Download Multiple Inheritance Single Inheritance Interfaces

CS257 Computer Science I
Kevin Sahr, PhD
Lecture 11: Interfaces
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Multiple Inheritance
✴ Some programming languages allow multiple inheritance
• a class can inherit from multiple parent classes
✴ For example, say we had a Student class and an
Employee class
• we might want to create a StudentWorker class to represent
students on work study
• StudentWorker is-a Student and StudentWorker is-a
Employee
• multiple inheritance would allow us to inherit from both classes
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Single Inheritance
✴ But, multiple inheritance adds subtle complexities to a
language
• for example, if both Student and Employee have an instance
variable name, which one does StudentWorker use?
✴ The designers of Java decided not to include multiple
inheritance
• in Java every class can derive from only one parent class
• this is called single inheritance
✴ However, some of the functionality of multiple inheritance
is provided in Java by interfaces
Interfaces
✴ A Java interface is a collection of abstract methods and/or
constants
✴ An abstract method is a method header without a method
body
• An abstract method can be declared in any class using the modifier
abstract, but because all methods in an interface are abstract,
usually it is left off
✴ An interface is used to establish a set of methods that a
class will implement
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Interfaces
interface is a reserved word
None of the methods in
an interface are given
a method body
public interface Doable
{
public void doThis();
public int doThat();
public void doThis2 (float value, char ch);
public boolean doTheOther (int num);
}
A semicolon immediately
follows each method header
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Interfaces
✴ An interface cannot be instantiated
• that is, we can’t create an object with an interface as it’s “class”
✴ A class formally implements an interface by:
• stating so in the class header
• providing implementations for each method in the interface
✴ If a class asserts that it implements an interface, it must
define all methods in the interface
Interfaces
public class CanDo implements Doable
{
public void doThis ()
implements is a
{
reserved word
// whatever
}
public void doThat ()
{
// something else
}
Each method listed
in Doable is
given a definition
// etc. for other methods in Doable
}
Interfaces
✴ As with any class, a class that implements an
interface can implement other methods as well
✴ In addition to (or instead of) abstract methods, an
interface can contain constants
✴ When a class implements an interface, it gains
access to all its constants
Interfaces
✴ A class can implement multiple interfaces
✴ All interfaces are listed in the implements clause,
separated by commas
✴ The class must implement all methods in all interfaces
listed in the header
class ManyThings implements Interface1, Interface2
{
// define all methods of both interfaces
}
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Interfaces
✴ The Java standard class library contains many helpful
interfaces
✴ The Comparable interface contains one abstract method
called compareTo, which is used to compare two objects
• it has the method header: public int compareTo (Object obj)
✴ We previously discussed the compareTo method of the
String class
• The String class implements Comparable, giving us the ability to put
strings in lexicographic order
The Comparable Interface
✴ Any class can implement Comparable to provide a
mechanism for comparing objects of that type
if (obj1.compareTo(obj2) < 0)
System.out.println ("obj1 is less than obj2");
✴ When implementing a Comparable interface your
compareTo method should follow the convention of
returning:
• negative value if this is “less than” the argument obj
• 0 if they are equal, and
• positive value if this is “greater than” the argument obj
The Comparable Interface
✴ It's up to the programmer to determine what makes one
object less than another
✴ For example, you may define the compareTo method of an
Employee class to order employees by name
(alphabetically) or by employee number
✴ The implementation of the method can be as
straightforward or as complex as needed for the situation
✴ Note that the argument to compareTo is an Object
• you will usually need to cast this object to the appropriate class in
order to invoke methods on it
✴ See CD.java and CompareCD.java
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Why Use Interfaces?
✴ Note that you could write a class that implements certain
methods (such as compareTo) without formally
implementing the interface (Comparable)
✴ However, formally implementing the interface provides a
number of benefits
• users of your class will already have some idea about how your
compareTo should operate, based on their experience with other
Comparable’s
• enables polymorphism (discussed in the next lecture)
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Lecture 11 Vocabulary
multiple inheritance
single inheritance
interface
abstract method
Comparable interface