Puzzle 1 what does the following program print?

public class Puzzle01

{

public static void main(String[] args)

{

System.out.print("C" + "S" + "E");

System.out.println('1' + '0' + '3' + '0' + 'z');

}

}

1

Solution

2

the program prints CSE318 instead of CSE1030Z

the problem occurs because the operator + is defined only for Strings and primitive numeric types the + operator concatenates Strings if and only if

at least one of the arguments is a String char is a primitive numeric type

Utilities

Implementing static features

3

Goals for Today initiate the design of simple class learn about class attributes

public static final

learn about preventing class instantiation

4

Motivation

5

you want to produce a software product that is to be used in many different countries

many different systems of measurement; for example distance

metre/kilometre versus yard/mile volume

teaspoon/tablespoon/cup versus millilitre/litre force

newton versus pound-force currency

CAN dollar versus US dollar versus euro versus …

Errors in Converting Units

6

errors in converting units can have catastrophic consequences NASA Mars Climate Orbiter

destroyed by navigation error caused by one subcontractor using Imperial units (pound-force) instead of metric (newton)

drug over/underdosing “Tenfold errors in pediatric doses are not uncommon. ”

Pediatric medication errors: predicting and preventing tenfold disasters. Journal of Clinical Pharmacology, 34(11):1043-5, 1994.

Review: Java Class

7

a class is a model of a thing or concept

in Java, a class is the blueprint for creating objects attributes

the structure of an object; its components and the information (data) contained by the object

methods the behaviour of an object; what an object can do

Designing a Class

8

to decide what attributes and methods a class must provide, you need to understand the problem you are trying to solve the attributes and methods you provide (the

abstraction you provide) depends entirely on the requirements of the problem

Person

appearancevoice

…

draw()talk()

…

Person

agephotograph

…

compatibleWith(Person)

contact ()…

video game person dating service person

class name

attributes

methods

Designing a Class to Convert Distances

9

design a class to convert between kilometres and miles

what attributes are needed? number of kilometres per mile

note: the number of kilometres in a mile never changes; it is genuinely a constant value

attributes that are constant have all uppercase names

DistanceUtility

KILOMETRES_PER_MILE : double attribute type

Version 1

10

public class DistanceUtility

{

// attributes

public static final

double KILOMETRES_PER_MILE = 0.621371192;

}

lay out of a typical class : see [AJ 4.1], [notes 1.2]

Attributes

11

an attribute is a member that holds data a constant attribute is usually declared by

specifying1. modifiers

1. access modifier public

2. static modifier static

3. final modifier final

2. type double

3. name KILOMETRES_PER_MILE

4. value 0.621371192

public static final double KILOMETRES_PER_MILE = 0.621371192;

Attributes

12

attribute names must be unique in a class the scope of an attribute is the entire class [JBA] and [notes] call public attributes fields

public Attributes

13

a public attribute is visible to all clients

public attributes break encapsulation a NothingToHide object has no control over the

value o f x clients can put a NothingToHide object into an

invalid state

public class NothingToHide { public int x; // always positive}

// client of NothingToHideNothingToHide h = new NothingToHide();h.x = 100;

h.x = -500; // x not positive

public Attributes

14

a public attribute makes a class brittle in the face of change

public attributes are hard to change they are part of the class API changing access or type will break exisiting client

code

public class NothingToHide { private int x; // always positive}

// existing client of NothingToHideNothingToHide h = new NothingToHide();h.x = 100; // no longer compiles

public Attributes

15

Avoid public attributes in production code except when you want to expose constant value types

static Attributes

16

an attribute that is static is a per-class member only one copy of the attribute, and the attribute is

associated with the class every object created from a class declaring a static

attribute shares the same copy of the attribute textbook uses the term static variable [AJ 256] also commonly called class variable

static Attributes

17

DistanceUtility u =

new DistanceUtility();

DistanceUtility v =

new DistanceUtility();

64 client invocation

u

see [JBA 4.3.3] for another example

500 DistanceUtility class

KILOMETRES_PER_MILE 0.621371192

1000 DistanceUtility object

???

1100 DistanceUtility object

???

v1000

1100

belongs to class

no copy ofKILOMETRES_PER_MILE

static Attribute Client Access

18

a client can access a public static attribute without requiring an object use the class name followed by a period followed

by the attribute name

it is legal, but considered bad form, to access a public static attribute using an object

// client of DistanceUtilitydouble kmPerMi = Distance.KILOMETRES_PER_MILE;

// client of DistanceUtility; avoid doing thisDistanceUtility u = new DistanceUtility();double kmPerMi = u.KILOMETRES_PER_MILE;

final Attributes

19

an attribute (or variable) that is final can only be assigned to once public static final attributes are typically

assigned when they are declared

public static final double

KILOMETRES_PER_MILE = 0.621371192;

public static final attributes are intended to be constant values that are a meaningful part of the abstraction provided by the class

final Attributes of Primitive Types

20

final attributes of primitive types are constantpublic class AlsoNothingToHide { public static final int x = 100;}

// client of AlsoNothingToHideAlsoNothingToHide.x = 88; // will not compile;

// attribute is final and// previously assigned

final Attributes of Immutable Types

21

final attributes of immutable types are constant

also, String is immutable it has no methods to change its contents

public class StillNothingToHide { public static final String x = "peek-a-boo";}

// client of StillNothingToHideStillNothingToHide.x = "i-see-you"; // will not compile;

// attribute is final and// previously assigned

final Attributes of Mutable Types

22

final attributes of mutable types are not logically constant; their state can be changed

public class ReallyNothingToHide { public static final int[] x = { 0, 1, 2 };}

// client of ReallyNothingToHideint[] y = { 100, 101, 102, 103 };ReallyNothingToHide.x = y; // will not compile;

// attribute is final and// previously assigned

ReallyNothingToHide.x[1] = 10000;// works!

final Attributes of Mutable Types

23

ReallyNothingToHide class

final x 192 700

:

700 int[] obj

:

not final 0

not final 1

not final 2

ReallyNothingToHide.x[1] = 10000;

10000

final Attributes of Mutable Types

24

final attributes of mutable types are not logically constant; their state can be changed

public class LastNothingToHide { public static final ArrayList<Integer> x =

new ArrayList<Integer>();}

// client of LastNothingToHideArrayList<Integer> y = new ArrayList<Integer>();LastNothingToHide.x = y; // will not compile;

// attribute is final and// previously assigned

LastNothingToHide.x.add( 10000 );// works!

final Attributes

25

Avoid using mutable types as public constants.

new DistanceUtility Objects

26

our DistanceUtility API does not expose a constructor but

DistanceUtility u = new DistanceUtility();

is legal

if you do not define any constructors, Java will generate a default no-argument constructor for you

our DistanceUtility API exposes only static constants (and methods later on) its state is constant

there is no benefit in instantiating a DistanceUtility object a client can access the constants (and methods)

without creating a DistanceUtility object

double kmPerMi = DistanceUtility.KILOMETRES_PER_MILE;

can prevent instantiation by declaring a private constructor

Preventing Instantiation

27

Version 2 (prevent instantiation)

28

public class DistanceUtility

{// attributes

public static final double KILOMETRES_PER_MILE = 0.621371192;

// constructors

// suppress default ctor for non-instantiation

private DistanceUtility()

{}

}

[notes 1.2.3]

Version 2.1 (even better)

29

public class DistanceUtility

{// attributes

public static final double KILOMETRES_PER_MILE = 0.621371192;

// constructors

// suppress default ctor for non-instantiation

private DistanceUtility()

{

throw new AssertionError();

}

}

[notes 1.2.3]

private

30

private attributes, constructors, and methods cannot be accessed by clients they are not part of the class API

private attributes, constructors, and methods are accessible only inside the scope of the class

a class with only private constructors indicates to clients that they cannot use new to create instances of the class

Puzzle 2

31

what does the following program print?

public class Puzzle02 { public static void main(String[] args) {

final long MICROS_PER_DAY = 24 * 60 * 60 * 1000 * 1000; final long MILLIS_PER_DAY = 24 * 60 * 60 * 1000; System.out.println(MICROS_PER_DAY / MILLIS_PER_DAY);}

}