!Warn your grandparents!
Historically, this class has been lethal to Historically, this class has been lethal to grandparents of students in the classgrandparents of students in the class– More often grandmothersMore often grandmothers
This happens most around test timeThis happens most around test time– Although occasionally around the times a big Although occasionally around the times a big
assignment is dueassignment is due
See See http://www.cis.gsu.edu/~dstraub/Courses/Grandma.htmhttp://www.cis.gsu.edu/~dstraub/Courses/Grandma.htm
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The if-else statement Syntax
if (Expression) Action1
else Action2
If Expression is true then execute Action1 otherwise execute Action2
The actions are either a single statement or a list of statements within braces
Expression
Action1 Action2
true false
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Finding the maximum of two valuesSystem.out.print("Enter an integer number: ");int value1 = stdin.nextInt();System.out.print("Enter another integer number: ");int value2 = stdin.nextInt();
int maximum;if (value1 < value2) { // is value2 larger? maximum = value2; // yes: value2 is larger}else { // (value1 >= value2) maximum = value1; // no: value2 is not larger}System.out.println("The maximum of " + value1
+ " and " + value2 + " is " + maximum);
But is it initialized?
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Finding the maximum of two valuesSystem.out.print("Enter an integer number: ");int value1 = stdin.nextInt();System.out.print("Enter another integer number: ");int value2 = stdin.nextInt();
int maximum;if (value1 < value2) { // is value2 larger? maximum = value2; // yes: value2 is larger}
System.out.println("The maximum of " + value1 + " and " + value2 + " is " + maximum);
But is it initialized?
57
Finding the maximum of two values
value1 < value2
maximum = value2 maximum = value1
true false
Is value2 larger than value1
Yes, it is . So value2 islarger than value1. Inthis case, maximum is
set to value2
No, its not. So value1is at least as large asvalue2. In this case,maximum is set to
value1
Either case, maximum is setcorrectly
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Why we use whitespace What does the following do?
System.out.print("Enter an integer number: ");int value1 = stdin.nextInt();System.out.print("Enter another integer number: ");int value2 = stdin.nextInt();if (value2 < value1) {int rememberValue1 = value1;value1 = value2;value2 = rememberValue1;}System.out.println("The numbers in sorted order are "+ value1 + " and then " + value2);
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How do you like your braces?
if (a == b){
//...} else {
//...}
if (a == b){
//...}else {
//...}
if (a == b){
//...} else {
//...}
if (a == b) {//...
} else {//...
}
if (a == b) {//...
} else {
//...}
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If-then-else precedence
if (number != 0)if (number > 0)
System.out.println("positive");else
System.out.println("negative");
Which if does this else refer to?
61
If-then-else precedence without whitespace
if (number != 0)if (number > 0)System.out.println("positive");elseSystem.out.println("negative");elseSystem.out.println("zero");
65
if (number == 0) {System.out.println("zero");
}else
If-else-if Consider
System.out.println("positive");}else { System.out.println("negative");}}
{ These braces aren’t needed
if (number > 0) {
Same results as previous segment – but this segment better expresses the meaning of what is going on
We can change the whitespace
of the code
66
Sorting three values For sorting values n1, n2, and n3 there are six possible
orderings n1 n2 n3 n1 n3 n2 n2 n1 n3 n2 n3 n1 n3 n1 n2 n3 n2 n1
Suppose s1, s2, s3 are to be a sorted version of n1, n2, and n3
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Sorting three valuesif ((n1 <= n2) && (n2 <= n3)) { // n1 <= n2 <= n3
s1 = n1; s2 = n2; s3 = n3;}else if ((n1 <= n3) && (n3 <= n2)) { // n1 <= n3 <= n2
s1 = n1; s2 = n3; s3 = n2;}else if ((n2 <= n1) && (n1 <= n3)) { // n2 <= n1 <= n3
s1 = n2; s2 = n1; s3 = n3;}else if ((n2 <= n3) && (n3 <= n1)) { // n2 <= n3 <= n1
s1 = n2; s2 = n3; s3 = n1;}else if ((n3 <= n1) && (n1 <= n2)) { // n3 <= n1 <= n2
s1 = n3; s2 = n1; s3 = n2;}else { // n3 <= n2 <= n1
s1 = n3; s2 = n2; s3 = n1;}
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Finding the minimum value Consider:
// z is to hold the minimum of x and yif ( x < y )
z = x;else
z = y;
Another way to do this:
z = (x<y) ? x : y;
Notice no braces!
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The ?: notation Only works when both “cases” return a value!
Meaning when both “cases” are expressions Example: z = (x<y) ? x : y; Thus, you can’t put a print statement in there!
Can be difficult to read
System.out.println ((number != 0) ? ((number > 0) ? "positive“ : "negative") : "zero“);
if (number != 0)if (number > 0)
System.out.println("positive");else
System.out.println("negative");else
System.out.println("zero");
73
Switch statement Software engineers often confronted with programming tasks
where required action depends on the values of integer expressions The if-else-if construct can be used
Separately compare the desired expression to a particular value If the expression and value are equal, then perform
the appropriate action
Because such programming tasks occur frequently Java includes a switch statement
The task is often more readable with the switch then with the if-else-if
74
A switch statement exampleif (a == ‘0’)
System.out.println (“zero”);else if (a == ‘1’)
System.out.println (“one”);else if (a == ‘2’)
System.out.println (“two”);else if (a == ‘3’)
System.out.println (“three”);else if (a == ‘4’)
System.out.println (“four”);else
System.out.println (“five+”);
switch (a) {case ‘0’:
System.out.println (“zero”);break;
case ‘1’:System.out.println (“one”);break;
case ‘2’:System.out.println (“two”);break;
case ‘3’:
System.out.println (“three”);break;
case ‘4’:
System.out.println (“four”);break;
default:System.out.println (“five+”);break;
}
76
Switch statement
switch ( SwitchExpression ) {
case CaseExpression1 :Action1 ;
case CaseExpression2 :Action2 ;
...
case CaseExpressionn :Actionn;
default :Actionn+1 ;
}
Constantintegral
expressionJavastatements
Integral expression tobe matched with a case
expression
77
Testing for vowel-nessswitch (ch) {
case 'a': case 'A':case 'e':case 'E':case 'i':case 'I':case 'o':case 'O':case 'u':case 'U':
System.out.println("vowel“);break;
default: System.out.println("not a vowel“);
}
78
Testing for vowel-ness
switch (ch) {case 'a': case 'A':case 'e': case 'E':case 'i': case 'I':case 'o': case 'O':case 'u': case 'U':
System.out.println("vowel“);break;
default: System.out.println("not a vowel“);
}
The break causes an exiting of the switch
Handles all of the other cases
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A better way to format that switch statement
switch (ch) {case 'a':
// FALL THRUcase 'A':
// FALL THRUcase 'e':
// FALL THRUcase 'E':
// FALL THRUcase 'i':
// FALL THRUcase 'I':
// FALL THRU
...
80
Processing a request
System.out.print("Enter a number: ");int n1 = stdin.nextInt();
System.out.print("Enter another number: ");int n2 = stdin.nextInt();
System.out.print("Enter desired operator: ");char operator = stdin.nextLine().charAt(0);
switch (operator) {case '+' : System.out.println((n1 + n2)); break;case '-' : System.out.println(n1 - n2); break;case '*' : System.out.println(n1 * n2); break;case '/' : System.out.println(n1 / n2); break;default: System.out.println(“Illegal request“);
}
81
SurveySurvey
How many people feel they How many people feel they understand the switch statement?understand the switch statement?
8282
Biggest software errorsBiggest software errors Ariane 5 rocket explosion (1996)Ariane 5 rocket explosion (1996)
– Due to loss of precision converting 64-bit double to 16-bit intDue to loss of precision converting 64-bit double to 16-bit int Pentium division error (1994)Pentium division error (1994)
– Due to incomplete look-up table (like an array)Due to incomplete look-up table (like an array) Patriot-Scud missile error (1991)Patriot-Scud missile error (1991)
– Rounding error on the timeRounding error on the time– The missile did not intercept an incoming Scud missile, leaving 28 dead and 98 The missile did not intercept an incoming Scud missile, leaving 28 dead and 98
woundedwounded Mars Climate Orbiter (1999)Mars Climate Orbiter (1999)
– Onboard used metric units; ground computer used English unitsOnboard used metric units; ground computer used English units AT&T long distance (1990)AT&T long distance (1990)
– Wrong break statement in C codeWrong break statement in C code Therac-25, X-ray (1975-1987)Therac-25, X-ray (1975-1987)
– Badly designed software led to radiation overdose in chemotherapy patientsBadly designed software led to radiation overdose in chemotherapy patients NE US power blackout (2003)NE US power blackout (2003)
– Flaw in GE software contributed to itFlaw in GE software contributed to it
References: References: http://www5.in.tum.de/~huckle/bugse.htmlhttp://www5.in.tum.de/~huckle/bugse.html, , http://en.wikipedia.org/wiki/Computer_bughttp://en.wikipedia.org/wiki/Computer_bug, , http://http://www.cs.tau.ac.il/~nachumd/verify/horror.htmlwww.cs.tau.ac.il/~nachumd/verify/horror.html
84
Testing variables for equality Consider
System.out.print("Enter an integer number: ");int n1 = stdin.nextInt();System.out.print("Enter another integer number: ");int n2 = stdin.nextInt();
if (n1 == n2) {System.out.println("Same");
}else {
System.out.println(“Different");}
What is the output if the user enters 88 and 3?
What is the output if the user enters 88 both times?
86
Testing objects for equality Consider
String s1 = “pastel”;String s2 = s1;
if (s1 == s2) {System.out.println("Same");
}else {
System.out.println("Different");}
87
Testing objects for equality
Memory looks like
The comparison is between the references!
Thus, s1 and s2 are the same (they refer to the same object)
"pastel"s1
s2
88
Testing objects for equality Consider
System.out.print("Enter a string: ");String s1 = stdin.nextLine();System.out.print("Enter another string: ");String s2 = stdin.nextLine();
if (s1 == s2) {System.out.println("Same");
}else {
System.out.println("Different");}
What is the output if the user enters "pastel" both times?
90
Testing objects for equality When it is executed
System.out.print("Enter a string: ");String s1 = stdin.nextLine();System.out.print("Enter another string: ");String s2 = stdin.nextLine();
Memory looks like
As a result no matter what is entered s1 and s2 are not the same They refer to different objects
"pastel"s1
"pastel"s2
91
Comparing strings for equality Consider:
String s = "hello";String t = "hello";System.out.println (s == t);
What gets printed? true
Consider:String u = new String("hello");String v = new String("hello");System.out.println (u == v);
What gets printed? false Huh?
These aren’t the exact same thing
93
Testing operators for equality Consider
System.out.print("Enter a string: ");String s1 = stdin.nextLine();System.out.print("Enter another string: ");String s2 = stdin.nextLine();
if (s1.equals(s2)) {System.out.println("Same");
}else {
System.out.println("Different");}
Tests whether s1 and s2 represent the same object
Most classes have a method equals(). It compares the objects themselves, not the references.
9494
All your base are belong to All your base are belong to usus
Flash animationFlash animation Reference: http://en.wikipedia.org/wiki/All_your_base_are_belong_to_usReference: http://en.wikipedia.org/wiki/All_your_base_are_belong_to_us
96
Short-circuit evaluation The value of a logical expression can be known before all the
operands have been considered If left operand of && is false, then the value must be false If left operand of || is true, then the value must be true
Java uses these properties to make logical operations efficient Evaluates left operand before it evaluates right operand If the operator value is determined from the left operand,
then the right operand is not evaluated The operation is short-circuited
97
Short-circuit evaluation Short-circuit evaluation is useful when some property must
be true for some other expression to be evaluated
Suppose you are interested in knowing whether scoreSum divided by nbrScores is greater than value The condition can be evaluated only if nbrScores is
nonzero
The following expression correctly represents the condition (nbrScores != 0) && ((scoreSum / nbrScores) > value)
98
Short-circuit evaluation Assume we have a returnsFalse()
method from a ‘foo’ object It returns false And it prints “returnsFalse() called”
And a returnsTrue() method
Consider:if ( foo.returnsFalse() && foo.returnsTrue() ) {}if ( foo.returnsTrue() && foo.returnsFalse() ) {}if ( foo.returnsFalse() || foo.returnsTrue() ) {}if ( foo.returnsTrue() || foo.returnsFalse() ) {}
returnsFalse() called
returnsTrue() calledreturnsFalse() called
returnsFalse() calledreturnsTrue() called
returnsTrue() called
Output
99
Quick surveyQuick survey
How comfortable do you feel with How comfortable do you feel with if’s and switches?if’s and switches?
100100
BiologyBiology
PhysicsPhysics
InterdisciplinaryInterdisciplinary ChemistryChemistry MathematicsMathematics LiteratureLiterature
PeacePeace
HygieneHygiene EconomicsEconomics
MedicineMedicine
The 2002 Ig Nobel PrizesThe 2002 Ig Nobel Prizes““Courtship behavior of ostriches towards humans underCourtship behavior of ostriches towards humans under
farming conditions in Britain”farming conditions in Britain”
““Demonstration of the exponential decay law using beer Demonstration of the exponential decay law using beer
froth”froth”
A comprehensive study of human belly button lintA comprehensive study of human belly button lint
Creating a four-legged periodic tableCreating a four-legged periodic table
““Estimation of the surface area of African elephants”Estimation of the surface area of African elephants”
““The effects of pre-existing inappropriate highlighting on The effects of pre-existing inappropriate highlighting on
reading comprehension”reading comprehension”
For creating Bow-lingual, a computerized dog-to-human For creating Bow-lingual, a computerized dog-to-human
translation devicetranslation device
For creating a washing machine for cats and dogsFor creating a washing machine for cats and dogs
Enron et. al. for applying imaginary numbers to the Enron et. al. for applying imaginary numbers to the
business worldbusiness world
““(male) asymmetry in man in ancient sculpture”(male) asymmetry in man in ancient sculpture”