Abstract Data Types

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Abstract Data Types
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Data types I

• We type data--classify it into various
  categories--such as int, boolean, String, Applet
• A data type represents a set of possible values,
  such as ..., -2, -1, 0, 1, 2, ..., or true,
  false
• By typing our variables, we allow the computer to
  find some of our errors
• Some operations only make sense when applied to
  certain kinds of data (you cant divide by a
  boolean)
• Some methods only make sense for certain objects
• Typing simplifies internal representation
• A String requires more and different storage than
  a boolean

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Data types II

• A data type is characterized by
• a set of values
• a data representation, which is common to all
  these values, and
• a set of operations, which can be applied
  uniformly to all these values

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Primitive types in Java

• Java provides eight primitive types
• boolean
• char, byte, short, int, long
• float, double
• Each primitive type has
• a set of values
• a data representation
• a set of operations
• These are set in stonethere is nothing the
  programmer can do to change anything about them

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Primitive types as data types
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Classes in Java

• A class defines a data type
• The possible values of a class are called objects
• The data representation is a reference (pointer)
  to a block of storage
• The structure of this block is defined by the
  fields (both inherited and immediate) of the
  class
• The operations on the objects are called methods
• Many classes are defined in Javas packages
• To program in Java, you can (and must) define
  your own classes, as well as use the ones provided

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Methods and operators

• An operator typically
• Is written with non-alphabetic characters , ,
  , , , etc.
• Is written as prefix, infix, or postfix -x, xy,
  x
• Has only one or two arguments, or operands
• A method (or function) typically
• Is written with letters, and its arguments are
  enclosed in parentheses toString(), Math.abs(n)
• Has a fixed, predetermined number of arguments

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Methods are operators

• The differences between methods and operations
  are only syntactic differences, not fundamental
  ones
• Many languages (not including Java) let you
  define new operators, that is, new syntax
• When you define a new class and its methods, you
  are, fundamentally, defining a new data type and
  its operators
• Suppose a language defines the operator _at_ to mean
  times 3 plus 1 for example _at_7 is 22
• Would you consider this a good operation to have
  in the language?
• What does this suggest about defining classes and
  their methods?

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Insertion into a list

• A list is just a sequence of values
• There are many ways you could insert a new item
  into a list

As the new first item As the new last item
Before a given item After a given item In
place of a given item
Before the nth element After the nth
element Before the nth from the end After the
nth from the end In the correct location to
keep the list in sorted order

• Is it a good idea to supply all of these?
• If not, why not?

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Cognitive load

• Human minds are limitedyou cant remember
  everything
• You probably dont even remember all the Java
  operators for integers
• Whats the difference between gtgt and gtgtgt ?
• What about between ltlt and ltltlt ?
• We want our operators (and methods) to be useful
  and worth remembering
• The methods we define should be both necessary
  and sufficient to use the class for its intended
  purpose
• Of course, we cant always foresee how the class
  will be used
• In addition, we typically want some convenience
  methods

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Efficiency

• A list is just a sequence of valuesit could be
  implemented by a linked list or by an array
• Inserting as a new first element is efficient for
  a linked list representation, inefficient for an
  array
• Accessing the nth element is efficient for an
  array representation, inefficient for a linked
  list
• Inserting in the nth position is efficient for
  neither
• Do we want to make it easy for the user to be
  inefficient?
• Do we want the user to have to know the
  implementation?

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Abstract Data Types

• An Abstract Data Type (ADT) is
• a set of values
• a set of operations, which can be applied
  uniformly to all these values
• To abstract is to leave out information, keeping
  (hopefully) the more important parts
• What part of a Data Type does an ADT leave out?

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Data representation in an ADT

• An ADT must obviously have some kind of
  representation for its data
• The user need not know the representation
• The user should not be allowed to tamper with the
  representation
• Solution Make all data private
• But what if its really more convenient for the
  user to have direct access to the data?
• Solution Use setters and getters

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Example of setters and getters

• class Pair
• private int first, last
• public getFirst() return first
• public setFirst(int first) this.first first
  
• public getLast() return last
• public setLast(int last) this.last last

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Aside naming setters and getters

• Setters and getters should be named by
• Capitalizing the first letter of the variable
  (first becomes First), and
• Prefixing the name with get or set (setFirst)
• For boolean variables, you can replace get with
  is (for example, isRunning)
• This is more than just a conventionif and when
  you start using JavaBeans, it becomes a
  requirement

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Whats the point?

• Setters and getters allow you to keep control of
  your implementation
• For example, you decide to define a Point in a
  plane by its x-y coordinates
• class Point public int x public int y
• Later on, as you gradually add methods to this
  class, you decide that its more efficient to
  represent a point by its angle and distance from
  the origin, ? and ?
• Sorry, you cant do thatyoull break too much
  code that accesses x and y directly
• If you had used setters and getters, you could
  redefine them to compute x and y from ? and ?

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Contracts

• Every ADT should have a contract (or
  specification) that
• Specifies the set of valid values of the ADT
• Specifies, for each operation of the ADT
• Its name
• Its parameter types
• Its result type, if any
• Its observable behavior
• Does not specify
• The data representation
• The algorithms used to implement the operations

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Importance of the contract

• A contract is an agreement between two parties
  in this case
• The implementer of the ADT, who is concerned with
  making the operations correct and efficient, and
  also with preserving the flexibility to make
  changes later
• The applications programmer, who just wants to
  use the ADT to get a job done
• It doesnt matter if you are both of these
  parties the contract is still essential for good
  code
• This separation of concerns is essential in any
  large project

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Promise no more than necessary

• For a general API, the implementer should provide
  as much generality as feasible
• But for a specific program, the class author
  should provide only what is essential at the
  moment
• In Extreme Programming terms, You aint gonna
  need it!
• In fact, XP practice is to remove functionality
  that isnt currently needed!
• Your documentation should not expose anything
  that the application programmer does not need to
  know
• If you design for generality, its easy to add
  functionality laterbut removing it may have
  serious consequences

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Implementing an ADT

• To implement an ADT, you need to choose
• a data representation that
• must be able to represent all possible values of
  the ADT
• should be private
• an algorithm for each of the possible operations
  that
• must be consistent with the chosen representation
• all auxiliary (helper) operations that are not in
  the contract should be private

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Writing the contract

• In most cases, the Javadoc for a class is the
  contract
• This means
• The Javadoc documentation should describe what
  the class is for and how it should be used
• The Javadoc documentation should not describe
  implementation details
• Also, now is a good time to read Documentation
  Comments (rules 38 to 58) in The Elements of Java
  Style
• Sometimes, however...
• The particular implementation makes certain
  operations efficient at the cost of making others
  inefficient
• For example, Java provides both ArrayList (fast
  random access) and LinkedList (fast insertions
  and deletions)
• The user needs to know this information, but
  doesnt need detailed implementation information

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Contract and implementation in Java,method I

• Express the contract as an outline class
  declaration, showing only
• public fields
• headings of constructors and methods
• (this is what you would describe in javadoc
  comments)
• Express the implementation as a completed class
  declaration
• You cant write the outline class declaration
  directly, but Javadoc can create it from your
  code

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Contract and implementation in Java,method II

• Express the contract as an interface
• Express the implementation as a class that
  implements the interface
• Disadvantage you cant describe constructors in
  an interface
• Nevertheless, this is a good technique, and is
  used heavily in Java

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Example contract (method I)

• General description of class
• public class Die // Each value is a die
  (singular of dice) with n sides, // numbered
  1 to n, with one face showing
• Constructor
• public Die(int numberOfSides) //
  constructs a die with faces 1 thru numberOfSides
• Accessor
• int lastRoll() // returns the
  result of the previous roll
• Transformer (mutative)
• int roll() // returns the result
  of a new roll of the die

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Implementation, method I, page 1

• import java.util.
• public class Die
• private int numberOfSides
• private static Random random new Random()
• private int face
• public Die(int numberOfSides)
• this.numberOfSides numberOfSides
• face roll() // construct in a valid state!

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Implementation, method I, page 2

• int lastRoll() return face
• int roll() face
  random.nextInt(numberOfSides) 1 return
  face
• // class Die

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Contract, method II

• interface DieRoller
• int lastRoll()
• int roll()
• Notice
• There is no way to define a constructor
• However, this should be part of the contract!
• We need two names one for the interface, and one
  for the class that implements it
• Usually more than one class will implement an
  interface

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Implementation, method II

• import java.util.
• public class Die implements DieRoller
• // Everything else is the same
• Frequently more than one class will implement an
  interface
• The interface describes the general (more
  abstract) case
• Implementations are designed for specific classes

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Responsibilities

• A class is responsible for its own values
• It should protect them from careless or malicious
  users
• Ideally, a class should be written to be
  generally useful
• The goal is to make the class reusable
• The class should not be responsible for anything
  specific to the application in which it is used
• In practice, most classes are application-specific
  
• Javas classes are, on the whole, extremely well
  designed
• They werent written specifically for your
  program
• Strive to make your classes more like Javas!

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Aside an interesting bug

• Originally, I left the word static out ofprivate
  static Random random
  new Random()
• Then I created a Vector of ten dice (Dies)
• When I printed the Vector, I got 2 2 2
  2 2 2 2 2 2 2
• Why?
• These really were ten different Die objects
• Hint How does Java initialize its random number
  generator?

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Summary

• A Data Type describes values, representations,
  and operations
• An Abstract Data Type describes values and
  operations, but not representations
• An ADT should protect its data and keep it valid
• All, or nearly all, data should be private
• Access to data should be via getters and setters
• An ADT should provide
• A contract
• A necessary and sufficient set of operations

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The End