TeachScheme, ReachJava

Adelphi University

Friday morning

July 16, 2010

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).

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.

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.

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.

Definition by choices

interface Shape

{

}

…

class Circle implements Shape

{

…

}

interface Shape { }

class Circleimplements Shape

{

Posn center;

double radius;

String color;

…

}

class Rectangle implements Shape

{

Posn topLeft;

double width;

double height;

String color;

…

}

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");

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.

To fix this…

interface Shape

{

public double area ();

public boolean contains (Posn other);

public Shape scale (double factor);

}

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.

time check

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).

Lists in Java

Write classes ESL and NESL, and interface StringList. For each class, provide

• a constructor

• examples

• a toString method

Lists in Java

Write the following methods on StringLists:

• countStrings : nothing -> int

• contains : String -> boolean

• countMatches : String -> int

Recall add-up function

(define (add-up nums)

(cond [(empty? nums) 0]

[(cons? nums)

(+ (first nums)

(add-up (rest nums)))]))

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

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 +

Another approachOf course, add-up-accum is less convenient to use.Easy fix:

(define (add-up nums)(add-up-accum nums 0))

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]))]))

Generalize

Both functions look pretty similar. They differ in

the answer to the "empty?" case, andhow to combine (first nums) with so-far

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

Are we done yet?

• Fill out end-of-day survey

• Fill out end-of-workshop survey

• Eat

• Go home

• Sleep

• Teach