Download Primitive Data Types and Operations

Chapter 2
Elementary Programming
(Primitive Data Types and
Operations*)
1
Objectives
• Basic java program structure .
• Getting input from user : using Scanner class
• Java identifiers (names)
(§2.3).
– used to name variables, constants, methods, and classes .
• Java Variables
– use variables to store data .
• Assignment statements / assignment expressions .
• Java constants .
• Java primitive data types: boolean, byte, short, int, long, float,
double, and char .
– Java operators for primitive types .
• Java string (§2.18)
• Getting input from user
– using the JOptionPane input dialog (§2.19).
– using Scanner class (§2.3).
• Java documentation, programming style, and naming conventions .
• Types of errors
– To distinguish syntax errors, runtime errors, and logic errors .
• To debug logic errors .
2
Java Program Structure
• A program is made up of one or more files (compilation
units)
• A file consists of one or more classes (or interfaces), among
which there is at most one public one.
• Classes are grouped into packages.
• A class belongs to one package and contains one or more
methods and fields
• A method contains program statements
• A Java application always executes the main method of a
class via a command like this:
>java -cp . example.Person
package example;
public class Person {
public static void main (String[] args) {
System.out.println (“Hi Man!");
} // method main
int age;
String name;
} // class Person
Java Program Structure
class modifier
class name
// comments about the class
public class MyProgram
{
class Header
class body
Comments can be added almost anywher
}
4
Java Program Structure
// comments about the class
public class MyProgram
Method modifiers
{
// comments about the method
public static void main (String[] args)
{
method body
}
Return type
}
5
method header
Method name
Introducing Programming with an
Example
Listing 2.1 Computing the Area of a
Circle
This program computes the area of
the circle.
ComputeArea
6
animation
Trace a Program Execution
public class ComputeArea {
/** Main method */
public static void main(String[] args) {
double radius;
double area;
allocate memory
for radius
radius
no value
// Assign a radius
radius = 20;
// Compute area
area = radius * radius * 3.14159;
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
7
animation
Trace a Program Execution
public class ComputeArea {
/** Main method */
public static void main(String[] args) {
double radius;
double area;
// Assign a radius
radius = 20;
// Compute area
area = radius * radius * 3.14159;
memory
radius
no value
area
no value
allocate memory
for area
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
8
animation
Trace a Program Execution
public class ComputeArea {
/** Main method */
public static void main(String[] args) {
double radius;
double area;
assign 20 to radius
radius
area
20
no value
// Assign a radius
radius = 20;
// Compute area
area = radius * radius * 3.14159;
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
9
animation
Trace a Program Execution
public class ComputeArea {
/** Main method */
public static void main(String[] args) {
double radius;
double area;
memory
radius
area
20
1256.636
// Assign a radius
radius = 20;
// Compute area
area = radius * radius * 3.14159;
compute area and assign it
to variable area
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
10
animation
Trace a Program Execution
public class ComputeArea {
/** Main method */
public static void main(String[] args) {
double radius;
double area;
memory
radius
area
20
1256.636
// Assign a radius
radius = 20;
// Compute area
area = radius * radius * 3.14159;
print a message to the
console
// Display results
System.out.println("The area for the circle of radius " +
radius + " is " + area);
}
}
11
Getting Input Using Scanner
1. Create a Scanner object
Scanner scanner = new Scanner(System.in);
2. Use the methods next(), nextByte(), nextShort(),
nextInt(), nextLong(), nextFloat(), nextDouble(),
nextBoolean(), or nextLine() to obtain to a string, byte,
short, int, long, float, double, boolean value or a whole line.
For example,
Scanner scanner = new Scanner(System.in);
System.out.print("Enter a double value: ");
double d = scanner.nextDouble();
ComputeCircleArea
ComputeAverage
TestScanner
12
Kinds of tokens in Java
• Token:
– Identifier
• used for the name of package, type, method, (local)variable,
parameter,etc.
– Keyword (reserved words)
• if, then, else, while, do, switch, case,
• class, method, public, static, final,
abstract,…
• int, boolean, double, long, …
– Literal
• user for representing constant values.
• ‘c’, 123, 123L, true, 0.2e23, ”s String”, …
– Separator
• { } ( ) [ ] , ; .
– Operator:
• +, -, *, /, %, <=, >, …
Identifiers(識別字;名稱)
• An identifier is a sequence of characters that
consist of letters, digits, underscores (_), and dollar
signs ($).
– may contain non-English letters like ‘變’ ‘數’ ‘一’ etc.
• An identifier must start with a letter, an underscore
(_), or a dollar sign ($). It cannot start with a digit.
– An identifier cannot be a reserved word. (See Appendix A,
“Java Keywords,” for a list of reserved words).
– $ is used by java tools, programmers should not use it.
• An identifier cannot be true, false, or null.
• An identifier can be of any length.
14
Variables
// Compute the first area
radius = 1.0;
area = radius * radius * 3.14159;
System.out.println("The area is “ +
area + " for radius "+radius);
// Compute the second area
radius = 2.0;
area = radius * radius * 3.14159;
System.out.println("The area is “ +
area + " for radius "+radius);
15
Declaring Variables
int x;
// Declare x to be an
// integer variable;
double radius; // Declare radius to
// be a double variable;
char a;
// Declare a to be a
// character variable;
/* int, double and char are types
instead of variables */
16
Assignment Statements
x = 1;
// Assign 1 to x;
radius = 1.0;
// Assign 1.0 to radius;
a = 'A';
// Assign 'A' to a;
17
Declaring and Initializing
in One Step
• int x = 1;
• double d = 1.4;
18
Constants
Format:
final datatype CONSTANTNAME = VALUE;
Examples:
final double PI = 3.14159;
final int SIZE = 3;
final String LAST_NAME = “Wang”;
final String FULL_NAME ="John "+NAME; //ok!
LAST_NAME = "Chen"; //error!
final boolean IT_IS_COLD = false;
cf: Constants v.s. literals
Constants are variables bound to a constant
value determined by programmers while
literals are tokens mapped to a constant
value designated by the language.
19
Java Data Types
•
primitive types
– numeric types
•
integer types: byte, char, short, int, long
•
floating-point types: float, double
– boolean
• reference types
– class
– interface
– array
Notes: instances of primitive types are not objects; only
instances of reference types are objects.
Integer types
• name
• byte
• short
• int
• long
• char
•
representation
range
8-bit 2's complement -128~127
16-bit 2's complement -32768~32767
(-215 ~ 215 -1)
32-bit 2's complement -2147483648 to
2147483647
64-bit 2's complement -263~263-1(19 digits)
16-bit Unicode
'\u0000' to '\uffff‘
(as well as unsigned short) 0 to 65536
floating-point types
• name
• float
representation
range
32-bit, IEEE 754 1.40239846e-45 to
3.40282347e+38
• double 64-bit, IEEE 754 4.94065645841246544e-324 to
1.79769313486231570e+308
Representation:
• non-zero value: v = S x M x 2^E
– For float: S is +/- 1, M is a positive integer less than 2^24, and E is an
integer in the inclusive range -149 to 104.
– For double: S is +/- 1, M is a positive integer less than 2^53, and E is an
integer in the inclusive range -1045 to 1000.
• special values defined in IEEE 754:
– +0, -0, +infinity, -Infinity, NaN.
– note: 0/0.0 => NaN,
–
1/0.0 => Infinity instead of overflow or divide by zero.
IEEE 754 float-point single precision
layout
•
•
•
•
•
0x7f800000 => positive infinity.
0xff800000 => negative infinity.
0x7f800001 ~ 0x7fffffff or 0xff800001~0xffffffff => NaN.
Java use 0x7fc00000 as the canonical value of NaN.
Distinct values of NaN are only accessible by use of the
Float.floatToRawIntBits(float)
• In all other cases, let s, e, and m be three values that can be
computed from the argument:
–
–
–
–
int s = ((bits >> 31) == 0) ? 1 : -1;
int e = ((bits >> 23) & 0xff); // s bit is ignored by mask of 0xff
int m = (e == 0) ? (bits & 0x7fffff) << 1 : (bits & 0x7fffff) | 0x800000;
the floating-point result is s ·m ·2 e-150.
IEEE 754 float-point single precision layout
(exponent)
s
e
b31 b30 b29 b28 b27 b26 b25 b24 b23 b22 b21 b20 b19 b18 b17 b16
(mantissa)
m
b15 b14 b13 b12 b11 b10
b9
b8
b7
b6
b5
b4
b3
b2
b1
b0
value is determined as follows:
0. s = (b31 == 0 ? 1: -1);
1. 255> e > 0
=> value = s x (1.m)2 x 2 e –127 = s x (1m) x 2 e - 150
2. E = 255 && m == 0
=> value = ( s == 0 ? Infinity : -Infinity)
3. e = 255 && m != 0
=> value = NaN; canonical NaN = 0181022
4. e = 0
=> value = s x (b22.b21---b00)2 x 2 e-127 = s x (m 0)2 x 2 e - 127-23
5.
double -> e has 11 bits and m has 52 bits
Numerical Data Types
Name
Range
Storage Size
byte
–27 (-128) to 27–1 (127)
8-bit signed
short
–215 (-32768) to 215–1 (32767)
16-bit signed
int
–231 (-2147483648) to 231–1 (2147483647) 32-bit signed
long
–263 to 263–1
(i.e., -9223372036854775808
to 9223372036854775807)
64-bit signed
float
Negative range:
-3.4028235E+38 to -1.4E-45
Positive range:
1.4E-45 to 3.4028235E+38
32-bit IEEE 754
double
Negative range:
-1.7976931348623157E+308 to
-4.9E-324
Positive range:
4.9E-324 to 1.7976931348623157E+308
64-bit IEEE 754
25
TIP
An excellent tool to demonstrate how
numbers are stored in a computer was
developed by Richard Rasala. You can access
it at
http://www.ccs.neu.edu/jpt/jpt_2_3/bitdisplay/applet.htm
Quiz:
1. What is the minimum distance between
which two adjacent float numbers?
2. What is the maximum distance between
which two adjacent float numbers ?
26
Numeric Operators
Name
Meaning
Example
Result
+
Addition
34 + 1
35
-
Subtraction
34.0 – 0.1
33.9
*
Multiplication
300 * 30
9000
/
Division
1.0 / 2.0
0.5
%
Remainder
20 % 3
2
• All operators are overloaded for all numeric types
27
Division(/) and remainder(%)
• can be used for both integer and float-point
numbers
5 / 2 yields an integer 2.
5.0 / 2 yields a double value 2.5
5 % 2 yields 1 (the remainder of the division)
5.0 % 2 = 1.0
2.2f/2 = 1.1f //2.2 = (10.0011 0011 0011 …)2
2.2f % 2 = 0.20000005 //!= 0.2
28
Remainder Operator
•Remainder is very useful in programming.
• can be used to determine if a number x is even or odd:
– x % 2 == 0 // => even
– x % 2 == 1 // => odd
• Suppose today is Saturday, then what day is it after 10
days?
Saturday is the 6th day in a week
A week has 7 days
(6 + 10) % 7 is 2
The 2nd day in a week is Tuesday
After 10 days
29
The rules
• integer % in java is not the same as in normal arithmetic
• What are x % y and x/y if x or y is negative?
• The rule:
– x / y = sign(x/y) | x/ y| = x/y with fraction part removed;
– x%y = x – (x/y) *y = sign(x) (|x|%|y|) always holds
• invariant: x = (x/y) * y + (x%y)
• Examples:
• -10 / -3 = 3;
-10 /3 = ________; 10 / -3 = _________
• -10 % -3 = -1;
-10 % 3 = _______; 10 % -3 = _________;
Example: Displaying Time
Write a program that obtains hours and
minutes from seconds.
DisplayTime
31
NOTES
• Calculations involving floating-point numbers are
approximated because these numbers are not stored
with complete accuracy. For example,
System.out.println(1.0 - 0.1 - 0.1 - 0.1 - 0.1 - 0.1);
displays 0.5000000000000001, not 0.5, and
System.out.println(1.0 - 0.9);
displays 0.09999999999999998, not 0.1. Integers are
stored precisely. Therefore, calculations with integers yield
a precise integer result.
• There is no errors/exceptions reported for overflow of
integer operations. Ex:
– Integer.MAX_VALUE * 2 return -2 // MAX_VALUE = 231 -1.
32
Number Literals
A literal is a constant value that appears directly
in the program. For example, 34, 1,000,000, and
5.0 are literals in the following statements:
int i = 34;
long x = 1000000;
double d = 5.0;
33
Integer Literals
•An integer literal can be assigned to an integer variable as
long as it can fit into the variable.
–Ex int i1 = 1000; short s = 1000 ; // ok!
–
byte b = 1000 ;
// compilation error!!
• An integer literal is assumed to be of the int type, whose
value is between -231 to 231–1.
–123, 456, // all are int type
•To denote an integer literal of the long type, append it
with the letter L or l.
– 123L, 456l // use L instead of l to avoid confusion!!
34
Floating-Point Literals
•Floating-point literals are written with a decimal
point.
–234 // int
234.0 // double
•By default, a floating-point literal is treated as a
double type value.
– 5.0 // a double
• You can make a number a float by appending the
letter f or F, and make a number a double by
appending the letter d or D.
– Ex: 100.2f , 100.2F // float
–
100.2d , 100.2D, 100.2 // double.
35
Scientific Notation
•Floating-point literals can also be specified in scientific
notation.
•Ex:
– 1.23456e+2, same as 1.23456e2, is equivalent to 123.456,
–1.23456e-2 is equivalent to 0.0123456.
•E(or e) represents an exponent and it can be either in
lowercase or uppercase.
•Notes:
– e or E means 10 instead of the natural exponent(2.71828)
–1.23 e 10 // error! Spaces before/after e not allowed!!
36
Arithmetic Expressions
3  4 x 10( y  5)(a  b  c)
4 9 x

 9( 
)
5
x
x
y
is translated to
(3+4*x)/5 – 10*(y-5)*(a+b+c)/x + 9*(4/x + (9+x)/y)
37
Example: Converting Temperatures
Write a program that converts a Fahrenheit degree
to Celsius using the formula:
celsius  ( 95 )( fahrenheit  32)
FahrenheitToCelsius
38
Shortcut Assignment Operators
Operator Example
Equivalent
+=
i += 8
i = i + 8
-=
f -= 8.0
f = f - 8.0
*=
i *= 8
i = i * 8
/=
i /= 8
i = i / 8
%=
i %= 8
i = i % 8
39
Increment and
Decrement Operators
Operator
++var
Name
preincrement
var++
postincrement
--var
predecrement
var--
postdecrement
Description
The expression (++var) increments var by 1 and
evaluates to the new value in var after the
increment.
The expression (var++) evaluates to the original
value in var and increments var by 1.
The expression (--var) decrements var by 1 and
evaluates to the new value in var after the
decrement.
The expression (var--) evaluates to the original
value in var and decrements var by 1.
40
Increment and
Decrement Operators, cont.
int i = 10;
int newNum = 10 * i++;
Same effect as
int i = 10;
int newNum = 10 * (++i);
int newNum = 10 * i;
i = i + 1;
Same effect as
i = i + 1;
int newNum = 10 * i;
41
Increment and
Decrement Operators, cont.
•Using
increment and decrement operators makes
expressions short, but it also makes them complex and
difficult to read.
•Avoid
using these operators in expressions that modify
multiple variables, or the same variable for multiple times
such as this: int k = ++i + i.
42
Assignment Expressions and Assignment
Statements
•Prior to Java 2, if expr is an expression, then
expr;
is a statement.
•Since Java 2, only the following types of expressions (which
may have side effects) can be statements:
variable op= expression; // Where op is +, -, *, /, or %
++variable;
variable++;
--variable;
variable--;
// or method invocation ex: x – 2; => error
43
Numeric Type Conversion
Consider the following statements:
byte i = 100;
long k = i * 3 + 4;
double d = i * 3.1 + k / 2;
44
Conversion Rules
When performing a binary operation e1 op e2 , Java
automatically converts the operands based on the following
rules: (numeric type promotion)
1. If one of the operands is double, the other is converted into
double.
2. Otherwise, if one of the operands is float, the other is
converted into float.
3. Otherwise, if one of the operands is long, the other is
converted into long.
4. Otherwise(int, char, short, byte), both operands are converted
into int.
Rules: x op y => ((w) x) op ((w) y) where w is the wider of (type(x),
type(y), int) and the reuslt type is w..
ex: byte i = j = 12;
byte x = i + j // error ! since i + j has type int, cast is needed!
45
Type Casting (改型)
Implicit casting(widening done by javac
automatically)
double d = 3; (type widening)
Explicit casting (narrowing must be
specified by programmers)
int i = (int) 3.0; (type narrowing)
int i = (int) 3.9; (Fraction part is
truncated)
What is wrong?
int x = 5 / 2.0;
range increases
byte, short, int, long, float, double
char
46
Type conversions
• Java allows conversions between values of various numeric types.
– Except for boolean, values of all primitive types can be converted.
• Basic types of conversions:
– Widening conversion:
– int  long;
float double;
char  int; …
• always safe (without loss of data) except for int float; longfloat, double.
• automated performed by Java
– Narrowing conversion: long  int; double  float;…
•
must use the cast () operator; (except for int literal, which can be downcasted
to the required integral type automatically if it is inside the range of the left
variable ).
• not always safe.
• Ex: -- int i =13;
– byte b = i; // compile error!!
– short s = 134 ; // ok!! though 134 is int type , it is a literal.
– short s2 = b + b; // error, must use (short) (b+b); since b+b has type int.
Use cast for narrowing conversion
• Ex: int i = 13;
–
–
–
–
–
–
–
byte b = (byte) i; // force i to be converted to a byte
i = (int) 13.456 // force double literal 13.456 to int 13
i = (int) –12.6 // i == -12
i = Integer.MAX_VALUE // i = 2147483647
float j = i // need not cast, but data loss; j = 2.14748365E9
j == i ? true : false
// will return true! why ?
(long) j == i ? true :false // return false since (long)j = 2147483648
• Math.round(), Math.floor(), Math.ceil() perform other types
of conversion.
• short v.s. char:
–
–
–
–
short s = (short) 0xffff; // s = -1; 2’s complement
char c = ‘\uffff’;
// char behaves like unsigned short
int i1 = s;
// i1 = -1
int i2 = c;
// i2 = 65535
Java Primitive Type Conversion rules
LR
boolean
byte
short char int long
float
double
boolean
-
N
N
N
N
N
N
N
byte
N
-
Y
C
Y
Y
Y
Y
short
N
C
-
C
Y
Y
Y
Y
char
int
N
N
C
C
C
C
C
Y
-
Y
Y
Y
Y*
Y
Y
long
N
C
C
C
C
-
Y*
Y*
float
N
N
C
C
C
C
C
C
C
C
C
C
C
Y
-
double
N: Not allowed;
C: Cast needed;
Y: automatically casted w/o loss of data; Y*: automatically casted with loss of data
Example: Keeping Two Digits After
Decimal Points
Write a program that displays the sales tax with two digits
after the decimal point.
tax = purchaseAmount * 0.06
Note: in (int) (tax * 100) / 100.0,
it is (tax*100) that is casted to int
SalesTax
50
Character Data Type
char letter = 'A'; // (ASCII)
char numChar = '4'; // (ASCII)
Four hexadecimal digits.
char letter = '\u0041'; // (Unicode; 4 hexidecimal )
char numChar = '\u0034'; // (Unicode)
NOTE: The increment and decrement operators can also be used
on char variables.
char ch = 'a';
System.out.println(++ch);
51
Unicode Format
 Java characters use Unicode
 16-bit encoding scheme
 support the interchange, processing, and display of
written texts in the world’s diverse languages.
 Unicode takes two bytes, preceded by \u, expressed in
four hexadecimal numbers that run from '\u0000' to
'\uFFFF'.
 Unicode can represent 65535 + 1 characters.
Unicode \u03b1 \u03b2 \u03b3 for three Greek
letters
52
Example: Displaying Unicodes
Write a program that displays two Chinese
characters and three Greek letters.
歡迎(\u6b6e\u8fce)
DisplayUnicode
53
Escape Sequences for Special Characters
Description
Escape Sequence
Unicode
Backspace
\b
\u0008
Tab
\t
\u0009
Linefeed
\n
\u000A
Carriage return \r
\u000D
Backslash
\\
\u005C
Single Quote
\'
\u0027
Double Quote
\"
\u0022
54
Appendix B: ASCII Character Set
ASCII Character Set is a subset of the Unicode from \u0000 to \u007f
55
ASCII Character Set, cont.
ASCII Character Set is a subset of the Unicode from \u0000 to \u007f
56
Casting between char and Numeric
Types
int i = 'a'; // Same as int i = (int)'a';
char c = 97; // Same as char c = (char)97;
int j = 97;
char c2 = j;
// error! why ?
57
The String Type
•The char type only represents one character.
• To represent a string of characters, use the data type called
String. Ex:
String message = "Welcome to Java";
•String is actually a predefined class in the Java library just like the
System class and JOptionPane class.
• The String type is not a primitive type.
–It is known as a reference type. Any Java class can be used as a reference
type for a variable.
–Reference data types discussed in later Chapter: “Classes and Objects.”
–For the time being, you just need to know how to declare a String
variable, how to assign a string to the variable, and how to concatenate
strings.
58
String Concatenation
// Three strings are concatenated
String message = "Welcome " + "to " + "Java";
// String Chapter is concatenated with number 2
String s = "Chapter" + 2; // s becomes Chapter2
// String Supplement is concatenated with character B
String s = "Supplement" + 'B'; // s becomes SupplementB
59
Obtaining Input
This book provides two ways of obtaining input.
1. Using JOptionPane input dialogs (§2.19)
2. Using the Scanner class (discussed in §2.3)
60
Getting Input from Input Dialog Boxes
String string = JOptionPane.showInputDialog(
null, “Prompting Message”, “Dialog Title”,
JOptionPane.QUESTION_MESSAGE));
61
Two Ways to Invoke the Method
•There are many ways to use the showInputDialog method.
•usage 1 :
String string = JOptionPane.showInputDialog(null, x,
y, JOptionPane.QUESTION_MESSAGE));
where x is a string for the prompting message, and y is a string for
the title of the input dialog box.
•Usage 2 :
JOptionPane.showInputDialog(x);
where x is a string for the prompting message.
62
Converting Strings to Integers
•The input returned from the input dialog box is a string.
–If you enter a numeric value such as 123, it returns “123”.
•To obtain the input as a number, you have to convert a
string into a number.
•To convert a string into an int value, you can use the
static parseInt method in the Integer class as follows:
int intValue = Integer.parseInt(intString);
where intString is a numeric string such as “123”.
63
Converting Strings to Doubles
•To convert a string into a double value, you can use the
static parseDouble method in the Double class as follows:
double doubleValue =Double.parseDouble(doubleString);
where doubleString is a numeric string such as “123.45”.
64
Example:
Computing Loan Payments
This program lets the user enter the interest
rate, number of years, and loan amount and
computes monthly payment and total
payment.
loanAmount  monthlyInt erestRate
1
1
numberOfYears12
(1  monthlyInt erestRate )
ComputeLoan
ComputeLoanUsingInputDialog
65
• The derivation of loan payment (skipped)
• Let an = loan not paid yet at time n.
r = monthly interest rate
p = payment per month
t = total months of payment
• Then a0 = load(L),
a1 = a0 (1+r) – p, … ,at = at-1(1+r) – p = 0.
• Hence a k+1 = (1+r) ak – p =>ak+1-p/r = (1+r) (ak – p/r).
=> ak = b(1+r)k + p/r for some b. By initial condiction,
a0 = b + p/r = L => b = (L-p/r)
at = (L-p/r)(1+r)k + p/r = 0
=> (L- p/r) (1+r)t + p/r = 0 =>L(1+r)t = p ((1+r)t -1)/r
66
=> p = Lr / (1–1/(1+r)t)
Example: Monetary Units
input x.yy and compute
x.yy dollars =
? dollar(100) + ? quarters(25) + ? dimes(10) +
? nickles(5) + ? pennies(1cent).
ComputeChange
67
Trace ComputeChange
Suppose amount is 11.56
int remainingAmount = (int)(amount * 100);
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100;
remainingAmount = remainingAmount % 100;
remainingAmount
1156
remainingAmount
initialized
// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25;
remainingAmount = remainingAmount % 25;
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10;
remainingAmount = remainingAmount % 10;
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
68
animation
Trace ComputeChange
Suppose amount is 11.56
int remainingAmount = (int)(amount * 100);
remainingAmount
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100;
remainingAmount = remainingAmount % 100;
numberOfOneDollars
// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25;
remainingAmount = remainingAmount % 25;
1156
11
numberOfOneDollars
assigned
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10;
remainingAmount = remainingAmount % 10;
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
69
animation
Trace ComputeChange
Suppose amount is 11.56
int remainingAmount = (int)(amount * 100);
remainingAmount
56
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100;
remainingAmount = remainingAmount % 100;
numberOfOneDollars
11
// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25;
remainingAmount = remainingAmount % 25;
remainingAmount
updated
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10;
remainingAmount = remainingAmount % 10;
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
70
animation
Trace ComputeChange
Suppose amount is 11.56
int remainingAmount = (int)(amount * 100);
remainingAmount
56
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100;
remainingAmount = remainingAmount % 100;
numberOfOneDollars
11
// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25;
remainingAmount = remainingAmount % 25;
numberOfOneQuarters
2
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10;
remainingAmount = remainingAmount % 10;
numberOfOneQuarter
s assigned
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
71
animation
Trace ComputeChange
Suppose amount is 11.56
int remainingAmount = (int)(amount * 100);
remainingAmount
6
// Find the number of one dollars
int numberOfOneDollars = remainingAmount / 100;
remainingAmount = remainingAmount % 100;
numberOfOneDollars
11
// Find the number of quarters in the remaining amount
int numberOfQuarters = remainingAmount / 25;
remainingAmount = remainingAmount % 25;
numberOfQuarters
2
// Find the number of dimes in the remaining amount
int numberOfDimes = remainingAmount / 10;
remainingAmount = remainingAmount % 10;
remainingAmount
updated
// Find the number of nickels in the remaining amount
int numberOfNickels = remainingAmount / 5;
remainingAmount = remainingAmount % 5;
// Find the number of pennies in the remaining amount
int numberOfPennies = remainingAmount;
72
Example: Displaying Current Time
Write a program that displays current time in GMT in the
format hour:minute:second such as 1:45:19.
The currentTimeMillis method in the System class returns
the current time in milliseconds since the midnight, January
1, 1970 GMT. (1970 was the year when the Unix operating
system was formally introduced.) You can use this method
to obtain the current time, and then compute the current
second, minute, and hour as follows.
ShowCurrentTime
Elapsed
time
Time
Unix Epoch
01-01-1970
00:00:00 GMT
Current Time
System.CurrentTimeMills()
73
Programming Style and
Documentation
• Appropriate Comments
• Naming Conventions
• Proper Indentation and Spacing
Lines
• Block Styles
74
Appropriate Comments
•Include a summary at the beginning of the
program to explain
–what the program does,
–its key features,
–its supporting data structures, and
–any unique techniques it uses.
•Include your name, class section, instructor, date,
and a brief description at the beginning of the
program.
75
Naming Conventions
• Choose meaningful and descriptive names.
• Variables and method names:
– Use lowercase.
– If the name consists of several words,
concatenate all in one, use lowercase for the
first word, and capitalize the first letter of each
subsequent word in the name. (camelCase)
– For example, the variables radius and area,
and the method solveTwoProblems.
76
Naming Conventions, cont.
• Class names:
– Capitalize the first letter of each word in
the name. For example, the class name
ComputeArea.
• Constants:
– Capitalize all letters in constants, and use
underscores to connect words.
– For example, the constant PI and
MAX_VALUE
77
Proper Indentation and Spacing
• Indentation
– Indent two spaces.
• Spacing
– Use blank line to separate segments of the code.
78
Block Styles
Use end-of-line style for braces.
Next-line
style
public class Test
{
public static void main(String[] args)
{
System.out.println("Block Styles");
}
}
public class Test {
public static void main(String[] args) {
System.out.println("Block Styles");
}
}
End-of-line
style
79
Programming Errors
• Syntax Errors
– Detected by the compiler
• Runtime Errors
– Causes the program to abort
• Logic Errors
– Produces incorrect result
80
Syntax Errors
public class ShowSyntaxErrors {
public static void main(String[] args) {
i = 30;
System.out.println(i + 4);
}
}
81
Runtime Errors
public class ShowRuntimeErrors {
public static void main(String[] args) {
int i = 1 / 0;
}
}
82
Logic Errors
public class ShowLogicErrors {
// Determine if a number is between 1 and 100 inclusively
public static void main(String[] args) {
// Prompt the user to enter a number
String input = JOptionPane.showInputDialog(null,
"Please enter an integer:",
"ShowLogicErrors", JOptionPane.QUESTION_MESSAGE);
int number = Integer.parseInt(input);
// Display the result
System.out.println("The number is between 1 and 100, " +
"inclusively? " + ((1 < number) && (number < 100)));
System.exit(0);
}
}
83
Debugging
•Logic errors are called bugs.
•The process of finding and correcting errors is called
debugging.
•A common approach to debugging is to use a
combination of methods to narrow down to the part of
the program where the bug is located.
–You can hand-trace the program (i.e., catch errors by reading
the program), or
–you can insert print statements in order to show the values of
the variables or the execution flow of the program. This
approach might work for a short, simple program.
–But for a large, complex program, the most effective approach
for debugging is to use a debugger utility.
84
Debugger
•Debugger is a program that facilitates debugging.
• You can use a debugger to
–Execute a single statement at a time.
–Trace into or stepping over a method.
–Set breakpoints.
–Display variables.
–Display call stack.
–Modify variables.
85
Eclipse
Optional
Debugging in Eclipse
The debugger utility is integrated in Eclipse. You
can pinpoint bugs in your program with the help of
the Eclipse debugger without leaving the IDE. The
Eclipse debugger enables you to set breakpoints
and execute programs step by step. As your
program executes, you can watch the values stored
in variables, observe which methods are being
called, and know what events have occurred in the
program.
86
Eclipse
Optional
Setting Breakpoints
A breakpoint is a stop sign placed on a line of source code
that tells the debugger to pause when this line is
encountered. The debugger executes every line until it
encounters a breakpoint, so you can trace the part of the
program at the breakpoint. Using the breakpoint, you can
quickly move over the sections you know work correctly
and concentrate on the sections causing problems.
How to set/remove a breakpoint on a line.
–double click the ruler area of the line on which you want to put
a breakpoint.
– position the curser to the ruler area of the line, open context
menu(right click mouth) and choose toggling breakpoint.
87
Eclipse
Optional
Setting Breakpoints, cont.
ruler area
As you debug your program, you can set as
many breakpoints as you want, and can
remove breakpoints at any time during
debugging. The project retains the
breakpoints you have set when you exit the
project. The breakpoints are restored when
you reopen it.
Breakpoint
88
Eclipse
Optional
Starting the debugger
Choose the program (e.g., ShowCurrentTime.java)
in the package explorer view, or in the editor
area and right-click the mouse button to
display the context menu. Click Debug As
JavaApplication in the context menu to start
debugging.
89
Eclipse
Optional
Debugger perspective
90
JBuilder
Optional
Console View
Console view displays
output and errors. You can
also enter input from the
console view.
Console view
91
Eclipse
Optional
Debug View
Debug view displays the
methods in the call stack.
Viriable view show
variables of methods in
the call stack.
Call stacks
92
Eclipse
Optional
Expression View
You can add variables to the Expression view. Expression view
displays the contents of the expression in the view.
Data watches
93
Eclipse
Optional
Adding Expressions to Expression View
There are several ways to add expressions to the watch view. A
simple way is to select the expression and then right-click the mouse
to display the context menu. Choose Watch in the context menu to
add the expression to the expression view.
94
Eclipse
Optional
Controlling Program Execution
The program pauses at the first breakpoint line encountered. This
line, called the current execution point, is highlighted and has a green
arrow to the left. The execution point marks the next line of source
code to be executed by the debugger.
When the program pauses at the execution point, you can issue
debugging commands to control the execution of the program. You
also can inspect or modify the values of variables in the program.
When Eclipse is in the debugging mode, the Run menu contains the
debugging commands. Most of the commands also appear in the
toolbar. Here are the commands for controlling program execution:
95
Eclipse
Optional
Debugger Commands in the view Toolbar
Resume
program
suspend
terminate
Step
into
Step
overStep
return
96
Eclipse
Optional
Debugger Commands in the Run Menu
97
Eclipse
Optional
Debugger Commands
Step Over executes a single statement. If the statement contains a call to a method, the entire
method is executed without stepping through it.
Step Into executes a single statement or steps into a method.
Step return executes all the statements in the current method and returns to its caller.
Run to Line runs the program, starting from the current execution point, and pauses and places
the execution point on the line of code containing the cursor, or at a breakpoint.
Run to End of Method runs the program until it reaches the end of the current method or a
breakpoint.
Resume Program continues the current debugging session or restarts one that has finished or
been reset.
Terminate Program ends the current program and releases it from memory. Use Reset to
restart an application from the beginning, as when you make a change to the code and want to
run again from the beginning, or if variables or data structures become corrupted with
unwanted values. This command terminates debugging and returns to the normal editing
session.
98