Java Class Composition Examples with Shapes

1. TeachScheme, ReachJava Adelphi University Friday morning July 16, 2010

2. Class composition Define a class LogEntry to represent a runner's daily log. It contains the Date of the run, the distance in miles, the time in minutes, and a free-form comment. Include • a constructor • several examples • a toString method • an avgSpeed method • an addComment method (which takes in a String and returns a LogEntry just like the old one but with the String added onto whatever comments were already there).

3. Class composition Define a class Circle to represent a circle on the screen. It contains a center (of type Posn), a radius (double), and a color (String). Include • a constructor • several examples • a toString method • an area method • a contains method that takes in another Posn and returns a boolean indicating whether that Posn is inside the circle • a scale method that takes in a double scaling factor and returns a new Circle like this one but with the radius multiplied by the scaling factor.

4. Class composition Define a class Rectangle to represent a rectangle on the screen. It contains a top-left corner (of type Posn), a width and height (both double), and a color (String). Include • a constructor • several examples • a toString method • an area method • a contains method that takes in another Posn and returns a boolean indicating whether that Posn is inside the rectangle • a scale method that takes in a double scaling factor and returns a new Circle like this one but with the width and height multiplied by the scaling factor.

5. Definition by choices Define a data type Shape which is either a Circle or a Rectangle. Since Circle and Rectangle both have constructors, Shape doesn't need one.

6. Definition by choices interface Shape { } … class Circle implements Shape { … }

7. interface Shape { } class Circleimplements Shape { Posn center; double radius; String color; … } class Rectangle implements Shape { Posn topLeft; double width; double height; String color; … }

8. What can you do with this? • A variable of type Shape can hold either a Circle or a Rectangle: Shape shape1 = new Circle(new Posn(3,4),5,"blue"); Shape shape2 = new Rectangle(new Posn (50,20), 30, 40, "orange");

9. What can't you do with this? shape1.area() doesn't compile! Why not? In Java, every variable has two types: the static type from its declaration, and the dynamic type from what it actually contains. shape1 was declared as a Shape, so that's its static type. Static type is used to decide what's a legal call and what isn't.

10. To fix this… interface Shape { public double area (); public boolean contains (Posn other); public Shape scale (double factor); }

11. What can you do with this? Now you can call the area, contains, and scale methods on a Shape variable shape1.area() // should return c. 78.54 shape2.area() // should return 1200 shape1.scale(2.0) // should return// new Circle(new Posn(3,4), 10, "blue") etc.

12. time check

13. Lists in Java A StringList is either an EmptyStringList or a NonEmptyStringList (ESL or NESL for short). An ESL has no parts. A NESL has two parts: first (a String) and rest (a StringList).

14. Lists in Java Write classes ESL and NESL, and interface StringList. For each class, provide • a constructor • examples • a toString method

15. Lists in Java Write the following methods on StringLists: • countStrings : nothing -> int • contains : String -> boolean • countMatches : String -> int

16. Recall add-up function (define (add-up nums) (cond [(empty? nums) 0] [(cons? nums) (+ (first nums) (add-up (rest nums)))]))

17. Trace add-up function (add-up (list 3 5 2 1)) (+ 3 (add-up (list 5 2 1))) (+ 3 (+ 5 (add-up (list 2 1)))) (+ 3 (+ 5 (+ 2 (add-up (list 1))))) (+ 3 (+ 5 (+ 2 (+ 1 (add-up empty))))) ; lots of pending +'s (+ 3 (+ 5 (+ 2 (+ 1 0)))) (+ 3 (+ 5 (+ 2 1)))) (+ 3 (+ 5 3)) (+ 3 8) 11

18. Another approach (define (add-up-accum nums so-far) (cond [(empty? nums) so-far] [(cons? nums) (add-up-accum (rest nums) (+ (first nums) so-far))])) (add-up-accum (list 3 5 2 1) 0) (add-up-accum (list 5 2 1) (+ 3 0)) (add-up-accum (list 5 2 1) 3) (add-up-accum (list 2 1) (+ 5 3)) (add-up-accum (list 2 1) 8) (add-up-accum (list 1) (+ 2 8)) (add-up-accum (list 1) 10) (add-up-accum empty (+ 1 10)) (add-up-accum empty 11) 11 ; never more than 1 pending +

19. Another approach Of course, add-up-accum is less convenient to use. Easy fix: (define (add-up nums) (add-up-accum nums 0))

20. Another example ; multiply-positives : list-of-numbers -> number (define (multiply-positives nums) (mp-accum nums 1)) (define (mp-accum nums so-far) (cond [(empty? nums) so-far] [(cons? nums) (mp-accum (rest nums) (cond [(> (first nums) 0) (* (first nums) so-far)] [else so-far]))]))

21. Generalize Both functions look pretty similar. They differ in the answer to the "empty?" case, and how to combine (first nums) with so-far

22. In Java… See projects June26 v1 through June26 v5 v1: addUp written by structural recursion v2: addUp written by accumulative recursion v3: addUp becomes static, in a separate class v4: addUp written using for-each loop v5: addUp written using while-loop

23. Are we done yet? • Fill out end-of-day survey • Fill out end-of-workshop survey • Eat • Go home • Sleep • Teach