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Puzzle 1

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'); } }. Solution. the program prints CSE318 instead of CSE1030Z

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Puzzle 1

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  1. 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'); } }

  2. Solution • 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

  3. Utilities Implementing static features

  4. Goals for Today • initiate the design of simple class • learn about class attributes • public • static • final • learn about preventing class instantiation

  5. Motivation • 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 …

  6. Errors in Converting Units • 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.

  7. Review: Java Class • 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

  8. Designing a Class • 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 video game person dating service person

  9. Designing a Class to Convert Distances • 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 attribute type

  10. Version 1 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]

  11. Attributes • an attribute is a member that holds data • a constant attribute is usually declared by specifying • modifiers • access modifier public • static modifier static • final modifier final • type double • name KILOMETRES_PER_MILE • value 0.621371192 public static final double KILOMETRES_PER_MILE = 0.621371192;

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

  13. public Attributes • 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 aNothingToHideobject into an invalid state public class NothingToHide { public int x; // always positive } // client of NothingToHide NothingToHide h = new NothingToHide(); h.x = 100; h.x = -500; // x not positive

  14. public Attributes • 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 NothingToHide NothingToHide h = new NothingToHide(); h.x = 100; // no longer compiles

  15. public Attributes • Avoid public attributes in production code • except when you want to expose constant value types

  16. static Attributes • 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

  17. static Attributes DistanceUtility u = new DistanceUtility(); DistanceUtility v = new DistanceUtility(); 1000 1100 belongs to class no copy of KILOMETRES_PER_MILE see [JBA 4.3.3] for another example

  18. static Attribute Client Access • a client can access a public staticattribute 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 DistanceUtility double kmPerMi = Distance.KILOMETRES_PER_MILE; // client of DistanceUtility; avoid doing this DistanceUtility u = new DistanceUtility(); double kmPerMi = u.KILOMETRES_PER_MILE;

  19. final Attributes • an attribute (or variable) that is final can only be assigned to once • public static finalattributes are typically assigned when they are declared public static final double KILOMETRES_PER_MILE = 0.621371192; • public static finalattributes are intended to be constant values that are a meaningful part of the abstraction provided by the class

  20. final Attributes of Primitive Types • final attributes of primitive types are constant public class AlsoNothingToHide { public static final int x = 100; } // client of AlsoNothingToHide AlsoNothingToHide.x = 88; // will not compile; // attribute is final and // previously assigned

  21. final Attributes of Immutable Types • 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 StillNothingToHide StillNothingToHide.x = "i-see-you"; // will not compile; // attribute is final and // previously assigned

  22. final Attributes of Mutable Types • 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 ReallyNothingToHide int[] y = { 100, 101, 102, 103 }; ReallyNothingToHide.x = y; // will not compile; // attribute is final and // previously assigned ReallyNothingToHide.x[1] = 10000; // works!

  23. final Attributes of Mutable Types 10000 ReallyNothingToHide.x[1] = 10000;

  24. final Attributes of Mutable Types • 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 LastNothingToHide ArrayList<Integer> y = new ArrayList<Integer>(); LastNothingToHide.x = y; // will not compile; // attribute is final and // previously assigned LastNothingToHide.x.add( 10000 ); // works!

  25. final Attributes • Avoid using mutable types as public constants.

  26. new DistanceUtility Objects • 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

  27. Preventing Instantiation • 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

  28. Version 2 (prevent instantiation) 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]

  29. Version 2.1 (even better) 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]

  30. private • privateattributes, constructors, and methods cannot be accessed by clients • they are not part of the class API • privateattributes, 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

  31. Puzzle 2 • 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); } }

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