Class 20: Anonymous Inner Classes

Held: Wednesday, March 8, 2006

Summary: Today we consider Java's anonymous inner classes, how we create them, and why we'd want to create them.

Related Pages:

• EBoard.
• Notes on Readings.

Notes:

• There is no reading for Friday. However, I would like you to email me a list of three important things you learned from Friday's video.
• There will be no class next Monday.
• Are there questions on Homework 2?

Overview:

• Anonymous values.
• Encapsulation and inner classes.
• Anonymous inner classes.
• Applications.

Anonymity

• An important issue in the design of languages is that you relieve the programmer from too many bookkeeping tasks.
• One such task is the need to name intermediate or temporary values.
• For example, we'd rather write

  z = (-b + Math.sqrt(b*b - 4*a*c))/2a;
  

  than

  double tmp1 = -b;
  double tmp2 = b*b;
  double tmp3 = a*c;
  double tmp4 = 4*tmp3;
  double tmp5 = tmp2-tmp4;
  ...
  

• You've seen in Scheme that it's useful to have anonymous functions, functions without a name.
  • If you only use the function once, why bother naming it?
• In object-oriented languages, it is useful to have anonymous classes, classes without a name.

Encapsulation and Inner Classes

• As we've said in the past, encapsulation is one of the key object-oriented design principles.
• It's easy to encapsulate data (fields) and capabilities (methods).
• However, when we're implementing one class with another class (e.g., lists with nodes), we may want to restrict access to the whole class to the using class.
• Inner classes provide one mechanism for doing so.

Java's Anonymous Inner Classes

• Create an object in a new class with

  new NameOfSuperClass() {
    code-for-overridden methods;
  }
  

• Why? Your reasons:
  • Callbacks
  • Event handling
  • Co-routining
• Why? My reasons
  • Functional reasons, e.g., Comparators for sorting routines.
  • Alternate mechanism for conditional access to fields.
  • ...