Class 16: Standard Object Methods

Held: Wednesday, 22 September 2004

Summary: Today we visit some of the standard methods that most of your model classes should use.

Related Pages:

• EBoard.
• Sample Code.

Due

• Homework 16: Labs.

Assignments

• Homework 17: Extending Dates.

Notes:

• I've decided to use "Mold class" rather than "Template class" for "class that is used to create lots of objects".
• I hope to have everything graded for Friday.
• I plan to distribute the take-home exam on Friday.
• It sounds like the labs were helpful. I'll try to incorporate more.

Overview:

• Why have standard methods?
• The toString method.
• The equals method.
• The compareTo method.
• The clone method.
• The hashCode method.

Why Have Standard Methods

• There are a number of methods that most mold classes provide.
• You have already encountered one such method, toString.
• Why have such standard methods? Because we can rely on them elsewhere.
• Some methods are implicitly there for every class (toString, equals, etc.)
• Some you state are there by indicating that your mold class implements a particular interface.

The Equals Method

• One simple method is equals, which you use to determine whether one object is "naturally" the same as another object.
• You first have a responsibility to determine what "naturally" means.
  • Are two Vec2D's the same if they have the same X and Y coordinate, but one is a Polar and one is a Point?
  • Are two Polars the same if their radiuses are within 1/1000 of each other and their angles are also within 1/1000 of each other?
• The standard signature for the equals method is

  public boolean equals(Object other)
  

• Note that it is somewhat difficult to compare a specific object to a general object.
• The solution? You can cast the object to another type. For example

  Vec2D ovec = (Vec2D) other;
  

• What happens if the cast fails? Right now, your program crashes.
• What's the solution? You can check whether the other object is in the appropriate class with the instanceof operation

  if (other instanceof Vec2D) {
  	Vec2D ovec = (Vec2D) other;
  	...
  }
  else {
  	return false;
  }
  

Comparing objects with compareTo

• The compareTo method is a more general form of comparison.
• This method has the signature

  public boolean compareTo(Object other)
  

• If you write the compareTo method, you should indicate that your mold class implements Comparable.
• compareTo returns one of three kinds of values
  • a negative number, if this object naturally preceeds the other object
  • zero, if this object is naturally the same as the other object
  • a positive number, if this object naturally follows the other object
• What should you do if the other object has no natural relationship? Give up.
  • We'll learn more about "giving up" tomorrow.

The clone Method

• The clone method makes another copy of the current object.
• It is most useful for mutable objects, since it lets you make a copy of the current state of an object before changing the object.
• If you write the clone method, you should indicate that your mold class implements Cloneable.
• The signature of clone is

  public Object clone()
  

The hashCode Method

• The last of the standard methods is somewhat strange. The hashCode method returns some integer that represents this object.
• Different objects in the same class should be likely to have different numbers.
• Since there are often more objects in a class than valid integers, different objects can have the same number.
• Two equal objects should have the same number.
• We'll use the hashCode method in the second half of the semester, but I'd like you to start using it now.