CIS 3020 Part 3

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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

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
  • 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

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