Transcript Lecture PowerPoint Chapter 2-2

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Identifiers - Naming
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Identifier, the name of a Java programming component.
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Identifier naming rules . . .
 Must start with letter, underscore (_) or dollar sign ($)
 Can be of any length
 Cannot contain “+” or “-”
 Cannot be a keyword used by Java (see Appendix A of text)
 Should have meaning (“radius” preferred to “r”)
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Identifier naming guidelines must be followed . . .
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Package Names - A collection of classes
 First word lower case
 Additional words lower case, separated by “.”
 Example geometry.areas
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Identifiers - Naming cont.
Identifier naming guidelines cont.
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Class Names - A collection of data and methods
 First word upper case
 Additional words upper case
 Example ShellSort
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Method Names - A collection of statements
 Are verbs that represent actions
 First word lower case
 Additional words upper case
 Example linearSearch
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Identifiers - Naming cont.
Identifier naming guidelines cont.
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Variables - Used to store data
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Are nouns that represent items or things
First word lower case
Additional words upper case,
Example circleArea
Constants - Used to store data that does not change
 All characters upper case
 Additional words separated by underscore _
 Example PI, FLOW_FACTOR
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Programs with Simple Algorithms
Our first programs will implement simple algorithms.
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Example: Find the area of a circle.
 Given radius = 2 and PI = 3.14156
 Find circleArea = PI * radius * radius
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We will need to learn how Java handles . . .
 Inputting data, variables, and constants
 Calculations with expressions
 Outputting to PC Monitor - System.out object
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Data and
Input
Variables
Radius r
How to Input Something?
Calculate Area
of Circle
Circle Area
A = PI * r2
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Handling Data
Java handles data as primitive data types.
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Java provides for three primitive data types . . .
1. Numeric data - Numbers
2. Character data - Text
3. Boolean data - Logical states, true or false
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Data typing provides increased . . .
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Efficiency of memory use
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Ease of programming
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Speed of program execution
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Keyword
Primitive Data Types
Description
Size/Format
(integers)
byte
Byte-length integer 8-bit [-128 to 127]
short
Short integer
16-bit [-32768 to 32767]
int
Integer
32-bit [approx.  2*109]
long
Long integer
64-bit [approx.  9*1018]
float
double
char
boolean
(real numbers)
Single-precision
32-bit [ approx. 3.4*1038]
floating point
Double-precision
64-bit [ approx. 1.7*10308]
floating point
(other types)
A single character
A Boolean value
(true or false)
16-bit Unicode character
true or false
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Data Types Literals
A literal is a primitive data type appearing directly in a
program.
123.4, ‘b’, “Hello World”, false are literals.
The type of a literal is as follows:
 The type of an integer literal that ends with L or l is
long; the type of any other integer literal is int; e.g. 10.
 The type of a floating-point literal that ends with F or f
is float; the type of any other floating-point literal is
double. (Floating-point literals contain a decimal point;
e.g., 2.0)
 The type of a character literal is char; e.g., ‘p’.
 The type of a Boolean literal is Boolean; e.g., true.
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Data Types Literals Examples
Examples:
Literal
Data Type
178
int
8864L
37.266
37.266f
37.266F
26.77e3
long
double
float
float
double
'c'
char
true
false
boolean
boolean
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Unicode Characters
Character data holds a single character value like ‘H’ or ‘5’.
Character data is stored as unicode, a 16 bit encoding scheme.
• Includes ASCII character set plus many other alphabets
and symbols - See appendix B of text.
• “\u0041” represents the letter “A” and is stored in
memory as two hexadecimal bytes, “00” & “41”.
• Examples:
 \u0041 is equivalent to the character ‘A’.
 \u0042 is equivalent to the character ‘B’.
 \u0041 + 1 is equivalent to the character ‘B’.
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Escapes Sequences
Escape sequence is available for convenience.
Description
Character Sequence
Unicode
Backspace
\b
\u0008
Tab
\t
\u0009
New line
\n
\u000A
Carriage return
\r
\u000D
Backslash
\\
\u005C
Single quote
\’
\u0027
Double quote
\”
\u0022
Example:
• ‘\’’ is the literal for a single quote. You cannot use ‘’’ !
• ‘\u0022’ will also work.
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Boolean Data and Literals
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Boolean data holds only one of two values, true or false.
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Boolean representations are useful to compare two values.
 Is iValue less than jValue?
 The answer is either true or false.
 And, is representable as Boolean.
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Boolean is also used to represent logical states.
 Is the intrusion detector switch open?
 It is either, true - open or false - closed.
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Handling Variables
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Variables are used to store data.
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Java provides for three basic variable types . . .
1. Numeric variable - Holds a numeric value.
2. Character variable - Holds a single character value.
3. Boolean variable - Holds a logical value, true or false.
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Creating a Java variable involves two steps . . .
1. Declaring the name and data type of the variable
2. Assigning the variable a value
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Note: Variables must be declared before used.
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Creating a Variable - Declaring
Step 1 - To declare a variable . . .
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Use the declaration statement . . .
datatype varName1, varName2, . . ., varNamen;
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The declaration statement consists of two parts . . .
 The datatype (byte, int, float, char, etc.).
 Name(s) for variable(s) separated by commas.
 Meaningful noun
 First word lower case
 Additional words upper case,
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Note that all Java statements end with a semicolon ;.
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Declaring Variables - Examples
Examples of declaring numeric variables . . .
• int count;
where data type is integer and name is count.
• float bulletSpeed, time, distance;
where data type is floating point and multiple variables
names are bulletSpeed, time, and distance.
• char startingLetter;
where data type is character and name is startingLetter
• boolean doorAjar, trunkOpen;
where data type is Boolean and names are doorAjar and
trunkOpen.
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Creating a Variable - Assigning
Step 2 - To assign a value to a variable,
• Use the assignment statement.
• variable = expression;
• The assignment statement consists of three parts . . .
• The variable whose value is to be assigned.
• The assignment symbol, =.
• An expression to be evaluated or a literal value to be
assigned to the variable.
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Creating a Variable - Examples
Examples of assigning values to variables . . .
• time = 4.0;
assigns the value 4.0 to numeric variable time.
• distance = time * 60.0 + 15.0;
evaluates the expression to 255.0 and assigns the value to
numeric variable distance.
• firstLetter = ‘F’;
assigns the character F to the character variable firstLetter.
• gateOpen = true;
assigns the logical condition true to the Boolean variable
gateOpen.
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Note About the “=“ Sign
Note: The = sign means assign a value, not equals.
• Variable (on the left) is replaced by evaluated result of the
expression (on the right).
• Consider the code below.
count = 4;
count = count + 1;
After executing, count is 5.
• It may look like an equation, but it is not!
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Multiple Assignments
To assign a value to a several variables, use . . .
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variable1 = variable2 = . . . = variablen = expression;
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Example:
iCount = jCount = kCount = 4;
is equivalent to . . .
iCount = 4;
jCount = 4;
kCount = 4;
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Combining Declare and Assign
To declare and assign a value to a variable at the same time,
combine the declaration and assignments statements.
• datatype variable = expression;
• variable is . . .
• Declared as datatype
• Assigned the value of expression
• Examples:
• int count = 4; integer count is assigned value of 4.
• double rate = 0.15, maxRate = 2.0 * rate;
rate and maxRate are declared as double and assigned
values 0.15 and 0.30 (calculated) respectively.
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Declaring a Constant
To declare and assign a value to a constant, use . . .
final datatype CONSTANT_NAME = VALUE;
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final means exactly that. Once the constant is assigned a value,
it cannot change!
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Examples:
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final double PI = 3.14159; assigns double constant PI the
value 3.14159.
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final int I_COUNT_MAX = 1000; assigns constant
I_COUNT_MAX the value 1000.
Remember that constant names are all caps with multiple names
separated by underscores!
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How to Calculate Something?
Assignment
Calculate
Statement
Data and Variables
Radius r
Calculate Area
of Circle
Circle Area
A = PI * r2
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How to Output Something?
Expressions
Data & Variables
Radius r
Calculate Area
System.out
OutputObject
of Circle
Circle Area
A = PI * r2
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print and println Methods
To output something with Java:
1. Invoke the out object of the System class (program).
2. Use println method to print and move to the next line
3. System.out.println(“Hello World”); prints Hello World.
By default out outputs to the PC monitor.
The println method is the action (or service) the object out
performs for us.
Method print is similar to println but without next line.
The actual Java statement looks like this . . .
• System.out.println(parameter);
• Parameter is the piece of data we want to output.
• Parameter can be a variable or literal.
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print and println Examples
Examples:
1. System.out.println(“Hello World”);
Outputs the string literal Hello World to the PC monitor.
2. Assume iCount is 10,
System.out.println(“iCount = ” + iCount);
Outputs the string literal iCount = 10 to the PC monitor.
Note: The plus + concatenates (ties together) the character
literal and the numeric variable value.
3. System.out.print(“Hello ”);
System.out.println(“World”);
Outputs identically to example 1.
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Putting It All Together
Expressions
Data & Variables
Radius r
Calculate Area
System.out Object
of Circle
Circle Area
A = PI * r2
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Example Procedural Program
public class CircleArea {
public static void main(String[] args) {
// Declare variables
double radius, circleArea;
final double PI = 3.14159;
// Input data
radius = 2.0;
// Process data
circleArea = PI * radius * radius;
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Example Procedural Program
// Output result
System.out.println("Let’s begin.");
System.out.print("A circle with radius = " + radius);
System.out.println(" has an area = " + circleArea + ".");
}
}
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