Programming in Java (COP 2250)
Lecture 12 & 13
Chengyong Yang
Fall, 2005
12-2
Outline
Comparing Data
The while Statement
Iterators
Other Repetition Statements
Decisions and Graphics
More Components
12-3
Comparing Data
•When comparing data using boolean expressions,it's important to understand the nuances of certaindata types
•Let's examine some key situations:
–Comparing floating point values for equality
–Comparing characters
–Comparing strings (alphabetical order)
–Comparing object vs. comparing object references
12-4
Comparing Float Values
•You should rarely use the equality operator (==)when comparing two floating point values (floator double)
•Two floating point values are equal only if theirunderlying binary representations match exactly
•Computations often result in slight differences thatmay be irrelevant
•In many situations, you might consider twofloating point numbers to be "close enough" evenif they aren't exactly equal
12-5
Comparing Float Values
•To determine the equality of two floats, you maywant to use the following technique:
if (Math.abs(f1 - f2) < TOLERANCE)
System.out.println ("Essentially equal");
•If the difference between the two floating pointvalues is less than the tolerance, they areconsidered to be equal
•The tolerance could be set to any appropriatelevel, such as 0.000001
12-6
Comparing Characters
•As we've discussed, Java character data is basedon the Unicode character set
•Unicode establishes a particular numeric value foreach character, and therefore an ordering
•We can use relational operators on character databased on this ordering
•For example, the character '+' is less than thecharacter 'J' because it comes before it in theUnicode character set
•Appendix C provides an overview of Unicode
12-7
Comparing Characters
•In Unicode, the digit characters (0-9) arecontiguous and in order
•Likewise, the uppercase letters (A-Z) andlowercase letters (a-z) are contiguous and in order
Characters
Unicode Values
0 – 9
48 through 57
A – Z
65 through 90
a – z
97 through 122
12-8
Comparing Strings
•Remember that in Java a character string is anobject
•The equals method can be called with strings todetermine if two strings contain exactly the samecharacters in the same order
•The equals method returns a boolean result
if (name1.equals(name2))
System.out.println ("Same name");
12-9
Comparing Strings
•We cannot use the relational operators to comparestrings
•The String class contains a method calledcompareTo to determine if one string comesbefore another
•A call to name1.compareTo(name2)
–returns zero if name1 and name2 are equal (contain thesame characters)
–returns a negative value if name1 is less than name2
–returns a positive value if name1 is greater than name2
12-10
Comparing Strings
if (name1.compareTo(name2) < 0)
System.out.println (name1 + "comes first");
else
if (name1.compareTo(name2) == 0)
System.out.println ("Same name");
else
System.out.println (name2 + "comes first");
•Because comparing characters and strings isbased on a character set, it is called alexicographic ordering
12-11
Lexicographic Ordering
•Lexicographic ordering is not strictly alphabeticalwhen uppercase and lowercase characters aremixed
•For example, the string "Great" comes before thestring "fantastic" because all of the uppercaseletters come before all of the lowercase letters inUnicode
•Also, short strings come before longer stringswith the same prefix (lexicographically)
•Therefore "book" comes before "bookcase"
12-12
Comparing Objects
•The == operator can be applied to objects – itreturns true if the two references are aliases ofeach other
•The equals method is defined for all objects, butunless we redefine it when we write a class, it hasthe same semantics as the == operator
•It has been redefined in the String class tocompare the characters in the two strings
•When you write a class, you can redefine theequals method to return true under whateverconditions are appropriate
12-13
Outline
Comparing Data
The while Statement
Iterators
Other Repetition Statements
Decisions and Graphics
More Components
12-14
Repetition Statements
•Repetition statements allow us to execute astatement multiple times
•Often they are referred to as loops
•Like conditional statements, they are controlled byboolean expressions
•Java has three kinds of repetition statements:
–the while loop
–the do loop
–the for loop
•The programmer should choose the right kind ofloop for the situation
12-15
The while Statement
•A while statement has the following syntax:
while ( condition )
statement;
•If the condition is true, the statement isexecuted
•Then the condition is evaluated again, and if it isstill true, the statement is executed again
•The statement is executed repeatedly until thecondition becomes false
12-16
Logic of a while Loop
statement
true
false
condition
evaluated
12-17
The while Statement
•An example of a while statement:
int count = 1;
while (count <= 5)
{
System.out.println (count);
count++;
}
•If the condition of a while loop is false initially, thestatement is never executed
•Therefore, the body of a while loop will executezero or more times
12-18
The while Statement
•Let's look at some examples of loop processing
•A loop can be used to maintain a running sum
•A sentinel value is a special input value thatrepresents the end of input
•A loop can also be used for input validation,making a program more robust
12-19
Infinite Loops
•The body of a while loop eventually must makethe condition false
•If not, it is called an infinite loop, which willexecute until the user interrupts the program
•This is a common logical error
•You should always double check the logic of aprogram to ensure that your loops will terminatenormally
12-20
Infinite Loops
•An example of an infinite loop:
int count = 1;
while (count <= 25)
{
System.out.println (count);
count = count - 1;
}
•This loop will continue executing until interrupted(Control-C) or until an underflow error occurs
12-21
Nested Loops
•Similar to nested if statements, loops can benested as well
•That is, the body of a loop can contain anotherloop
•For each iteration of the outer loop, the inner loopiterates completely
12-22
Nested Loops
•How many times will the string "Here" be printed?
count1 = 1;
while (count1 <= 10)
{
count2 = 1;
while (count2 <= 20)
{
System.out.println ("Here");
count2++;
}
count1++;
}
10 * 20 = 200
12-23
Outline
Comparing Data
The while Statement
Iterators
Other Repetition Statements
12-24
Iterators
•An iterator is an object that allows you to processa collection of items one at a time
•It lets you step through each item in turn andprocess it as needed
•An iterator object has a hasNext method thatreturns true if there is at least one more item toprocess
•The next method returns the next item
•Iterator objects are defined using the Iteratorinterface, which is discussed further in Chapter 6
12-25
Iterators
•Several classes in the Java standard class libraryare iterators
•The Scanner class is an iterator
–the hasNext method returns true if there is more data tobe scanned
–the next method returns the next scanned token as astring
•The Scanner class also has variations on thehasNext method for specific data types (such ashasNextInt)
12-26
Iterators
•The fact that a Scanner is an iterator isparticularly helpful when reading input from a file
•Suppose we wanted to read and process a list ofURLs stored in a file
•One scanner can be set up to read each line of theinput until the end of the file is encountered
•Another scanner can be set up for each URL toprocess each part of the path
12-27
Outline
Comparing Data
The while Statement
Iterators
Other Repetition Statements
12-28
The do Statement
•A do statement has the following syntax:
do
{
statement;
}
while ( condition )
•The statement is executed once initially, and thenthe condition is evaluated
•The statement is executed repeatedly until thecondition becomes false
12-29
Logic of a do Loop
true
condition
evaluated
statement
false
12-30
The do Statement
•An example of a do loop:
•The body of a do loop executes at least once
int count = 0;
do
{
count++;
System.out.println (count);
} while (count < 5);
12-31
Comparing while and do
statement
true
false
condition
evaluated
The while Loop
true
condition
evaluated
statement
false
The do Loop
12-32
The for Statement
•A for statement has the following syntax:
for ( initialization ; condition ; increment )
statement;
The initialization
is executed once
before the loop begins
The statement is
executed until the
condition becomes false
The increment portion is executed atthe end of each iteration
12-33
Logic of a for loop
statement
true
condition
evaluated
false
increment
initialization
12-34
The for Statement
•A for loop is functionally equivalent to thefollowing while loop structure:
initialization;
while ( condition )
{
statement;
increment;
}
12-35
The for Statement
•An example of a for loop:
for (int count=1; count <= 5; count++)
System.out.println (count);
•The initialization section can be used to declare avariable
•Like a while loop, the condition of a for loop istested prior to executing the loop body
•Therefore, the body of a for loop will execute zeroor more times
12-36
The for Statement
•The increment section can perform any calculation
•A for loop is well suited for executing statementsa specific number of times that can be calculatedor determined in advance
for (int num=100; num > 0; num -= 5)
System.out.println (num);
12-37
The for Statement
•Each expression in the header of a for loop isoptional
•If the initialization is left out, no initialization isperformed
•If the condition is left out, it is always consideredto be true, and therefore creates an infinite loop
•If the increment is left out, no increment operationis performed