CSE115: Introduction to Computer Science I

CSE115: Introduction to Computer Science I Dr. Carl Alphonce 219 Bell Hall 645-4739 alphonce@buffalo.edu

Announcements • No classes on Monday before 6:00 PM • No office hours on Monday • New lab starting next week • Exam grading should be finished tonight 

What is a class definition? • A class definition is a description of the properties and behaviors that instances of the class will have. • Recall that we said a running OO program is a system of interacting objects. • Possible relationships between objects are determined by relationships between classes. (Important – we’ll return to this!)

Where do objects come from? • Objects are instances of classes • We instantiate classes: • e.g. new example1.Terrarium() • There are three parts to this expression: • new • example1.Terrarium • ()

‘new’ is a “reserved word” in Java. This means that the word ‘new’ has a special meaning in the Java language. example1.Terrarium() is a constructor call. A constructor initializes the state of a newly created object. new example1.Terrarium() ‘new’ is the name of an operator whose job it is to create an instance of a given class The parentheses delimit the argument list of the constructor call. In this case there are no arguments being passed along to the constructor, so the argument list is empty. example1.Terrarium is the name of the class we are instantiating. It is a compound name, consisting of a package name (example1) and the name of the class’ constructor (Terrarium), separated by a dot ‘.’ A constructor initializes the state of a newly created object.

Object creation(revisited & refined) • new determines size of object • reserves a block of memory for object • calls constructor to initialize the object • we will consider a constructor to be a special case of a method • returns the starting address of block of memory (a reference to the object)

Packages – 1 • A package is an organizational mechanism • related classes are grouped together • allows class names to be re-used in different packages • reduces chance of naming conflicts

Packages – 2 • One analogy: • package::class • area code::phone number • A class’ fully qualified name consists of its package name and its (unqualified) class name: • example1.Terrarium is a fully qualified class name • Terrarium is an unqualified class name

Folder structure on disk • each package corresponds to a folder on the disk • packages can be nested within each other • corresponds to nested folder on disk • examples: • java.rmi.registry • javax.swing.text.html.parser • com.sun.accessibility.internal.resources.accessibility

Defining a class • Let us define a class which, when instantiated, creates a Terrarium, adds a Caterpillar, and makes the Caterpillar move. • This is similar to what you will do for lab 2. • We start with a minimal class definition.

Our first class definition! Here’s a minimal class definition. We will label and discuss each part of it: package lab2; public class EcoSystem { public EcoSystem() { } }

Syntax package lab2; public class EcoSystem { public EcoSystem() { } } Package declaration is shown in green:

Syntax package lab2; public class EcoSystem { public EcoSystem() { } } package is a reserved word:

Syntax package lab2; public class EcoSystem { public EcoSystem() { } } lab2 is the name of the package – you choose this (we’ll cover naming rules and conventions later):

Syntax package lab2; public class EcoSystem { public EcoSystem() { } } A semicolon ‘;’marks the end of the declaration:

Syntax package lab2; public class EcoSystem { public EcoSystem() { } } The class definition is shown in green:

Syntax package lab2; public class EcoSystem{ public EcoSystem() { } } The class definition consists of a header . . .

Syntax package lab2; public class EcoSystem { public EcoSystem() { } } . . . and a body:

Syntax package lab2; public class EcoSystem{ public EcoSystem() { } } The class header consists of an access control modifier . . .

Syntax package lab2; public classEcoSystem{ public EcoSystem() { } } . . . the reserved word class . . .

Syntax package lab2; publicclass EcoSystem{ public EcoSystem() { } } . . . and a class name:

Syntax package lab2; publicclass EcoSystem { public EcoSystem() { } } The class body begins with an opening brace ‘{’ . . .

Syntax package lab2; publicclass EcoSystem{ public EcoSystem() { } } . . . and ends with the matching closing brace ‘}’ :

Syntax package lab2; publicclass EcoSystem{ public EcoSystem() { } } In this example, the body consists of a single constructor definition:

Syntax package lab2; publicclass EcoSystem{ public EcoSystem() { } } The constructor definitions consists of a header . . .

Syntax package lab2; publicclass EcoSystem{ public EcoSystem() { } } . . . and a body:

Syntax package lab2; publicclass EcoSystem{ publicEcoSystem() { } } The constructor header consists of an access control modifier . . .

Syntax package lab2; publicclass EcoSystem{ public EcoSystem() { } } . . . the constructor name (which is the same as the class name) . . .

Syntax package lab2; publicclass EcoSystem{ public EcoSystem() { } } . . . and a parameter list:

Syntax package lab2; publicclass EcoSystem{ public EcoSystem() { } } The constructor body begins with an opening brace ‘{’ . . .

Syntax package lab2; publicclass EcoSystem{ public EcoSystem() { } } . . . and ends with the matching closing brace ‘}’ :

Let’s return to:defining a class • We set out to define a class which, when instantiated, creates a Terrarium, adds a Caterpillar, and makes the Caterpillar move. • We started with a minimal class definition, let’s move beyond that now.

Variable declaration A variable can be declared inside the body of a method – it is called a local variable. package lab2; publicclass EcoSystem{ public EcoSystem() { example1.Terrarium t; } }

Variables’ properties • name • location • type • value (contents) • scope • lifetime

Variable scope • The scope of a variable is the part of a program where a variable declaration is in effect. • Variables declared in different ways have different scope.

Local variables • A variable declared within a constructor (method) is called a local variable. • The scope of a local variable is from the point of the declaration to the end of the method body.

Assignment statement Any statement must be inside the body of a method: package lab2; publicclass EcoSystem{ public EcoSystem() { example1.Terrarium t; t = new example1.Terrarium(); } }

The complete solution package lab2; publicclass EcoSystem{ public EcoSystem() { example1.Terrarium t; t = new example1.Terrarium(); example1.Caterpillar c; c = new example1.Caterpillar(); t.add(c); c.start(); } }

Every time class is instantiated, the constructor is executed. • This creates a new Terrarium with a new moving Caterpillar.

CSE115: Introduction to Computer Science I