Week 3 Lecture slides

1. Cosc 1P02 Week 3 Lecture slides Birthdays are good for you. Statistics show that the people who have the most live the longest. (Rev. Larry Lorenzoni)

2. Review • Memory Representation of: • Value variables • Reference variables • Assignment of reference variables • Tiling patterns

3. Dealing with Complexity • Limits on composition • Can only go so deep before the nesting becomes un-manageable. • Requires separate index variables for each loop • Repetition of code • Duplicating code in a program is a bad thing • Increases complexity unnecessarily • More work. • Abstraction • Divide and conquer • Break problems into chunks • Let the process (code) be independent from operation of calling the code. • E.g. Yertle.forward(40);runs magic code to move yertle.

4. Two Squares • Consider drawing two squares at different places on the screen • repeated code • increased possibility of error • Drawing a square could be considered an operation (like drawing a line) • drawing a line done by a method (forward) • write a method to draw a square (drawSquare) • Example

5. Methods • Method declaration • syntax • name • verb • body • Method call • syntax • Method execution • current code suspended • body of method executed • current code continues

6. Eight Squares RevisitedeightSquares2.java • Composition via methods • use method call instead of nesting • Example • Reduced complexity • Can think of drawing a square as a basic operation • don’t have to worry about how it is done • code for EightSquares simpler • don’t have to worry about different index variables

7. Scope • Declarations occur at different places • in class • in method • in for statement • Scope • Where are names defined in declarations useable? • Rule • in class – from { in class header to } at end of class • includes methods names • in method – from { in method header to } at end of method • in for – body of for (statements between { }) • Example • Composition via nesting vs via methods • non-interference

8. What is an Object • An Object has 2 parts • Memory • What it remembers • State information • Data • Behaviour • What an object can do • Actions which can be performed • A Class defines what an object is: • Instance Variables represent the memory • Methods define the behaviour

9. Object Creation • New causes an object to be created from the template (class) • E.g. new Turtle(); • An Object is initialized when it is created. • Initial memory and behaviour are set • New causes the constructor to run to create the object • Constructor • Is a method (initial behaviour) • Has the same name as the class • New causes the constructor to execute.

10. Constructors, Methods & Objects • Methods are executed by an object • who is executing drawSquare? • no object specified • Turtle (yertle) doesn't know how • main method • is a method declaration • every Java program must have ate least one class (called the main class) that has a main method • Execution begins in body of main method • body includes object creation (new) • creates a new EightSquares object • on creation object executes its constructor • Local method call • when no object specified method is executed by same object as is executing the method call (e.g. the EightSquares object) • drawSquare() is shorthand for this.drawSquare()

11. Eight Squares One More Time • Constructor is meant to initialize an object • Separate initialization from operation • Puts object into a known state. • write operations as methods • e.g. Turtle • Example • constructor simply creates display, turtle and connects them • method draw performs the operation to draw the eight squares • draw is executed by the EightSquares object created in main • E.g. new EightSquares3().draw(); • New EightSquares3() creates an object returning a reference • .draw then calls the behavior of the new object.

12. Hexagon Revisited • Complex scene with many figures • hard to keep track of where drawing begins & ends • Centre figure on a point with a size of the figure (rather than a side) • method to draw figure starts from center & leaves pen in center • i.e. leaves turtle in same state as it started • Turtle state becomes independent from drawing. • Geometry of a hexagon • Drawing a hexagon • move out from center • position down first side • draw sides • return to center & pen orientation • Example

13. Expressions • Computations specified by expressions • like algebraic expressions in Mathematics • written on one line • Composed of • operators • constants • variables • functions • Evaluation order • by priority, left to right • parentheses to force order

14. Two Hexagons • Consider drawing two hexagons of different size • Size of hexagon fixed in method • different methods for different sizes? • essentially identical code • differs only by value of radius • Allow the method to vary depending on value of radius • Example • radius is parameter

15. Parameters • Allows a method to be generalized over one or more values • radius for drawHexagon • Parameter (formal parameter) • declared in method header • behaves like a local variable within method body • scope is method body • is initialized to value of argument at call • Argument (actual parameter) • an expression that produces a value of the declared type • constant • variable • expression • Method execution • value of argument computed • calling code suspended • value of argument copied to parameter • body of method executed • calling code continues

16. Polygons • Code for drawing all regular closed figures (polygons) similar • move out from center • rotate to facing down first side • for each side • draw side • rotate to next side • move back to center • Geometry of a polygon • Code would depend on two values: radius and number of sides • method with two parameters • Example • Note: number of times in for loop now a variable • index i counts from 1 to value of nSides

17. The end