Interfaces

1
Interfaces

• In Java, interfaces declare methods but they do
  not provide any implementation.
• A Java interface is a collection of abstract
  methods and constants.
• interface interfacename
• - constant declarations
• - abstract methods
• An interface looks like a class, but it is not a
  class. It is not in the class hierarchy.

2
Interfaces

• interface I1 Although we do not write here, it
  is assumed
• that CONST1 is declared as a constant (with
• int CONST15 keywords public, final and
  static)
• void m1() Although we do not write here, it
  is assumed
• that m1 is declared with keywords public
• and abstract.

3
Implementing Interfaces

• Interfaces are intended to capture the common
  characteristics and behavior of the classes that
  implement the interfaces.
• A class that implements an interface must provide
  implementations for all of the methods in that
  interface.
• A class can implement an interface as follows
• class classname implements interfacename
• .
• . implementations of all methods in the
  interface must be provided here.

4
Implementing Interfaces - Example

• class C1 implements I1
• .
• . implementations of all methods in the
  interface I1
• . must be provided here.
• .
• All constants in I1 can be accessible in C1
  as
• its own variables.

5
Implementing Interfaces (cont.)

• An interface can be implemented by multiple
  classes.
• Each implementing class can provide their own
  unique versions of the method
  definitions.
• interface I1
• void m1()
• class C1 implements I1
• public void m1() System.out.println(Implement
  ation in C1)
• class C2 implements I1
• public void m1() System.out.println(Implement
  ation in C2)

6
Single Inheritance versus Multiple Inheritance

• Single inheritance means that each subclass has
  exactly one superclass, and that subclass can
  only inherit from its single superclass.
• Multiple inheritance means that each subclass can
  have more than one superclass, and that subclass
  can inherit from all its superclasses.
• C1 C1 C2 C3
• C2 C4
• single inheritance multiple inheritance
• Java supports only single inheritance among
  classes. (C support multiple inheritance.).
  Multiple inheritance is difficult to use.

7
Weak Form of Multiple Inheritance (!)

• Using interfaces, Java supports somehow a weak
  form of multiple inheritance (it is not actual
  multiple inheritance).
• In Java, a class may implement more than one
  interface, and this can be seen as a weak form
  of multiple inheritance (but it is not multiple
  inheritance).
• class classname implements interface1,,interface
  n
• .
• .
• . implementations of all methods in the all
  interfaces
• must be provided here.

8
Implementing More Than One Interface

• interface I1
• void m1()
• interface I2
• void m2() C must implement all methods in I1
  and I2.
• void m3()
• class C implements I1, I2
• public void m1() System.out.println(C-m1)
• public void m2() System.out.println(C-m2)
• public void m3() System.out.println(C-m3)
  

9
Resolving Name Conflicts Among Interfaces

• Since a class may implement more than one
  interface, the names in those interfaces may
  collide.
• To solve name collisions, Java use a simple
  mechanism.
• Two methods that have the same name will be
  treated as follows in Java
• If they are different signature, they are
  considered to be overloaded.
• If they have the same signature and the same
  return type, they are considered to be the same
  method and they collapse into one.
• If they have the same signature and the different
  return types, a compilation error will occur.

10
Resolving Name Conflicts Among Interfaces

• interface I1
• void m1()
• void m2()
• void m3()
• interface I2
• void m1(int a) There will be a compilation
  error for m3.
• void m2()
• int m3()
• class C implements I1, I2
• public void m1() // implementation of
  m1 in I1
• public void m1(int x) // implementation
  of m1 in I2
• public void m2() // implementation of
  m1 in I1 and I2

11
Inheritance Relation Among Interfaces

• Similarly for classes, interfaces can hold an
  inheritance relation among them.
• interface I2 extends I1
• Now, I2 contains all abstract methods of I1 plus
  its own abstract methods.
• The classes implementing I2 must implement all
  methods in I1 and I2.

12
Interfaces as Data Types

• Just as classes are data types, so are
  interfaces.
• Different from classes We cannot create an
  instance of an interface.
• interface I1
• class C1 implements I1
• class C2 extends C1
• // a variable can be declared as type I1
• I1 x
• A variable declared as I1, can store objects of
  C1 and C2.

13
Subtypes

• Each interface also defines a data type.
• The interface relation among interfaces, and the
  implementation relation between a class and
  interfaces create subtype relations.
• Complete subtype relations in Java as follows
• If class C1 extends class C2, then C1 is a
  subtype of C2.
• If interface I1 extends interface I2, then I1 is
  a subtype of I2.
• If class C implements interface I, then C is a
  subtype of I.
• For every interface I, I is a subtype of Object.
• For every type T (reference or primitive), T
  (array type of T) is a subtype of Object.
• If type T1 is a subtype of T2, then T1 is a
  subtype of T2.

14
Interface Example

• public interface Juggleable
• void tossIt()
• void catchIt()
• public class Fruit implements Juggleable
• public void tossIt() ...
• public void catchIt() ...
• public class Apple extends Fruit ...
• public class Orange extends Fruit ...
• public class Knife implements Juggleable ...
• Juggleable thingsToJuggle new Juggleable3
• thingsToJuggle0 new Apple()
• thingsToJuggle1 new Orange()
• thingsToJuggle2 new Knife()