1. The Java Alternative to Multiple Inheritance
Interfaces
The Java Alternative to Multiple Inheritance
Copyright © Curt Hill

2. Introduction
There are two similar but different useful techniques in Java
Derivation
Interfaces
The good Java programmer needs both
Derivation is widely used to add specialization to classes
Interfaces are required for Layout Managers, among others
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3. Derivation & Interfaces
Derivation allows a new class to inherit properties and methods from an ancestor
Inheritance is discussed at length in a previous presentation
Interfaces allow a class to implement common features without inheriting code or data
Used to replace multiple inheritance
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4. Multiple inheritance C++ and Eiffel and other languages allow
The iostreams are classic examples
The formatting comes from one class
The I/O from another
Works very nicely
SmallTalk had it then removed it
Unfortunately there are problems
Recall the person inheritance hierarchy
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5. person set_name employee student set_wage set_major grad set_degree
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6. Multiple Inheritance Problems
Suppose that we wanted a student employee
We could then derive a class from both student and employee
The class would have two name properties
One from person through student
One from person through employee
How do we distinguish between the two
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7. Multiple Inheritance
Problems like this have not been resolved well in any programming language
Eiffel comes closest
Thus Java. C# and Smalltalk disallow multiple inheritance
Java and C# substitute the Interface
This gives many of the advantages
None of the problems
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8. Interfaces An interface is somewhat like a pure abstract base class
It defines no code and only constant data
It defines what methods are needed, without giving any indication at all about the implementation
Provides signatures only
If a class claims that it implements an interface it must implement these defined methods
Otherwise compile errors
Copyright © Curt Hill

9. Using an interface
Using an interface requires the implements reserved words: class xyz implements abc {…}
abc must be a defined interface
A class may implement several interfaces but only be derived from one class
Derivation uses the extend reserved word
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10. Defining an interface Similar to a class:
public interface Employee { void set_wage (double w); }
Notice this does not set the wage field
This is only the signature – no code
Multiple methods are possible
An interface may also extend another interface
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11. Creating the employee
Here we have both derivation and interface
class employee extends person implements Employee{
class stu_emp extends student implements Employee{
Either one will be abstract if it does not implement set_wage
An abstract class may not be instantiated
Like an abstract base class
Useful but not as common as instantiable classes
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12. Why use interfaces? Several common examples that we will use
Layout Managers
Event handlers in AWT 1.1 (or later) event model
Image handlers
Especially in Applets
Multi-threaded classes
Each of these that we need will be considered in a separate presentation
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13. Abstract Base Class or Interface?
An interface tells us all that we need to know: the method signature
There is no common code or data
An abstract base class usually adds common code and data
A class may only have one base class, but many interfaces
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14. Finally We have looked at this in a somewhat formal sense
What we next need is both the usage of an interface and later the definition of a worth while interface
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