Interfaces

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Section 6

• Interfaces
• Upcasting
• Downcasting

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Interfaces

• inter (Lat.) - between.
• face (Eng.) - the front part of the human head
  including the chin, mouth, nose, cheeks, eyes,
  and usually the forehead.

interface
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Interface

• The interface is a list of methods and/or
  constants.
• Class implements the interface I, if it has all
  Is methods.
• public interface A ...
• class B implements A ...

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Interfaces Example

• public interface Comparable
• public int compareTo(Object o)
• The implementation should return
• negative value when the object is lt o
• 0 when it is o
• positive value when if it is gt o

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Interfaces Comparable

• class Student implements Comparable
• public int IQ
• ...
• public int compareTo(Object o)
• return IQ - ((Student)o).IQ

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Fancier way

• class Student implements Comparable
• public int IQ
• ...
• public int compareTo(Object o)
• return (new Integer(IQ)).compareTo(
• new Integer(((Student)o).IQ))

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Usages

• Object max(Comparable a,
• Comparable b)

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Usages

• Object max(Comparable a,
• Comparable b)
• if (a.compareTo(b) lt 0)
• return b
• else
• return a

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Containers

• public interface Playable
• void play()
• class Song implements Playable ...
• void play(Playable a)

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Containers

• public interface Playable
• void play()
• class Song implements Playable ...
• void play(Playable a)
• if (a null) return
• for (int i 0 i lt a.length i)
• ai.play()

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Moral

• We can write generic functions, which operate on
  containers of objects implementing interfaces,
  using interface methods.

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Interfaces Extension

• As one class may extend other, one interface may
  also extend other.
• interface BritneySong extends Playable
• void showBritney()
• The extending interface inherits all
  fields/methods of the base interface.

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Interfaces Upcasting

• Take int f(Comparable c).
• What arguments can f take?
• Answer Objects of any class implementing
  Comparable, or any interface extending
  Comparable.

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Upcasting

• Rationale This is, in a way, looking at only the
  part of the object. The fact that the class
  implements interface assures us that methods we
  call will be present.
• void f( ) - f needs only the part of
  the object

class A implements Comparable
int compareTo(...)
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Upcasting

• Function having an interface as an argument can
  accept any object implementing this interface.

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Upcasting

• How it works?
• void f(Comparable c)
• ... c.compareTo(...)
• During the execution of f, the method compareTo
  is being looked for in the actual object passed
  to the method.

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Downcasting

• Object max(Comparable c1, Comparable c2)
• The code using this function could look like
• Integer i max(new Integer(5),
• new Integer(23))

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Downcasting

• WRONG!
• We need to downcast Object explicitly to Integer.
• Integer i (Integer)(max(new Integer(5), new
  Integer(23)))
• Note that
• Float f (Float)(max(new Integer(5), new
  Integer(23)))
• would cause a runtime error (exception).

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Downcasting

• So
• max looks at only the parts of its arguments
  which are the part of the interface
• However, the original objects are still there and
  we can get access to them by downcasting.

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Summary

• Upcasting
• Automatic (implicite)
• Function with interfaces as arguments.
• Arrays of interfaces.
• Runtime errors impossible.
• Downcasting
• Have to downcast explicite.
• Can cause runtime error.