Lecture 4 Objects and Classes Classes Objects Data

Lecture 4: Objects and Classes • • Classes Objects Data fields (instance variables) Methods – Instance methods – Static methods – Constructors • But first…

Problems with Homework 1 • In some cases, misunderstanding of what to do, pls come to talk to me after class • How to convert the input strings for the old and new base to int: – – not Integer. parse. Int not Math. pow can just call str 2 int or, special method for converting base since it cannot be more than 2 characters • Bugs: easy to check correctness by converting output string back to original base • Program was supposed to keep prompting for input until user enters empty string • Nice output format is very helpful • Identifier names are important • The way errors are detected is important: does not have to be complicated, but should not be misleading to user

How we can best learn • I propose to show the best program that was submitted, we test it, look at the code, comment on its good and bad points • Idea is not to put anyone down, but to learn to be a better programmer • I also expect to learn things from you during this course… • How do you like this idea? • Rule about homework: talking to people is good, acknowledging it is even better, but copying (or editing) other students’ solutions is absolutely forbidden (even with their permission)

Classes • All Java programs are organized into classes • Most classes have instance variables and instance methods, allowing us to construct objects of the class • Exception: some classes just have static variables and static methods, such as the Math class. You cannot construct an object of the Math class.

An Example: the Player class int heads; // data field (instance variable) Player() { // constructor // initialize heads to 0 } void flip() { // instance method // increment heads with probability 1/2 }

Constructing 2 Player objects • Player jack = new Player(); • Or Player jack; jack = new Player(); • Player jill = new Player(); • while jack. heads < 10 & jill. heads < 10 { jack. flip(); jill. flip(); • }

Private vs Public • Considered good practice to make data fields of an object private • Then they can only be accessed or changed by methods of the same class • Methods that do this are called accessor and mutator methods (getter and setter) methods respectively • Likewise private methods cannot be called outside the class • Public data fields and methods can be called from anywhere; those that are neither public nor private can be called anywhere from with the same package

Object variables are really references • What does the following do: Player jack = New Player(); Player jill = New Player(); jill = jack; while jack. heads < 10 & jill. heads < 10 { jack. flip(); jill. flip(); }