Class design: Common errors

Identified by AP administrators from the 2004 free-response answers:

• Not recognizing when to use information inherited from superclasses.

• Failing to use super in the first line of a constructor method. (It's worth teaching students to always use super, and not even mentioning that sometimes it's optional.)

• Not understanding distinction between classes and interfaces. For example:

  Set s = new Set(); // illegal
  

Identified by me after grading over a thousand responses to 2005's A2 question.

• Conflating the declaration of a class with the declaration of a constructor method.

  public class Point(double myX, double myY) {
  	double x = myX;
  	double y = myY;
  
  	public double getX() { return x; } //...
  

• Not labeling instance variables as private (or protected). I would guess that this comes from not understanding the distinction between instance variables and local variables, syntactically and semantically.

• Accessing constructor method parameters within other methods.

• The super line in a constructor doesn't include arguments corresponding to a superclass constructor.

• Not understanding that inherited variables shouldn't be initialized in subclass's constructor methods.

• Not understanding that inherited methods shouldn't be reimplemented unless a new behavior is necessary.

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