Class 15: Conditionals (2)

Held: Tuesday, 20 September 2005

Summary: Today we continue our basic exploration of conditionals.

Overview:

• EBoard.
• Lab.

Assignments

• Read When Things Go Wrong: Exceptions in Java in Espresso.

Notes:

• Do you prefer the lecture-like or the lab-like experiences? Why?
• Are there more questions on homework 2?
• I expect to return homework 1 tomorrow.

Overview:

• Review
• Standard comparison methods
• Lab

Standard Comparison Methods

• Most Java classes provide two standard comparison methods, equals(Object other) and compareTo(Object other)
• The equals method returns true if other is naturally the same as the current object.
  • The particulars of naturally the same as vary from class to class.
• The compareTo method returns a negative integer if the current object is less than other, 0 if the two are naturally the same, and a positive number if the current object is greater than other.
• You may wonder how we can compare to generic other objects.
• The answer is that we can't, we must cast them to the appropriate type by prefixing it with the name of the desired type in parentheses.
  • In order to cast an object, you should make sure that it can be of the appropriate class with the instanceof operation.
• For example, here's an equals method for the Fraction class.

  public boolean equals(Object other)
  {
    if (other instanceof Fraction) {
      Fraction f = (Fraction) other;
      return (this.numerator.equals(f.numerator))
             && (this.denominator.equals(f.denominator));
    }
    else {
      // Can only compare to other fractions.
      return false;
    }
  } // equals(Object)
  

• You should make it a habit to write these two methods.

Lab

• Do the lab.
• Be prepared to reflect.