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1. UNIVERSITY COLLEGE OF
COMPUTER
APPLICATIONS
WELCOME YOU ALL

2. INTERFACES

3. JAVA INTERFACE
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◉ A J ava interface is a collection of constants and
abstract methods
◼ abstract method: a method header without a method
body; we declare an abstract method using the
modifier a b s t ra c t
◼ since all methods in an interface are abstract, the
a b s t ra c t modifier is usually left off
◉ Methods in an interface have public visibility by
default

4. INTERFACE: SYNTAX
interfaceisa reserved word
public interface Doable
{
public static f i n a l String NAME;
public void
doThis();
public i n t doThat();
public void doThis2 ( f l o a t value, char ch);
public boolean doTheOther ( i n t num);
}
A semicolon immediately
followseach method
header
No method in an
interfacehas a definition
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5. IMPLEMENTING AN INTERFACE
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◉ A class formally implements an interface by
◼ stating so in the class header in the implements
clause
◼ a class can implement multiple interfaces: the
interfaces are listed in the implements clause,
separated by commas
◉ If a class asserts that it implements an interface, it
must define all methods in the interface or the
compiler will produce errors

6. IMPLEMENTING INTERFACES
public class Something implements
Doable
{
public void doThis ( )
{
//
whateve
r
}
public void doThat ( )
{
// whatever
}
implements is
a reserved
word
Each method
listed
in Doable
is
given a
definition
// etc.
}
public c l a s s ManyThings implements Doable,
AnotherDoable 6

7. INTERFACES: AN EXAMPLE
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◉ A class that implements an interface can
implement other methods as well

8. UML DIAGRAM
<<interface>>
Complexity
+ getComplexity () : int
+ setComplexity (int) : void
Question
+ getQuestion () : String
+ getAnswer () : String
+ answerCorrect (String) : boolean
+ toString() : String
MiniQuiz
+ main(args : String[]) : void
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9. INTERFACES: EXAMPLES FROM
JAVA STANDARD CLASS LIBRARY
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◉ The Java Standard Class library defines many
interfaces:
◼ the I t e r a t o r interface contains methods that
allow the user to move through a collection of
objects easily
□ h a sN ex t (), n e x t ( ) , remove()
◼ the Comparable interface contains an abstract
method called compareTo, which is used to compare
two objects
i f (obj1.compareTo(obj2) < 0)
Syste m . o u t . pr i n t l n (“ o b j 1 i s l e s s than
o b j 2 ” ) ;

10. POLYMORPHISM VIA INTERFACES
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◉ Define a polymorphism reference through
interface
◼ declare a reference variable of an interface type
Doable o b j ;
◼ the obj reference can be used to point to any object
of any class that implements the Doable interface
◼ the version of doThis depends on the type of
object that obj is referring to:
o b j . d o T h i s ( ) ;

11. EXAMPLE: POLYMORPHISM VIA
INTERFACE
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◉ The payroll program revisited: we want to
sort the employees by name

12. MORE EXAMPLES
p u b l ic i n t e r f a c e Speaker
{
p u b l ic void sp e a k( ) ;
}
c l a s s Philosopher extends Human
implements Speaker
{
//
p u b l ic void speak()
{…}
p u b l ic void p o n t i f i c a t e ( )
{…}
}
c l a s s Dog extends Animal
implements Speaker
{
//
p u b l ic void speak()
{
…
}
Speaker g u e s t ;
guest = new Ph i l o sop he r( );
g u e s t . s p e a k ( ) ;
guest = Dog();
g u e s t . s p e a k ( ) ;
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Speaker s p e c i a l ;
s p e c i a l = new Ph i l o so phe r( ) ;
s p e c i a l . p o n t i f i c a t e ( ) ; /
/compilererror
Speaker s p e c i a l ;
s p e c i a l = new Ph i l o so phe r( ) ;
( ( P h i l o s o p h e r ) s p e c i a l ) . p o n t i f i c a t e ( ) ;

13. INTERFACE HIERARCHIES
◉ Inheritance can be applied to interfaces as well
as classes
◉ One interface can be used as the parent of
another
◉ The child interface inherits all abstract methods
of the parent
◉ A class implementing the child interface must
define all methods from both the parent and
child interfaces
◉ Note that class hierarchies and interface
hierarchies are distinct (they do not overlap)
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