Month: October 2005

Today I will continue my series on classes in Java. To review where we are, you can read this post.

First of all, we need to add a couple of methods to our class. There are two other types of methods in classes: reporters and mutators. A reporter does exactly that. As you might notice, our instance variables are private, meaning that only that class can see it. So if we want to know certain information about the student, we must write a method that looks at the variable in question and reports it. A mutator method changes the object in some way. So, that being said, lets write a couple of these methods.

Here, we have added two methods to our class- a reporter and a mutator. Our reporter, tellName, looks at the current value of String name and returns it. Since we are returning a value, the method must be of the type we are returning. It is public so it can be accessed from outside our class. Now, here I just wrote one for name, but to get the other values (age, gpa, status), you must write a reporter method for each variable, as it’s respective type (an age reporter would be of type int).

Our next method is a mutator that figures out a new gpa. This is a simple one that accepts a new GPA from the client and averages the new and old one together. This method is not returning anything, just doing a calculation, so it is a void method. It is accepting a variable of type float, which we declare in the parentheses before the do work in the method. Then we perform the calculation. We declare a new variable called oldAvg that gets the value of gpa, and then we assign the new value to gpa.

That is all there is to other types of methods. They can be as simple or as complicated as you see fit. Next time, I will talk about inheritance and extensions. Later!

Today and over the last couple of days we have been talking about classes in Java. I’m fairly certain I understand them enough to now talk about them here. Tonight I will introduce them and continue the discussion later.

Basically, a class “consists of a collection of types of encapsulated instance variables and types of methods, possibly with implementation of those types together with a constructor function that can be used to create objects of the class. A class is a cohesive package that consists of a particular kind of compile-time metadata. A Class describes the rules by which objects behave; these objects are referred to as “instances” of that class. …” In basic terms, it is the blue prints of an object. Inside of a class, you will find instance varibles with values, methods to manipulate these variables, and constructors of those objects. If we were to make a class called Student , it might look something like this:

This class has 2 methods- both constructors- and 4 instance variables in it. The variables are initialized, but have no values. This is where the constructors come in. The first constructor for Student has 4 arguments sent to it. One for name, age, gpa and status. The constructor takes these 4 values sent by the client (NOT necessarily a user) and applies them to our 4 instance variables. The second constructor, which has no arguments, simply creates an object where all of them variables are set to nothing values.

For right now we only have constructors in our class. Next time, I will talk about other possible methods for our Student class, and subclasses that might inherit out Student class. Later

PS- to read ahead, and check out the counter class we have been working on in class, you can find it here
I know this was a little rushed, so if you have any questions, feel free to comment them or email me.