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Java Software Solutions
Lewis and Loftus
Java Language Review
Joe Komar
1998
Java Software Solutions
Lewis and Loftus
Primitive Data Types
• A data type is defined by a set of values and the
operators you can perform on them
• Each value stored in memory is associated with a
particular data type
• The Java language has several predefined types, called
primitive data types
• The following reserved words represent eight different
primitive types:
– byte, short, int, long, float, double, boolean, char
Chapter 3
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Integers
• There are four separate integer primitive data types
• They differ by the amount of memory used to store them
Type
Storage
Min Value
Max Value
byte
short
int
long
8 bits
16 bits
32 bits
64 bits
-128
-32,768
-2,147,483,648
< -9 x 1018
127
32,767
2,147,483,647
> 9 x 1018
Chapter 3
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Floating Point
• There are two floating point types:
Type
Storage
Approximate
Min Value
float
double
32 bits
64 bits
-3.4 x 1038
-1.7 x 10308
Approximate
Max Value
3.4 x 1038
1.7 x 10308
• The float type stores 7 significant digits
• The double type stores 15 significant digits
Chapter 3
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Characters
• A char value stores a single character from the
Unicode character set
• A character set is an ordered list of characters
• The Unicode character set uses sixteen bits per character,
allowing for 65,536 unique characters
• It is an international character set, containing symbols
and characters from many world languages
Chapter 3
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Characters
• The ASCII character set is still the basis for many other
programming languages
• ASCII is a subset of Unicode, including:
uppercase letters
lowercase letters
punctuation
digits
special symbols
control characters
Chapter 3
A, B, C, …
a, b, c, …
period, semi-colon, …
0, 1, 2, …
&, |, \, …
carriage return, tab, ...
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Boolean
• A boolean value represents a true or false condition
• They can also be used to represent any two states, such
as a light bulb being on or off
• The reserved words true and false are the only
valid values for a boolean type
Chapter 3
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Wrappers
• For each primitive data type there is a corresponding
wrapper class. For example:
Primitive Type
Wrapper Class
int
double
char
boolean
Integer
Double
Character
Boolean
• Wrapper classes are useful in situations where you need
an object instead of a primitive type
• They also contain some useful methods
Chapter 3
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Variables
• A variable is an identifier that represents a location in
memory that holds a particular type of data
• Variables must be declared before they can be used
• The syntax of a variable declaration is:
data-type variable-name;
• For example:
int total;
Chapter 3
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Variables
• Multiple variables can be declared on the same line:
int total, count, sum;
• Variables can be initialized (given an initial value) in the
declaration:
int total = 0, count = 20;
float unit_price = 57.25;
• See Piano_Keys.java
Chapter 3
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Assignment Statements
• An assignment statement takes the following form:
variable-name = expression;
• The expression is evaluated and the result is stored in the
variable, overwriting the value currently stored in the
variable
• See United_States.java
• The expression can be a single value or a more
complicated calculation
Chapter 3
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Constants
• A constant is similar to a variable except that they keep
the same value throughout their existence
• They are specified using the reserved word final in
the declaration
• For example:
final double PI = 3.14159;
final int STUDENTS = 25;
Chapter 3
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Constants
• When appropriate, constants are better than variables
because:
– they prevent inadvertent errors because their value cannot
change
• They are better than literal values because:
– they make code more readable by giving meaning to a value
– they facilitate change because the value is only specified in one
place
Chapter 3
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Input and Output
• Java I/O is based on input streams and output streams
• There are three predefined standard streams:
• The print and println methods write to standard
output
Chapter 3
Stream
Purpose
Default Device
System.in
System.out
System.err
reading input
writing output
writing errors
keyboard
monitor
monitor
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Input and Output
• The Java API allows you to create many kinds of streams
to perform various kinds of I/O
• To read character strings, we will convert the
System.in stream to another kind of stream using:
BufferedReader stdin = new BufferedReader
(new InputStreamReader (System.in),1);
• This declaration creates a new stream called stdin
Chapter 3
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Escape Sequences
• See Echo.java
• An escape sequence is a special sequence of characters
preceded by a backslash (\)
• They indicate some special purpose, such as:
Chapter 3
Escape Sequence
Meaning
\t
\n
\"
\'
\\
tab
new line
double quote
single quote
backslash
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Buffers
• As you type, the characters are stored in an input buffer
• When you press enter, the program begins processing the
data
• Similarly, output information is temporarily stored in an
output buffer
• The output buffer can be explicitly flushed (sent to the
screen) using the flush method
• See Python.java
Chapter 3
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Numeric Input
• Converting a string that holds an integer into the integer
value can be done with a method in the Integer
wrapper class:
value = Integer.parseInt (my_string);
• A value can be read and converted in one line:
num = Integer.parseInt (stdin.readLine());
• See Addition.java and Addition2.java
Chapter 3
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Expressions
• An expression is a combination of operators and
operands
• The arithmetic operators include addition (+), subtraction
(-), multiplication (*), and division (/)
• Operands can be literal values, variables, or other sources
of data
• The programmer determines what is done with the result
of an expression (stored, printed, etc.)
Chapter 3
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Division
• If the operands of the / operator are both integers, the
result is an integer (the fractional part is truncated)
• If one or more operands to the / operator are floating
point values, the result is a floating point value
• The remainder operator (%) returns the integer remainder
after dividing the first operand by the second
• The operands to the % operator must be integers
• See Division.java
• The remainder result takes the sign of the numerator
Chapter 3
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Division
Chapter 3
Expression
Result
17 / 5
17.0 / 5
17 / 5.0
3
3.4
3.4
9 / 12
9.0 / 12.0
0
0.75
6 % 2
14 % 5
-14 % 5
0
4
-4
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Operator Precedence
• The order in which operands are evaluated in an
expression is determined by a well-defined precedence
hierarchy
• Operators at the same level of precedence are evaluated
according to their associativity (right to left or left to
right)
• Parentheses can be used to force precedence
• Appendix D contains a complete operator precedence
chart for all Java operators
Chapter 3
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Operator Precedence
• Multiplication, division, and remainder have a higher
precedence than addition and subtraction
• Both groups associate left to right
Expression:
5 + 12 / 5 - 10 % 3
3
Order of evaluation:
Result:
Chapter 3
1
4
2
6
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The if Statement
• The Java if statement has the following syntax:
if (condition)
statement;
• If the boolean condition is true, the statement is
executed; if it is false, the statement is skipped
• This provides basic decision making capabilities
Chapter 3
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The if Statement
condition
false
true
statement
Chapter 3
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Boolean Expressions
• The condition of an if statement must evaluate to a
true or false result
• Java has several equality and relational operators:
Chapter 3
Operator
Meaning
==
!=
<
<=
>
>=
equal to
not equal to
less than
less than or equal to
greater than
greater than or equal to
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Block Statements
• Several statements can be grouped together into a block
statement
• Blocks are delimited by braces
• A block statement can be used wherever a statement is
called for in the Java syntax
• See Temperature2.java
Chapter 3
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The if-else Statement
• An else clause can be added to an if statement to
make it an if-else statement:
if (condition)
statement1;
else
statement2;
• If the condition is true, statement1 is executed; if the
condition is false, statement2 is executed
• See Temperature3.java and
Right_Triangle.java
Chapter 3
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The if-else Statement
condition
false
true
statement1
Chapter 3
statement2
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Nested if Statements
• The body of an if statement or else clause can be
another if statement
• These are called nested if statements
• See Football_Choice.java
• Note: an else clause is matched to the last
unmatched if (no matter what the indentation implies)
Chapter 3
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The while Statement
• A while statement has the following syntax:
while (condition)
statement;
• If the condition is true, the statement is executed; then
the condition is evaluated again
• The statement is executed over and over until the
condition becomes false
Chapter 3
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The while Statement
condition
false
true
statement
Chapter 3
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The while Statement
• If the condition of a while statement is false initially,
the statement is never executed
• Therefore, we say that a while statement executes
zero or more times
• See Counter.java, Factors.java, and
Powers_of_Two.java
Chapter 3
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Factors.java
import java.io.*;
class Factors {
public static void main (String[] args) throws IOException {
BufferedReader stdin = new BufferedReader
(new InputStreamReader (System.in),1);
int count = 1, number;
System.out.println ("Enter a positive number: ");
number = Integer.parseInt (stdin.readLine());
System.out.println ("The factors of " + number + " are:");
while (count <= (number/2)) {
if (number%count == 0)
System.out.println (count);
count = count +1;
}
}// method main
}// class factors
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Infinite Loops
• The body of a while loop must eventually make the
condition false
• If not, it is an infinite loop, which will execute until the
user interrupts the program
• This is a common type of logical error -- always double
check that your loops will terminate normally
• See Forever.java
Chapter 3
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Average2.java
import java.io.*;
class Average2 {
public static void main (String[] args) throws IOException {
int grade = 9999, count=0, max =0;
float sum = 0, average;
BufferedReader stdin = new BufferedReader
(new InputStreamReader (System.in),1);
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Average2.java (cont’d)
while (grade >=0) {
System.out.print ("Enter a grade (-1 to quit): ");
System.out.flush();
grade = Integer.parseInt (stdin.readLine());
if (grade >= 0) {
count = count +1;
sum = sum + grade;
if (grade > max)
max = grade;
}
}
Java Software Solutions
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Average2.java (cont’d)
if (count == 0)
System.out.println ("No valid grades were entered!");
else {
average = sum / count;
System.out.println();
System.out.println ("Total number of students: " + count);
System.out.println ("Average grade: " + average);
System.out.println ("Highest Grade: " + max);
}
}// method main
}// Class Average2
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Average2.java Sample Output
Symantec Java! JustInTime Compiler Version 210.063 for JDK 1.1.3
Copyright (C) 1996-97 Symantec Corporation
Enter a grade (-1 to quit): 88
Enter a grade (-1 to quit): 66
Enter a grade (-1 to quit): -1
Total number of students: 2
Average grade: 77.0
Highest Grade: 88
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Conversions
• Each data value and variable is associated with a
particular data type
• It is sometimes necessary to convert a value of one data
type to another
• Not all conversions are possible. For example, boolean
values cannot be converted to any other type and vice
versa
• Even if a conversion is possible, we need to be careful
that information is not lost in the process
Chapter 5
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Widening Conversions
• Widening conversions are generally safe because they go
from a smaller data space to a larger one
• The widening conversions are:
Chapter 5
From
To
byte
short
char
int
long
float
short, int, long, float, or double
int, long, float, or double
int, long, float, or double
long, float, or double
float or double
double
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Narrowing Conversions
• Narrowing conversions are more dangerous because they
usually go from a larger data space to a smaller one
• The narrowing conversions are:
Chapter 5
From
To
byte
short
char
int
long
float
double
char
byte or char
byte or short
byte, short, or char
byte, short, char, or int
byte, short, char, int or long
byte, short, char, int, long, or float
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Performing Conversions
• In Java, conversion between one data type and another
can occur three ways
• Assignment conversion - when a value of one type is
assigned to a variable of another type
• Arithmetic promotion - occurs automatically when
operators modify the types of their operands
• Casting - an operator that forces a value to another type
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Assignment Conversion
• For example, if money is a float variable and
dollars is an int variable (storing 82), then
money = dollars;
converts the value 82 to 82.0 when it is stored
• The value in dollars is not actually changed
• Only widening conversions are permitted through
assignment
• Assignment conversion can also take place when passing
parameters (which is a form of assignment)
Chapter 5
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Arithmetic Promotion
• Certain operators require consistent types for their
operands
• For example, if sum is a float variable and count is
an int variable, then the statement
result = sum / count;
internally converts the value in count to a float then
performs the division, producing a floating point result
• The value in count is not changed
Chapter 5
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Casting
• A cast is an operator that is specified by a type name in
parentheses
• It is placed in front of the value to be converted
• The following example truncates the fractional part of
the floating point value in money and stores the integer
portion in dollars
dollars = (int) money;
• The value in money is not changed
• If a conversion is possible, it can be done through a cast
Chapter 5
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More Operators
• We've seen several operators of various types:
arithmetic, equality, relational
• There are many more in Java to make use of:
–
–
–
–
Chapter 5
increment and decrement operators
logical operators
assignment operators
the conditional operator
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The Increment and Decrement Operators
• The increment operator (++) adds one to its integer or
floating point operand
• The decrement operator (--) subtracts one
• The statement
count++;
is essentially equivalent to
count = count + 1;
Chapter 5
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The Increment and Decrement Operators
• The increment and decrement operators can be applied in
prefix (before the variable) or postfix (after the variable)
form
• When used alone in a statement, the prefix and postfix
forms are basically equivalent. That is,
count++;
is equivalent to
++count;
Chapter 5
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The Increment and Decrement Operators
• When used in a larger expression, the prefix and postfix
forms have a different effect
• In both cases the variable is incremented (decremented)
• But the value used in the larger expression depends on
the form
Expression
Operation
Value of Expression
count++
++count
count---count
add 1
add 1
subtract 1
subtract 1
old value
new value
old value
new value
Chapter 5
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The Increment and Decrement Operators
• If count currently contains 45, then
total = count++;
assigns 45 to total and 46 to count
• If count currently contains 45, then
total = ++count;
assigns the value 46 to both total and count
Chapter 5
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The Increment and Decrement Operators
• If sum contains 25, then the statement
System.out.println (sum++ + "
"
" + sum + "
" + ++sum +
" + sum--);
prints the following result:
25
27
27
27
and sum contains 26 after the line is complete
Chapter 5
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Logical Operators
• There are three logical operators in Java:
Operator
Operation
!
&&
||
Logical NOT
Logical AND
Logical OR
• They all take boolean operands and produce boolean
results
• Logical NOT is unary (one operand), but logical AND
and OR are binary (two operands)
Chapter 5
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Logical NOT
• The logical NOT is also called logical negation or logical
complement
• If a is true, !a is false; if a is false, then !a is true
• Logical expressions can be shown using truth tables
Chapter 5
a
!a
false
true
true
false
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Logical AND
• The expression a && b is true if both a and b are true,
and false otherwise
• Truth tables show all possible combinations of all terms
Chapter 5
a
b
a && b
false
false
true
true
false
true
false
true
false
false
false
true
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Logical OR
• The expression a || b is true if a or b or both are true,
and false otherwise
Chapter 5
a
b
a || b
false
false
true
true
false
true
false
true
false
true
true
true
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Logical Operators
• Conditions in selection statements and loops can use
logical operators to form more complex expressions
if (total < MAX && !found)
System.out.println ("Processing...");
• Logical operators have precedence relationships between
themselves and other operators
Chapter 5
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Logical Operators
• Full expressions can be evaluated using truth tables
total < MAX
found
!found
total < MAX
&& !found
false
false
true
true
false
true
false
true
true
false
true
false
false
false
true
false
Chapter 5
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Assignment Operators
• Often we perform an operation on a variable, then store
the result back into that variable
• Java provides assignment operators that simplify that
process
• For example, the statement
num += count;
is equivalent to
num = num + count;
Chapter 5
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Assignment Operators
• There are many such assignment operators, always
written as op= , such as:
Operator
+=
-=
*=
/=
%=
Chapter 5
Example
Equivalent To
x
x
x
x
x
x
x
x
x
x
+=
-=
*=
/=
%=
y
y
y
y
y
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
=
=
=
=
=
x
x
x
x
x
+
*
/
%
y
y
y
y
y
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Assignment Operators
• The right hand side of an assignment operator can be a
complete expression
• The entire right-hand expression is evaluated first, then
combined with the additional operation
• Therefore
result /= (total-MIN) % num;
is equivalent to
result = result / ((total-MIN) % num);
Chapter 5
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The Conditional Operator
• Java has a conditional operator that evaluates a boolean
condition that determines which of two expressions is
evaluated
• The result of the chosen expression is the result of the
entire conditional operator
• Its syntax is:
condition ? expression1 : expression2
• If the condition is true, expression1 is evaluated; if it is
false, expression2 is evaluated
Chapter 5
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The Conditional Operator
• It is similar to an if-else statement, except that it is an
expression that returns a value
• For example:
larger = (num1 > num2) ? num1 : num2;
• If num1 is greater that num2, then num1 is assigned to
larger; otherwise, num2 is assigned to larger
• The conditional operator is ternary, meaning it requires
three operands
Chapter 5
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The Conditional Operator
• Another example:
System.out.println ("Your change is " + count +
(count == 1) ? "Dime" : "Dimes");
• If count equals 1, "Dime" is printed, otherwise
"Dimes" is printed
Chapter 5
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Another Selection Statement
• The if and the if-else statements are selection
statements, allowing us to select which statement to
perform next based on some boolean condition
• Another selection construct, called the switch statement,
provides another way to choose the next action
• The switch statement evaluates an expression, then
attempts to match the result to one of a series of values
• Execution transfers to statement list associated with the
first value that matches
Chapter 5
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The switch Statement
• The syntax of the switch statement is:
switch (expression) {
case value1:
statement-list1
case value2:
statement-list2
case …
}
Chapter 5
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The switch Statement
• The expression must evaluate to an integral value, such
as an integer or character
• The break statement is usually used to terminate the
statement list of each case, which causes control to jump
to the end of the switch statement and continue
• A default case can be added to the end of the list of
cases, and will execute if no other case matches
• See Vowels.java
Chapter 5
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Vowels.java
class Vowels {
public static void main (String[] args) {
int acount = 0, ecount = 0, icount = 0, ocount=0;
int ucount = 0, other =0, index =0;
String quote = "We are the Borg. Resistance is futile.";
Java Software Solutions
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Vowels.java (cont’d)
while (index < quote.length()) {
switch (quote.charAt (index)) {
case 'a' :
acount++;
break;
Java Software Solutions
Lewis and Loftus
Vowels.java (cont’d)
case 'e' :
ecount++;
break;
case 'i' :
icount++;
break;
case 'o' :
ocount++;
break;
case 'u' :
ucount++;
break;
default:
other++;
}
index++;
}
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Vowels.java (cont’d)
System.out.println ("Quote: \"" + quote + "\"");
System.out.println ("length = " + quote.length());
System.out.println ("a: " + acount);
System.out.println ("e: " + ecount);
System.out.println ("i: " + icount);
System.out.println ("o: " + ocount);
System.out.println ("u: " + ucount);
System.out.println ("Other: " + other);
} // method main
}// Class Vowels
Java Software Solutions
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Vowels.java Output
Quote: "We are the Borg. Resistance is futile."
length = 39
a: 2
e: 6
i: 3
o: 1
u: 1
Other: 26
Java Software Solutions
Lewis and Loftus
More Repetition Constructs
• In addition to while loops, Java has two other
constructs used to perform repetition:
• the do statement
• the for statement
• Each loop type has its own unique characteristics
• You must choose which loop type to use in each situation
Chapter 5
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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The do Statement
• The do statement has the following syntax:
do
statement
while (condition);
• The statement is executed until the condition becomes
false
• It is similar to a while statement, except that its
termination condition is evaluated after the loop body
Chapter 5
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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The do Statement
statement
true
condition
false
Chapter 5
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The do Statement
• See Dice.java
• The key difference between a do loop and a while loop
is that the body of the do loop will execute at least once
• If the condition of a while loop is false initially, the
body of the loop is never executed
• Another way to put this is that a while loop will
execute zero or more times and a do loop will execute
one or more times
Chapter 5
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The for Statement
• The syntax of the for loop is
for (intialization; condition; increment)
statement;
which is equivalent to
initialization;
while (condition) {
statement;
increment;
}
Chapter 5
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The for Statement
• Like a while loop, the condition of a for statement is
tested prior to executing the loop body
• Therefore, a for loop will execute zero or more times
• It is well suited for executing a specific number of times,
known in advance
• Note that the initialization portion is only performed
once, but the increment portion is executed after each
iteration
Chapter 5
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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The for Statement
initialization
condition
false
true
statement
increment
Chapter 5
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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The for Statement
• Examples:
for (int count=1; count < 75; count++)
System.out.println (count);
for (int num=5; num <= total; num *= 2) {
sum += num;
System.out.println (sum);
}
• See Dice2.java
Chapter 5
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The for Statement
• Each expression in the header of a for loop is optional
– If the initialization is left out, no initialization is performed
– If the condition is left out, it is always considered to be true, and
therefore makes an infinite loop
– If the increment is left out, no increment opertion is performed
• Both semi-colons are always required
Chapter 5
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The break and continue statements
• The break statement, which we used with switch
statements, can also be used inside a loop
• When the break statement is executed, control jumps to
the statement after the loop (the condition is not
evaluated again)
• A similar construct, the continue statement, can also
be executed in a loop
• When the continue statement is executed, control
jumps to the end of the loop and the condition is
evaluated
Chapter 5
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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The break and continue Statements
• They can also be used to jump to a line in your program
with a particular label
• Jumping from one point in the program to another in an
unstructured manner is not good practice
• Therefore, as a rule of thumb, avoid the break
statement except when needed in switch statements,
and avoid the continue statement altogether
Chapter 5
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Arrays
• An array is an ordered list of values
• Each value has a numeric index
• An array of size N is indexed from zero to N-1
• The following array of integers has a size of 10 and is
indexed from 0 to 9
0
scores
Chapter 6
1
2
3
4
5
6
7
8
9
79 87 94 82 67 98 87 81 74 91
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Arrays
• A particular value in an array is referenced using the
array name followed by the index in brackets
• For example, the expression
scores[4]
refers to the value 67 (which is the 5th value in the array)
• That expression represents a place to store a single
integer and can be used wherever an integer variable can
• For example, it can be assigned a value, printed, used in
a calculation
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Arrays
• An array stores multiple values of the same type
• That type can be primitive types or objects
• Therefore, we can create an array of integers, or an array
of characters, or an array of String objects, etc.
• In Java, the array itself is an object
• Therefore the name of the array is a object reference
variable, and the array itself is instantiated separately
Chapter 6
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Declaring Arrays
• The scores array could be declared as follows:
int[] scores = new int[10];
• Note that the type of the array does not specify its size,
but each object of that type has a specific size
• The type of the variable scores is int[] (an array of
integers)
• It is set to a newly instantiated array of 10 integers
• See Basic_Array.java
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Declaring Arrays
• Some examples of array declarations:
float[] prices = new float[500];
boolean[] flags;
flags = new boolean[20];
char[] codes = new char[1750];
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Bounds Checking
• Once an array is created, it has a fixed size
• An index used in an array reference must specify a valid
element
• That is, they must be in bounds (0 to N-1)
• The Java interpreter will throw an exception if an array
index is out of bounds
• This is called automatic bounds checking
• Its common to inadvertently introduce off-by-one errors
when using arrays
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Bounds Checking
• Each array object has a public constant called length
that stores the size of the array
• It is referenced through the array name (just like any
other object):
scores.length
• Note that length holds the number of elements, not the
largest index
• See Reverse_Numbers.java and
Adjust_Test_Scores.java
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Array Declarations Revisited
• The brackets of the array type can be associated with the
element type or with the name of the array
• Therefore
float[] prices;
and
float prices[];
are essentially equivalent
• The first format is usually more readable
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Initializer Lists
• An initializer list can be used to instantiate and initialize
an array in one step
• The values are delimited by braces and separated by
commas
• Examples:
int[] units = {147, 323, 89, 933, 540,
269, 97, 114, 298, 476};
char[] letter_grades = {'A', 'B', 'C',
'D', 'F'};
Chapter 6
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Initializer Lists
• Note that when an initializer list is used:
– the new operator is not used
– no size value is specified
• The size of the array is determined by the number of
items in the initializer list
• An initializer list can only be used in the declaration of
an array
• See Primes.java and Sales_Analysis.java
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Sales_Analysis.java
class Sales_Analysis {
public static void main (String[] args) {
Monthly_Sales sales = new Monthly_Sales ();
int best;
System.out.println ("End of year Report");
System.out.println ();
sales.print_table();
System.out.println ("Total sales: " + sales.total());
System.out.println ("Average sales: " + sales.average());
best = sales.highest_month();
System.out.println ("Best month was " + best + " with revenue "
+ sales.months_revenue(best));
}// method main
}// class Sales_Analysis
Java Software Solutions
Lewis and Loftus
Sales_Analysis.java (cont’d)
class Monthly_Sales {
private final int JANUARY = 1;
private final int DECEMBER = 12;
private int[] revenue = {0, 1692, 2504, 2469, 1826, 2369
, 3699, 2383, 2697, 2569, 1986,
2692, 2536 };
public int total (){
int sum = 0;
for (int month = JANUARY; month <= DECEMBER; month++)
sum += revenue[month];
return sum;
}// method total
Java Software Solutions
Lewis and Loftus
Sales_Analysis.java (cont’d)
public int average () {
return total() / DECEMBER;
}// method average
public int highest_month () {
int highest = JANUARY;
for (int month = JANUARY+1; month <= DECEMBER; month++)
if (revenue[highest] < revenue[month])
highest = month;
return highest;
}//method highest month
Java Software Solutions
Lewis and Loftus
Sales_Analysis.java (cont’d)
public int months_revenue (int month) {
return revenue [month];
}// method months_revenue
public void print_table () {
System.out.println ("Month\tSales");
for (int month = JANUARY; month <= DECEMBER; month++)
System.out.println (" " + month + "\t" + revenue[month]);
System.out.println ();
}//method print_table
}// class Monthly_Sales
Java Software Solutions
Sales_Analysis.java Output
Lewis and Loftus
End of year Report
Month Sales
1 1692
2 2504
3 2469
4 1826
5 2369
6 3699
7 2383
8 2697
9 2569
10 1986
11 2692
12 2536
Total sales: 29422
Average sales: 2451
Best month was 6 with revenue 3699
Java Software Solutions
Lewis and Loftus
Arrays of Objects
• The elements of an array can be object references
• The declaration
String[] words = new String[25];
reserves space to store 25 references to String objects
• It does NOT create the String objects themselves
• Each object stored in an array must be instantiated
separately
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Arrays of Objects
• See Children.java and Presidents.java
• Objects can have arrays as instance variables
• Therefore, fairly complex structures can be created
simply with arrays and objects
• The software designer must carefully determine an
organization of data and objects that makes sense for the
situation
• See Roll_Call.java
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Arrays as Parameters
• An entire array can be passed to a method as a parameter
• Like any other object, the reference to the array is
passed, making the formal and actual parameters aliases
of each other
• Changing an array element in the method changes the
original
• An array element can be passed to a method as well, and
follow the parameter passing rules of that element's type
• See Array_Test.java
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Multidimensional Arrays
• A one-dimensional array stores a simple list of values
• A two-dimensional array can be thought of as a table of
values, with rows and columns
• A two-dimensional array element is referenced using two
index values
• To be precise, a two-dimensional array in Java is an array
of arrays, therefore each row can have a different length
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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Multidimensional Arrays
• An initializer list can be used to create and set up a
multidimensional array
• Each element in the list is itself an initializer list
• Note that each array dimension has its own length
constant
• See Multi_Array_Test.java and
Soda_Survey.java
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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The Vector Class
• An object of class Vector is similar to an array in that
it stores multiple values
• However, a vector
– only stores objects
– does not have the indexing syntax that arrays have
• Service methods are used to interact with a vector
• The Vector class is part of the java.util package
• See Beatles.java and ZZ_Top.java
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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The Vector Class
• An important difference between an array and a vector is
that a vector can be thought of as a dynamic, able to
change its size as needed
• Each vector initially has a certain amount of memory
space reserved for storing elements
• If an element is added that doesn't fit in the existing
space, more room is automatically acquired
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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The Vector Class
• A vector is implemented using an array
• Whenever new space is required, a new, larger array is
created, and the values are copied from the original to the
new array
• To insert an element, existing elements are first copied,
one by one, to another position in the array
• Therefore, the implementation of Vector in the API is
not very efficient
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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The StringTokenizer Class Revisited
• We've seen a StringTokenizer object separate a
string into separate tokens
• By default, those tokens are delimited by white space
• But by using other StringTokenizer constructors,
we can define the delimiters used to define a token
• We can also set whether we want the delimiters
themselves returned as tokens
• See Voltaire.java and URL_Tokens.java
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
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URL_Tokens.java
import java.util.StringTokenizer;
public class URL_Tokens {
public static void main (String[] args) {
URL_Tokenizer url = new URL_Tokenizer
("http://www.stthomas.edu/junk/index.htm");
System.out.println ("Protocol: " + url.get_protocol());
System.out.println ("Address: " + url.get_address());
System.out.println ("Resource: " + url.get_resource());
}//method main
}// class URL_Tokens
Java Software Solutions
Lewis and Loftus
URL_Tokens.java (cont’d)
class URL_Tokenizer {
private String protocol;
private String address;
private String resource;
public URL_Tokenizer (String URL_Text) {
StringTokenizer URL = new StringTokenizer (URL_Text, ":");
protocol = URL.nextToken();
address = URL.nextToken(":/");
resource = URL.nextToken("");
}// constructor
Java Software Solutions
Lewis and Loftus
URL_Tokens.java (cont’d)
public String get_protocol() {
return protocol;
}
public String get_address() {
return address;
}
public String get_resource() {
return resource;
}
}// class URL_Tokenizer
Java Software Solutions
Lewis and Loftus
URL_Tokens.java Output
Protocol: http
Address: www.stthomas.edu
Resource: /junk/index.htm
Java Software Solutions
Lewis and Loftus
The StringBuffer Class
• Recall that the value of a String object is immutable;
once set it cannot be changed
• The StringBuffer class can be used to define a
character string whose value can change
• It's service methods include the ability to append and
insert characters
• See Money.java
• However, most functionality defined by the
StringBuffer class can be accomplished with
String objects and string concatenation
Chapter 6
Copyright 1997 by John Lewis and William Loftus. All rights reserved.
21