Difference between revisions of "CIS 3020 Part 3"
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NOTE: BAD to use multiple returns!
NOTE: BAD to use multiple returns!
Line 217: | Line 217: | ||
* Diamonds - represent decision to make | * Diamonds - represent decision to make | ||
* Lines with arrows - detail the "flow" within the diagram | * Lines with arrows - detail the "flow" within the diagram | ||
+ | ===Example=== | ||
+ | * Suppose we have a value, x, and would like to calculate the absolute value of x, setting '''absol''' to this value: | ||
+ | <pre> | ||
+ | if (x>=0) { | ||
+ | absol=x; | ||
+ | } else { | ||
+ | absol=-x; | ||
+ | } | ||
+ | </pre> | ||
+ | * This is not real useful unless we need this calculated only once, so as a method we might write: | ||
+ | <pre> | ||
+ | public int abs(int) { | ||
+ | int absol; | ||
+ | if (x>=0) { | ||
+ | absol=x; | ||
+ | } else { | ||
+ | absol = -x; | ||
+ | } | ||
+ | return (absol); | ||
+ | } | ||
+ | </pre> | ||
+ | ====Alternative Implementation 1==== | ||
+ | <pre> | ||
+ | public int abs(int x) { | ||
+ | if (x<0) { | ||
+ | x=-x; | ||
+ | } | ||
+ | return (x); | ||
+ | } | ||
+ | </pre> | ||
+ | ====Alternative Implementation 2==== | ||
+ | <pre> | ||
+ | public int abs(int x) { | ||
+ | if (x<0) { | ||
+ | return (-x); | ||
+ | } else { | ||
+ | return (x); | ||
+ | } | ||
+ | } | ||
+ | </pre> | ||
+ | <center>''NOTE: BAD to use multiple returns!''</center> | ||
+ | ====Alternative Implementation 3==== | ||
+ | <pre> | ||
+ | public int abs(int x) { | ||
+ | if (x<0) { | ||
+ | return (-x); | ||
+ | } | ||
+ | return (x); | ||
+ | } | ||
+ | </pre> | ||
+ | <center>''NOTE: BAD to use multiple returns!''</center> |
Revision as of 12:25, 23 March 2007
Syntactic Elements
class SpecialTurtle extends Turtle { public void drawTriangle( double sideLength ) { final double DEGREES_IN_CIRCLE = 360.0; double turnAngle = DEGREES_IN_CIRCLE / 3; this.tailDown(); this.move(sideLength); this.turnLeft(turnAngle); this.move(sideLength); this.turnLeft(turnAngle); this.move(sideLength); return; } // end drawTriangle } // end class SpecialTurtle
- Keywords (a.k.a. Reserved Words):
- Examples: class, public, static, void, new, extends, double, this, return, const
- Have a predefined meaning
- Can only be used in specific locations within your program
- class - used to define a class
- public - allows everone to access
- static - belonds to the class as a whole, not to an instance of the class
- void - a "return value" of nothing
- new - instantiate an instance (object) of a class
- extends - define a specialized version of a class
- double - defines a type - real number
- this - refers to the current object
- return - stop processing and return the specifed value, if one is given:
- return;
- return( expr );
- final - used to define a constant:
- final double DEGREES_IN_CIRCL 360.0;
Identifiers
- Programmer selected symbolic names for data, operations, and classes
- Contain letters, numbers, underscore
- Start with letter or underscore
- Class names: Demo, Island, Turtle
- Start with capital letter (stylistic convention)
- Each subsequent word also is capitalized
- Method names: main, drawTriangle
- start with lower case letter
- Each subsequent word is capitalized
- Data names
- Constants - DEGREES_IN_CIRCLE
- values that cannot change
- All upper-case characters with underscores between words
- Variables - sideLength, turnAngle, tina
- Start with lower case
- Each subsequent word is capitalized
- Constants - DEGREES_IN_CIRCLE
Operators
- Predefined by the language
- [] - array index operator
- = - gets (the value of), assignment operator
- . - dot operator - send the message on the right to the object on the left. Not the same as decimal point in number.
- + - addition operator
- - - subtraction operator
- * - multiplication operator
- / - division operator
- % - modulus operator - remainder from division
Literals
- A.K.A. Manifest constants
- String
- Examples: "a" "1" "Hi!"
- Primitives
- boolean
- Used to represent the values true and false
- Char
- Uses 16-bits to represent a unicode glyph
- Examples: 'a' 'A' '1' '.' Note single quotes
- Is a number having a symbolic interpretation
- boolean
- Numeric Values
- Int
- 32 bit signed integer
- Represents numbers ~ +/- 2,147,000,000
- Double
- 64-bit signed real number
- 15 digits of accuracy
- Represents numbers ~ +/- 2.2*10^308
- Int
Delimiters
- Used to separate elements of the program
- White space:
- space, tab, newline
- Are sometimes required
- Use liberally to aid in readability
- Punctuation
- () - argument lists, parameter lists, subexpressions
- {} - defines logical block (scope), array list initializers
- ; - end of a statement
- , - separate elements in a list
Comments
- Used to provide documentation on what the code does
- Ignored by the compiler/interpreter
- // everything until end of line is comment
- /* everything between is comment */
- Example:
int a /* not a good comment */ = 23; int b = 47; // better comment /* * multi-line comment */
Expressions
- Produce a value when executed
- Operators combine values
- Evaluated left to right obeying operator precedence
- Subexpression with operator of higher precedence is evaluated prior to expression with lower precedence
- Example: a+b*c
- Best to always clarify precedence with ()'s: a+(b*c)
Structure of Class Definition
class Example { // define class data members // (constants and variables) // define instance data members // (constants and variables) // define class methods // define constructors // define instance methods }
Class vs. Instance
- Class data member:
- Associated with the class
- A single value shared by entire class
- Defined as a class value through use of "static" keyword
- Instance data member:
- Associated with an instance (object) of a class
- Each object has its own independant value
- Class method:
- Used independant of any object of the class
- Instance method:
- Use is associated with an object of the class
Example
class USCitizen { // class variables private static int numUSPresidents; private static String USPresident; // Instance variables private String name; private int age; private USCitzen sigOther; // Class method public static String getPresidentName() { return USCitizen.presidentName; } // Constructors public USCitizen(String name, int age) { this.name = name; this.age = age; } public USCitizen(String name) { this.name = name; } // Instance methods // Accessor public String getName() { return (this.name); } public String toString() { return "US Citizen named " + this.name; } // Mutators public void setName(String name) { this.name = name; } public void haveBirthday() { this.age = this.age + 1; } }
Selection Statements
- A selection statement allows us to conditionally determine which following statement should be executed
- if statement's general format:
if (expr) { statement_1; } else { statement_2; }
Note that the else clause is optional.
Selection Statement's expr
- This is a boolean value expression composed of:
- Literals: true false
- Boolean values variables and constants
- Operations returning a boolean value
- Compound expressions involving logical operators
Truth Tables
A B | !A | A && B | A || B |
---|---|---|---|
T T | F | T | T |
T F | F | F | T |
F T | T | F | T |
F F | T | F | F |
! is a Unary operator - takes a single value
&& and || are Binary operators - take two values
Flow Charts
Flow charts are used to visually express what a program does
Major components of a Flow Chart:
- Boxes - represent calculations to perform
- Diamonds - represent decision to make
- Lines with arrows - detail the "flow" within the diagram
Example
- Suppose we have a value, x, and would like to calculate the absolute value of x, setting absol to this value:
if (x>=0) { absol=x; } else { absol=-x; }
- This is not real useful unless we need this calculated only once, so as a method we might write:
public int abs(int) { int absol; if (x>=0) { absol=x; } else { absol = -x; } return (absol); }
Alternative Implementation 1
public int abs(int x) { if (x<0) { x=-x; } return (x); }
Alternative Implementation 2
public int abs(int x) { if (x<0) { return (-x); } else { return (x); } }
Alternative Implementation 3
public int abs(int x) { if (x<0) { return (-x); } return (x); }