Outline of Class 7: Review

Miscellaneous

• Any comments or questions on homework 2?
• Is anyone still missing the book?
• Reminder: if I'm going to fast, or too slow, let me know (in person, via email, whatever).
• I've noticed some problems with understanding of Java's model, so we'll go over it in a little bit more depth today.
• Charles Jepsen will be giving a talk at the Mathematics Journal Club Thursday in room 2413 at 4:30 pm on Equisum Sets. Food is available in room 2400 at 4:14 pm.
• Do you want "less talk, more labs"?
• I've been asked "Since you put almost everything you say on the web, do we really have to come to class?" Yes. Why? Because we'll cover things in class that won't make it on the web pages, because I'll be trying to get you to think about what's going on as we go (that's harder to do with Java, but will get easier as we go on to other topics), and because it provides me with some feedback as to how you're doing.
• I'm guessing that we'll need to spend one extra week on Java. It's not my ideal use of a week, but it does seem necessary.

Java's Program Model

• Java's model of what a program is differs significantly from that of the languages you've learned in the past (or at least from those of Scheme and Pascal).
• In Java, a program is a collection of independent "actors" each of which knows how to do certain things, has certain properties, and may communicate with other actors.
  • These actors are the classes and objects of your program.
  • The things an actor knows how to do are its methods.
  • Data pertainent to the actor are represented by its attributes/field.
• How does this differ from traditional procedural programming? In a number of ways.
  • You should try to model your algorithms in terms of communicating objects, rather than functions that call each other.
  • You need to remember that, in general, you will be telling your objects to do things to/with themselves, rather than telling the system as a whole to do things to/with your objects. For example, instead of saying "add two objects", you would tell one object to add another object to itself.
  • Instead of one mammoth program, you'll have a number of smaller class definitions, each in its own file.
  • Your programs should be more "modular" in that you will have developed a number of independent (or partially independent) actors.
• However, the Java interpreter still needs to know where to "start" setting up and manipulating the actors. Your main() method serves this purpose.

Variables, Objects, and References

• Java carefully distinguishes between the objects we create and the variables that refer to them.
• Each object occupies some area of memory. This area holds the fields of the object and additional information to help the system decide which method to execute.
• Instead of containing objects, variables hold references to objects.
  • We usually indicate this visually by drawing an arrow from a variable's name to the object it refers to.
  • These references are essentially the same as pointers, except that you may be more limited in what you can do with references.
• Because variables hold references, and not objects, it is possible for more than one variable to refer to the same object. A change through variable will be to the underlying object, so the change will be observable through all variables.
• Because variables hold references, and not objects, it is necessary to create new objects when we want independent "values" for our variables. This is why we use the new operation in conjunction with constructors.
• When you pass objects to methods, you are, in effect, passing references to those objects. This means that if a method modifies one of its parameters, it will modify it globally, and not just in the routine.

Exercises

• Create a new object of type Complex and have two variables reference it.
• Modify one, and see what happens if you then try to print the corresponding field of the other.
• Create a copyTo(Complex c) method that copies the value of the current complex number to its parameter. Your method will look something like

   /**
    * Copy the current number to c.
    */
   public void copyTo(Complex c) {
     c.real_part = real_part;
     c.imaginary_part = imaginary_part;
   } // copyTo
  

• Test your copyTo() method.