MIT AITI 2004 Lecture Supplement

1
MIT AITI 2004 Lecture Supplement

• Inner Classes

2
Calc Design Problems

• Calc has a separate small class for every
  listener Lots of classes!
• Every listener for Calc has package access so
  other classes inside the same package could
  potentially use them
• Couldn't we put the listener classes inside Calc
  somehow and make them private?

3
Inner Classes

• Inner classes are classes we put inside other
  classes.
• They are declared inside the braces of the class
  just like fields and methods

class MyClass // fields here // methods
here // inner classes here
4
Inner Class Syntax

• Syntax for declaring an inner class is the same
  as the syntax for a regular class, except inner
  classes can be static and private
• Examples

class MyClass private static class
MyInnerClass1 . . . public class
MyInnerClass2 . . . static class
MyInnerClass3 . . .
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Inner Class Accessibility

• The inner class has access to all the private
  members of the enclosing class
• But the enclosing class does not have access to
  the private members of the inner class
• This makes sense when you think of the definition
  of private accessible only within the class.

6
Static vs Non-Static Inner Class

• Non-static inner classes have access to all the
  members of the enclosing class
• Static inner classes have access to only the
  static members of the enclosing class
• If an inner class doesn't need access to a
  non-static member of the enclosing class, you
  should make it static.

7
Non-Static Inner Class Example
public class Calc private class
CalcListener implements
ActionListener public void
actionPerformed(ActionEvent e) // place
code from calculatePressed here

• Now you can delete the calculatePressed method
  and the old CalcListener class file.
• Why must this class be non-static?

8
Static Inner Class Example
public class Calc private static class
Quitter implements
ActionListener public void
actionPerformed(ActionEvent e)
System.exit(0)

• Create an instance of Quitter and add it as a
  listener to the quit button
• Why can this class be static?

9
Inner Class Challenge

• Make ClickPrinter an inner class and delete the
  separate ClickPrinter class file
• Should this inner class be static or non-static?
• Now your whole Calc program is located within a
  single file Calc.java