Data Structures & Algorithms Lecture Notes

26 January 2010 • Interfaces

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Outline

• Class descriptions.
• Interfaces.

An Array ADT

class ArrayADT {

  ArrayADT(int n) { ... }
  void resize(int n) { ... }

  int get(int i) { ... }
  int size() { ... }

  void put(int i, int v) { ... }
  void swap(int i, int j) { ... }

  // and so on.
  }

What’s Wrong With That?

• A class contains much information uninteresting to clients.
  • Class implementation details.
  • Non-public methods and data.
• Other array-like classes have to be dealt with separately, rather than as a related group.
• Is there a client-friendly way to specify the important parts of a class?
• Is there a way to specify array-like behavior without specifying an array?

Interfaces

• An interface specifies a class’s allowable operations.
  • Specifies, not defines.
  • All methods are non-static and public.
• An interface can also contain variables, classes, and other interfaces.

Example Interfaces

interface ArrayADT{
  public void put(int i, int v);
  public int get(int i);
  public boolean find(int v);

  // and so on
  }  

Interfaces and Classes

• A class implements one or more interfaces.

  class CustomArray
  implements ArrayADT {
  
    public void put(int i, int v) { ... }
  
    public int get(int i) { ... }
  
    public boolean find(int v) { ... }
  
    }
  

Classes and Interfaces

• A class may implement
  • more than one interface.
  • interfaces and extend a class.
  • its own methods.
• An interface may extend another interface.

  interface PrintableArrayADT
  extends ArrayADT {
    void print(Writer out);
    }
  

Interface and Class Types

• An interface defines a type.
• A class implementing an interface can considered to be of the interface’s type.

  A value of type CustomArray can also be of type ArrayADT.

• Interfaces have other uses (such as pseudo multiple inheritance).

Interface Advantages

• An interface precisely specifies the constituent operations required by a data structure.
  • A CustomArray behaves like an ArrayADT.
• A single interfaces covers many different but similarly behaved data structures.

Summary

• Interfaces specify expected class behavior.
• Specify a data structure’s constituent operations in terms of interfaces.

References

• Use interfaces only to define types, Item 43, from Effective Java, second edition, by Joshua Bloch, Addison-Wesley, 2008.

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This page last modified on 26 January 2010.