What is an Interface??

1
What is an Interface??

• An interface is a class which contains ONLY
  abstract methods.
• public interface Capitalizable
• public abstract String outCaps()
  
• //returns String representation of
  object in all capital letters
• public abstract String outFirstCap()
  
• //returns String representation of
  object w/ first char in caps
• public abstract String outNoCaps()
• // returns String representation of object
  in all lowercase

2
What is an Interface FOR??

• An interface can be used to FORCE a class to
  provide certain functionality.
• Suppose we needed to be certain that the class
  Phrase provided a way to output data
• in all capitals
• with first letter capitalized
• in all lower case

3

• public class Phrase implements Capitalizable
  
• String the_data
• public Phrase()
• // code for this method
• public void addWord(String word)
• // code for this method
• public int howMany()
• // code for this method
• //Phrase MUST fully define all methods which are
  in Capitalizable

4

• //the class will NOT compile without these
  methods!!
• public String outCaps()
• // code for this method
• public String outFirstCap()
• // code for this method
• 
  
• public String outNoCaps()
• // code for this method
• ADDITIONALLY, an object of class Phrase is ALSO
  an object of type Capitalizable!!!!!!!!!!!

5

• Why might I need to ensure that a class offers
  the methods
• listed in Capitalizable??
• Suppose we wanted to write a program which
  creates Sentence objects, Paragraph objects,
  Word object, and BlockText objects
• And our program was to store these objects and
  output them all in capital letters.

6

• Sentence sobj
• Paragraph pobj
• Word wobj
• Blocktext bobj
• sobj new Sentence 100
• pobj new Paragraph 100
• wobj new Word 100
• bobj new Blocktext 100
• sobj0 new Sentence()
• pobj0 new Paragraph()
• wobj0 new Word()
• bobj0 new BlockText()
• .. // other assignments
• sobj15 new Sentence()

7

• for (int index0 index lt 15 index)
• System.out.println( Object at index
  index is
• 
  sobjindex.outCaps() )
• for (int index0 index lt 15 index)
• System.out.println( Object at index
  index is
• 
  pobjindex.outCaps() )
• for (int index0 index lt 15 index)
• System.out.println( Object at index
  index is
• 
  wobjindex.outCaps() )
• for (int index0 index lt 15 index)
• System.out.println( Object at index
  index is
• 
  pobjindex.outCaps() )
• There will be a problem if any of the objects in
  the array does not provide a method called
  outCaps, with exactly that signature

8

• Rather than leaving this up to the individual
  programmers of the Sentence, Paragraph, Word and
  BlockText class, it would be nice if the
  COMPILER could force these classes to implement
  the methods we need
• We can, if the application program code insists
  that those classes must IMPLEMENT the
  Capitalizable interface..

9

• Capitalizable sobj
• Capitalizable pobj
• Capitalizable wobj
• Capitalizable bobj
• sobj new Capitalizable 100
• pobj new Capitalizable 100
• wobj new Capitalizable 100
• bobj new Capitalizable 100
• sobj0 new Sentence() // this is fine,
  these objects are Capitalizable
• pobj0 new Paragraph()
• wobj0 new Word()
• bobj0 new BlockText()
• .. // other assignments
• sobj15 new Sentence()

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• public class Sentence implements Capitalizable
• // this class will ONLY not compile if it
  does not contain
• the methods specified in the Capitalizable
  interface
• public class Paragraph implements Capitalizable
• // this class will ONLY not compile if it
  does not contain
• the methods specified in the Capitalizable
  interface
• public class BlockText implements Capitalizable
• // this class will ONLY not compile if it
  does not contain
• the methods specified in the Capitalizable
  interface

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• // one array can store varying types of objects
• // all objects can be output in capitals using
  the same code
• // because it is KNOWN that each object has an
  outCaps method
• Capitalizable textobj
• textobj new Capitalizable 60
• textobj0 new Sentence()
• textobj1 new Paragraph()
• textobj2 new Word()
• .. // other assignments
• textobj59 new BlockText()
• for (int index0 index lt 60 index)
• System.out.println( Object at index
  index is
• 
  textobjindex.outCaps() )

12

• for (int index0 index lt 60 index)
• System.out.println( Object at index
  index is
• 
  textobjindex.outCaps() )
• //different methods are being executed depending
  the type of each array element
• How does JAVA know which method outCaps() is
  being output at any one time??

13

• The Java language provides polymorphism,
• or late binding.
• A polymorphic language waits until RUN TIME to
  bind a method call to the actual code that will
  be run.
• (this allows Java to KNOW the type of
  object that is making the call)
• (An object oriented language provides
• encapsulation of data and
  methods via classes
• polymorphism
• inheritance
  )

14
You have already seen an interface. Shape

• The Shape interface provides a number of
  abstract methods, just like any other interface.
• Rectangle2D, Line2D, Ellipse2D,
  RoundRectangle2D, Rectangle2D all realize
  (implement) the Shape interface.
• So a object created from any of these classes
  is an object of type Shape.
• An array declared and instantiated as
• Shape shapelist new
  Shape40
• is capable of storing 40 objects instantiated
  from any of these classes.

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• For example
• import .
• public class MyShapes extends Applet
• private Shape shapelist new Shape4
• public MyShapes()
• //create
  the draw
• shapelist0 new Rectangle2D.Double(x,y
  ,width,height)
• shapelist1 new Ellipse2D.Double(x,y,w
  idth,height)
• shapelist2 new Line2D.Double(x,y,widt
  h,height)
• shapelist3 new RoundRectangle2D.Doubl
  e(x,y,width,height)

16

• MyShapes class continued.
• public void paint(Graphics g)
• Graphics2D g2 (Graphics2D) g
• // draw all the
  shapes
• for (int index 0 index lt 4 index
  )
• g2.draw(shapelistindex)

17

• The MyShapes class does not make use of the fact
  that all the objects in the array have many
  method names in common.
• This applet only uses their common type to
  allow one array to be
• used to store all the shapes.
• HOWEVER, the draw(Shape) method of the
  Graphics2D class probably does
• call upon methods of the passed objects.
• for example
• Any object of type Shape has a method called
  getBoundary() . This method would be quite useful
  in drawing any shape and Is probably called
  within the draw method code..

18
One method which was in the Array class is the
the Arrays class provided static void sort
(Object array) static int
binarySearch(Object array, Object key) This
means Dog values new Dog55
Dog adog . code which
initializes the array
Arrays.sort(values) ..
int position Arrays.binarySearch(values,aDog)

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How does sort know how to order an array of
Dogs?? How can binarySearch determine if 2 Dog
objects are equal?? In order to use these
methods with a class type, that class type must
define its ordering That class type must
implement the Comparable interface)
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• public interface Comparable
• public abstract int compareTo(Object
  obj)
• Usage
• // This method returns 0 if implicit object
  (this) is equal
• to obj
• // This method returns a negative value if
  implicit object
• is less than obj
• // This method returns a positive value if obj
  is larger than
• implicit object

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• Suppose we wanted to write an applet that would
  read Dog data from a file, store them in an
  array, sort them and draw them in order.
• Our Dog class would have to implement the
  Comparable interface..
• Lets code that

22

• another use of interfaces
• Suppose we had a class named BankAccounts, where
  an object of this class represented one bank
  account.
• (one method of this class is getBalance, which
  gives the
• account balance).
• Now we want to create a class named BADataSet,
  which can monitor a group BankAccount objects,
• keeping track of the total funds and largest
  account.

23
BADataSet might look like this

• public class BADataSet
• private double sum //total of all account
  balances
• private BankAccount maximum //account w/
  highest balance
• private int count //number of accounts in
  set
• public void add(BankAccount x) sum sum
  x.getBalance() if (count 0
  maximum.getBalance()lt x.getBalance()) maximum
  x count
• public BankAccount getMaximum() return
  maximum

24

• Suppose we had a class named Coin, where an
  object of this class represents one coin.
• (one method of this class is getTotal, which
  gives the
• value of the coins).
• Now we want to create a class named CDataSet,
  which can keeps track of a number of Coin
  objects,
• keeping track of the total cash and largest
  denomination of coin.

25
CDataSet might look like this

• public class CDataSet
• private double sum //total of all coin
  objects
• private Coin maximum //coin object with
  highest value
• private int count //number of coin objects
  in set
• public void add(Coin x) sum sum
  x.getTotal() if (count 0
  maximum.getTotal()lt x.getTotal()) maximum x
  count
• public Coin getMaximum() return
  maximum

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• Gee, both of those classes look quite alike.
  In fact,
• except for
• getBalance() and getTotal()
• parameter/return types
• it is the same code. How can we write ONE
  DataSet class which can handle both types of
  objects??

27

• Suppose both classes could agree on the same
  method name, say, getMeasure
• DataSet could call that method
• sum sum x.getMeasure()if (count 0
  maximum.getMeasure() lt x.getMeasure()) maximum
  x
• Define an interface
• public interface Measurable public
  abstract double getMeasure()

28
Remember .

• when a class implements an interface it must
  provide a full implementation of
• the abstract methods in the interface
• So, if both BankAccount and Coin classes
  implemented the measurable interface, we know
  they have the method getMeasure

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• class BankAccount implements Measurable
• double balance //additional variables and
  methods
• public double getMeasure() return balance
• class Coin implements Measurable
• double value
• //additional variables and methods
• public double getMeasure() return value

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• public class DataSet
• private double sumprivate 
  maximumprivate int count
• public void add( x) sum sum
  x.getMeasure() if (count 0
  maximum.getMeasure() lt x.getMeasure()) maximum
  x count
• public  getMaximum() return
  maximum
• WE have taken care of the method name, but what
  data
• type will work, cant be BankAccount and cant
  be Coin!!

31
Remember .

• when a class implements an interface, objects
  of that class are object of the interface type
  (in addition to also being objects of the
  specific class)
• So, if both BankAccount and Coin classes
  implemented the Measurable interface, we know
  they are both objects of class Measurable !

32

• public class DataSet
• private double sumprivate Measurable
  maximumprivate int count
• public void add(Measureable x) sum sum
  x.getMeasure() if (count 0
  maximum.getMeasure() lt x.getMeasure()) maximum
  x count
• public Measurable getMaximum() return
  maximum

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• // This program tests the DataSet class.
• public class DataSetTest
• public static void main(String args)
• DataSet bankData new DataSet()
• bankData.add(new BankAccount(0))
• bankData.add(new BankAccount(10000))
• bankData.add(new BankAccount(2000))
• System.out.println("Average balance "
• bankData.getAverage())
• Measurable max bankData.getMaximum()
• System.out.println("Highest balance "
  
• max.getMeasure())

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• DataSet coinData new DataSet()
• coinData.add(new Coin(0.25, "quarter"))
• coinData.add(new Coin(0.1, "dime"))
• coinData.add(new Coin(0.05, "nickel"))
• System.out.println("Average coin value
  "
• coinData.getAverage())
  
• max coinData.getMaximum()
• System.out.println("Highest coin value "
  
• max.getMeasure())

35
UML Diagram

• Note that DataSet is decoupled from BankAccount,
  Coin

36
Converting Between Types

• Can convert from class type to realized interface
  type BankAccount account new
• BankAccount(10000) Measurable x account
  // OK
• Same interface type variable can hold reference
  to Coinx new Coin(0.1, "dime") // OK
• Cannot convert between unrelated typesx new
  Rectangle(5, 10, 20, 30) // ERROR

37
Casts

• Add coin objects to DataSetDataSet coinData
  new DataSet()coinData.add(new Coin(0.25,
  "quarter"))coinData.add(new Coin(0.1,
  "dime"))...
• Get largest coin with getMaximum
  methodMeasurable max coinData.getMaximum()
• If the coin class also has a method called
  getName which
• returns the type of coin as a String
• You know it's a coin, but the compiler doesn't.
  Apply a castCoin maxCoin (Coin)maxString
  name maxCoin.getName()
• If you are wrong and max isn't a coin, the
  compiler throws an exception

38
The instanceof Operator

• Use instanceof for safe casts
• if (max instanceof Coin)   Coin maxCoin
  (Coin)max   . . .

39

• object instanceof ClassName
• Example
•  if (x instanceof Coin)   Coin c
  (Coin)x
• Purpose
• To return true if the object is an instance of
  ClassName (or one of its subclasses), false
  otherwise

40
Polymorphism

• Interface variable x holds reference to object of
  a class that realizes (implements) the
  interfaceMeasurable xx new
  BankAccount(10000)x new Coin(0.1, "dime")
• You can call any of the interface methods
  double m x.getMeasure()
• Which method is called?

41

• Which method is called?
• Depends on the actual object.
• If x refers to a bank account, calls
  BankAccount.getMeasure
• If x refers to a coin, calls Coin.getMeasure
• Java can handle this because it supports
  polymorphism (greek many shapes) gt one
  reference can refer to different types of objects
• In the case of a polymorphic object, the actual
  method which will be called is determined by the
  type of the object.
• How does this work?? The JVM uses the actual
  type of object to determine method to execute at
  runtime !! (dynamic or late binding) This is
  called polymorphism.
• Different from overloading. Overloading is
  resolved by the compiler (at compile time).