teachscheme, reachjava adelphi university thursday morning june 25, 2009

TeachScheme, ReachJava

Adelphi University

Thursday morning

June 25, 2009

The Transition to Java

Goals:• Teach enough [good] Java and OOP that

students can use it in subsequent courses• Maintain lessons learned about

specification, testing, data types, etc.• Teach techniques & idioms common to

Java that haven’t been covered in Scheme (e.g. loops)

Development environments• ProfessorJ: Java subset in DrScheme

– doesn't have a lot of expected features (System.out.println, assert, generics, collection classes…)

• DrJava: DrScheme's cousin– more complete Java; familiar user interface with

Definitions and Interactions panes• BlueJ

– larger user community, more support– automatic class diagrams– also has "CodePad", like Interactions pane– I'll use BlueJ this week

Expressions in Java: arithmetic

• Infix notation for arithmetic3 + 5

• Order of operationsDoes 3+4*5 mean (3+4)*5, or 3+(4*5)?PEMDAS: 3+(4*5)

• AssociativityDoes 3-4-5 mean (3-4)-5, or 3-(4-5)? Left associativity: (3-4)-5

• Operations on integers produce integersEven division? Yes: 5/3 = 1

Expressions in Java: Strings

• "abc" as in Scheme• "abc"+"def" returns "abcdef"

(equivalent to string-append in Scheme)• "abc"-"def" meaningless• 3 + " years" returns "3 years"• 4+5+" years" returns "9 years"• "I'm "+4+5+" years old"

returns "I'm 45 years old"… huh? Left associativity!

Variables in Java

• int age = 45;"I'm " + age + " years old"

• String name = "Chris";"My friend's name is " + namename + " is " + age + " years old"

• General syntax rule:type varname = expression ;

More numeric expressions in Java

• Math. method-name (args)Math.sqrt(2) returns 1.414…Math.cos(Math.PI) returns -1.0Math.max(3, -4) returns 3… lots of built-in mathematical functions …

Analogous to Scheme function calls from a "library" named Math

More String expressions in Java

• expression . method-name (args)"abc".length() returns 3"abc".toUpperCase() returns "ABC""abc".concat("def") returns "abcdef""abcde".substring(1) returns "bcde""abcde".substring(1,3) returns "bc""abcde".indexOf("d") returns 3"hello everybody".replaceAll("y","fnord")

returns "hello everfnordbodfnord"

More String expressions in Java

• expression . method-name (args)• These too are analogous to Scheme function calls,

but the expression in front is one of the arguments to the function

• (string-append "hello" "goodbye")becomes"hello".concat("goodbye")

• (string-length "hello")becomes"hello".length()

Two kinds of methods

• "static" methods called with a class name in front of the dot

• "non-static" methods called with an expression in front of the dot

• We'll learn to write both eventually. "static" first.

Exercises

• Write a Java expression using the int variable age and producing something like"I am ___ years old; next year I'll be ___."

• Write a Java expression using the String variable name and producing something like"Hi, __! There are __ letters in your name."

Writing functions in Java

• "function" in Scheme = "method" in Java• All method definitions must be inside a class• To create a class, select "New Class", and fill in a

class name (say, June25). Class names cannot contain spaces or punctuation, and should usually start with a capital letter.

• BlueJ gives you a bunch of boilerplate code. You can ignore it, or delete everything between the first "{" and the last "}".

Writing methods in Javapublic static String greet (String name) {

return "Hello, " + name + "!";}

analogous to

(define (greet name)(string-append "Hello, " name "!"))

Note: you have to declare the types of parameters and methods in Java. "String" must be capitalized.

The "public" is boilerplate; I'll explain later.The "static" means this method will be called with the class name

(June25) in front of the dot.

Testing manually

TypeJune25.greet("Steve")

in the CodePad. It should return "Hello, Steve!".Try some other examples.

Testing automatically

1) right-click on the class in the diagram and select "Create Test Class"

2) double-click the new class (June25Test)3) This is where we'll put all the test cases for June25.4) find the "testSomething" method and rename it "testGreet"5) inside it, put the line

t.checkExpect (June25.greet("Steve"), "Hello, Steve!");6) Click the "Compile" button. If it doesn't complain…7) right-click the test class and select "testEverything".Try adding some more tests, including some that fail.

The design recipe in Java

Contract (in June25 class):// static greet : String -> String

Test cases (in June25Test class):public void testGreet (Tester t){

t.checkExpect (June25.greet("Steve"), "Hello, Steve!");t.checkExpect (June25.greet("Albert Einstein"), "Hello, Albert Einstein!");

}


Skeleton (in June25 class):public static String greet (String name){

}


Inventory:public static String greet (String name){

// name String

}


Body:public static String greet (String name){

// name Stringreturn "Hello, " + name + "!";

}


Testing:Hit the "Compile" buttonIf it complains, fix the errors and try again.If not, right-click on "June25Test" and select

"testEverything".Read the report.

Exercises

Write a static method greet2 that takes in a String parameter and returns something like

"Hi, __! There are __ letters in your name."

Follow the design recipe.

Writing methods on numbersWrite a cube method on integers.Contract:// static cube : integer -> integer

Test cases (in June25Test class):public void testCube (Tester t){

t.checkExpect (June25.cube(0), 0);t.checkExpect (June25.cube(4), 64);t.checkExpect (June25.cube(-5), -125);

}

Writing methods on numbers

Skeleton:public static int cube (int x){

}

Note: Java has no "number" type; you have to decide between "int", for integers, and "double", for any other number.


Inventory:public static int cube (int x){

// x int

}


Body:public static int cube (int x){

// x intreturn x * x * x;

}

Test as before.

Inexact numbersWrite a static method circPerim that takes in the radius of a

circle and returns its perimeter.Contract:// circPerim : double -> double

Test cases (in June25Test):public void testCircPerim (Tester t){

t.checkInexact (June25.circPerim(0.0), 0.0, 0.01);t.checkInexact (June25.circPerim(1.0), 6.28, 0.01);

}

Inexact numbersSkeleton, inventory, body:

public static double circPerim (double radius){

// radius doublereturn radius * 2 * Math.PI;

}

Test as before.

Exercises

• Write a static method avgThree which takes in three double numbers and returns their average.

• Write a static method convert3digits which takes in three integers representing the "hundreds", "tens", and "ones" digits of a number, and returns the number.

• Write a static method largestRoot which takes in three double numbers a, b, and c, and returns(-b+ sqrt (b2-4ac))/(2a).

BooleansWrite a static method canVote which takes in a person's age and

returns whether the person is at least 18 years old.

Contract (in June25 class):

// static canVote : int -> boolean

Test cases (in June25Test class):

public void testCanVote (Tester t)

{

t.checkExpect (June25.canVote(17), false);

t.checkExpect (June25.canVote(18), true);

t.checkExpect (June25.canVote(19), true);

}

BooleansSkeleton, inventory, body (in June25 class):

public static boolean canVote (int age)

{

// age int

// 18 fixed int

return age >= 18;

}

Boolean operators

Scheme

(and foo bar)

(or foo bar)

(not foo)

Java

foo && bar

foo || bar

! foo

Conditionals

Scheme

(cond [q1 a1]

[q2 a2]

[q3 a3])

Java

if (q1)return a1;

else if (q2)

return a2;

else if (q3)

return a3;

Definition by choices

Definition by choices within a single type (String, int, double, etc.) is done by conditionals.

Definition by choices between types is done differently (see below)

Exercises

• Write a static method teenager that takes in an age and returns true if it's between 13 and 19 inclusive

• Write a static method roughAge that takes in an age and returns either "child", "teenager", or "adult" as appropriate

Definition by parts

In Scheme…

;; A Book is (make-book String String Number)

(define-struct book (title author pub-year))

;; examples

(define alice (make-book "Alice in Wonderland" "Carroll" 1866))

(define glass (make-book "TTLG" "Carroll" 1872))

(define cat-hat (make-book "The Cat in the Hat" "Seuss" 1957))

The same thing in Java

// to represent a book in the library

class Book

{

String title;

String author;

int pubYear;

// other stuff goes in here

}

About the "other stuff"…

In Scheme, we got a constructor, getters, and discriminator "for free":

; make-book : string string number -> book

; book-title : book -> string

; book-author : book -> string

; book-pub-year: book -> number

; book? : anything -> book

About the "other stuff"…

In Java, we have to write a constructor. Getters are free (I'm lying). We'll come back to discriminators.

Constructor (inside Book class):

public Book (String title, String author, int pubYear)

{

this.title = title;

this.author = author;

this.pubYear= pubYear;

}

Note: title is the name of a parameter; this.title is the name of a struct field.

A constructor looks sorta like a method, but it's not static, its return type is the class it's in, and it has no name at all.

Making examples in Java

Book alice = new Book("Alice", "Carroll", 1866);

Book glass = new Book("TTLG", "Carroll", 1872);

Book catHat = new Book("The Cat in the Hat", "Seuss", 1957);

Using examples in CodePad

alice.title // should return "Alice"

glass.author // should return "Carroll"

catHat.pubYear // should return 1957

Same syntax that we used in constructor to refer to the fields.

Using examples in test class

class BookTest {

Book alice = new Book("Alice", "Carroll", 1866);

Book glass = new Book("TTLG", "Carroll", 1872);

Book catHat = new Book("The Cat in the Hat", "Seuss", 1957);

…

public void testFields (Tester t)

{

t.checkExpect (this.alice.title, "Alice");

t.checkExpect (this.glass.author, "Carroll");

t.checkExpect (this.catHat.cnum, 1957);

}

}

Exercises

Write a Java class Date with three integer fields day, month, and year, a constructor, and examples.

Write a Java class Posn with two double fields x and y, a constructor, and examples.

Time check

• Snack break?

Non-static methods

Write a non-static method age which operates on a Book, takes in the current year, and returns how many years old the book is.

Contract:

// age : int -> int

Test cases (in BookTest class):

public void testAge (Tester t)

{

t.checkExpect (this.alice.age(2009), 143);

t.checkExpect (this.glass.age(2009), 137);

t.checkExpect (this.catHat.age(2009), 52);

}

Non-static methods

• Note that a non-static method has one "special" parameter (the "receiver"), passed in front of the dot, and possibly some others in between the parentheses.

• A non-static method with 3 parameters (plus the "receiver") corresponds to a Scheme function or a static method with 4 parameters

• Many non-static methods take no parameters at all, getting all the information they need from the receiver.

• age actually takes in a Book (as the receiver) and an int (as a parameter).

Methods in data classes

Skeleton (in Book class):

public int age (int currentYear)

{

}


Inventory


{

// currentYear int

// this Book

// this.title String

// this.author String

// this.pubYear int

}

Note: The inventory for a non-static method always includes this; its type is always whatever class it's in.


Body


{

// currentYear int

// this Book



// this.pubYear int

return currentYear – this.pubYear;

}

A standard method: toString

Write a non-static method toString that operates on a Book and combines all the information about it into one readable String.

Contract (in Book class):

// toString : nothing -> String

Test cases (in BookTest class):

public void testToString (Tester t)

{

t.checkExpect (this.alice.toString(), "Alice, by Carroll, published 1866");

t.checkExpect (this.glass.toString(), "TTLG, by Carroll, published 1872");

t.checkExpect (this.catHat.toString(), "The Cat in the Hat, by Seuss, published 1957");

}

A standard method: toStringSkeleton, inventory, body (in Book class):

public String toString ()

{

// this Book



// this.pubYearint

return this.title + ", by " + this.author +", published " + this.pubYear;

}

Exercises

• Add toString methods to the Date and Posn classes.

• Add a method distanceToOrigin, operating on a Posn and taking no parameters.

• Add a method swapCoords, operating on a Posn and taking no parameters; it returns a new Posn like this one but with x & y reversed.

Exercises

• Add a method dayInYear that operates on a Date, takes no parameters, and returns the number of days since the beginning of the year. (For simplicity, assume all months are 30 days long.)

• Add a method distance that operates on a Posn, takes in another Posn, and returns the Euclidean distance between them.

Time check

• lunch time?

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


• 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.


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.


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


Lists in Java

Write the following methods on StringLists:

• countStrings : nothing -> int

• contains : String -> boolean

• countMatches : String -> int

Are we done yet?

• Fill out end-of-day survey

• Eat

• Go home

• Sleep

• Come back for another day

teachscheme, reachjava adelphi university thursday morning june 25, 2009

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