• Classes and Objects
  • Blueprint / cookie cutter
  • product / cookie
• Instance vs class (a.k.a. “static”)
  • Instance variables are the properties that make one object distinct from another.
    • A date object, for example, may have month, day, and year as instance variables, as that is what we use to distinguish two dates.
  • Class (aka “static”) variables are shared among all objects in a class.
    • Common for constants (e.g., # of days in each month)
    • Can also be mutable
    • Sometimes classes need to keep track of the number of instances. That can be done with a class variable.
    • Suppose I write a date as “10/15/93”. What year is that?
    • Suppose I write a date as “10/15/22”. What year is that?
    • Suppose I write a date as “10/15/32”. What year is that?
    • Suppose I write a date as “10/15/42”. What year is that?
    • There has to be some rule for when you switch back to the 1900s. That cutoff could be a static variable: Changing it changes the rule for all dates.
  • Instance methods are methods that rely on instance variables. For example if I say dayOfWeek(), you can only provide an answer with respect to a specific object (i.e., a specific date). The question simply doesn’t make sense otherwise.
  • Class methods don’t rely on any instance data. For example, you can answer isLeapYear(1954) without any additional data.
    • Think of class methods as independent methods that are simply packaged up inside a given class for convenience / organization. Technically, they could be placed anywhere (although that would make the code hard to follow.)
  • Some software engineers prefer class methods over instance methods. Why?
    • They are easier to test: Why? No side effects.
  • Why is the main method static? Because when Java first begins, there are no objects. Thus, no objects on which to invoke an instance method.
• Method Overloading
  • A class can have several methods with the same name, as long as the parameter list is different. This is called Overloading.
  • Think of a class as a tool to help other programmers get the job done.
  • Overloading is a convenience to the programmer who uses that “tool.” In most cases, the overloading isn’t necessary — just convenient.
  • Look at the Java String API https://https://docs.oracle.com/javase/7/docs/api/java/lang/String.html
    • There are four different versions of indexOf.
    • indexOf(int ch) is not necessary. It just makes it so you can say str.indexOf('x') rather than str.indexOf('x', 0). Notice the convenience factor.
      • What’s the shortest way to write indexOf(char) if you have already completed indexOf(char, int)?
      • Make sure you do this when completing the project.
      • What does DRY stand for? Don’t repeat yourself.
      • Be sure to look for opportunities to apply DRY in 163. It’s part of the grading rubric.
    • indexOf(String str) does the same thing but matches strings instead of characters. We could have given it a different name; but, since it does the same thing, it’s easier to remember just one name.
    • Of the four indexOf methods, which would you implement first? Why? How could you write the other three in terms of the first?
  • The SimpleDate class has two different constructors SimpleDate(int, int, int) and SimpleDate(String). You will implement both.
    • In the case of this constructor, there isn’t a good way to write one in terms of the other. But, you can factor common code out into a private helper method.

Switch to testing.

Array tricks

• Discuss how to test ordinalDate (i.e., which test cases and why)?
  • Have students implement
  • After 5 or minutes ask to see some different approaches.
  • Show daysInMonth static final array. Ask for ideas of how this could make the source code more concise.
  • What else could be put in an array to make it even better?