CSE115: Introduction to Computer Science I Dr. Carl Alphonce 219 Bell Hall

Agenda Announcements – Cell phones / laptops off & away –Name signs out FAIR WARNING: First exam on Wednesday, Sept 28 Last time –expressions –references –variables –terminology Today –Object diagrams –More terminology –Class definitions

A bit of review example1.Terrarium t; example1.Caterpillar c; t = new example1.Terrarium(); c = new example1.Caterpillar(); t.add(c); c.start();

The variable declaration Consists minimally of: type & name What is a type? A class is a type. –Remember: objects are instantiated from classes. Examples of variable declarations: example1.Terrariumt; example1.Caterpillar c;

Syntax (a little more formal) variable declarations appear in many places in Java programs a variable declaration in Java –ALWAYS has: type identifer In our example –type identifier ; –semicolon is a terminator

assignment statement variable = expression ; ‘=’ is the ASSIGNMENT OPERATOR Example: t = new example1.Terrarium();

A bit of review example1.Terrarium t; example1.Caterpillar c; t = new example1.Terrarium(); c = new example1.Caterpillar(); t.add(c); c.start(); What happens in memory?

available 112 available 108 available used used Terrarium object Caterpillar object available Something like this!

Object diagram (corresponding to memory diagram on previous slide) example1.Terrarium example1.Caterpillar t c Boxes denote variables Ovals denote objects Arrows denote references This diagram is an abstraction of the one on the previous slide: it ignores irrelevant details, such as the addresses and sizes of the two objects being shown. An abstraction is thus a simplification. Simpler!

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 example1.Terrarium() ‘new’ is a “reserved word” in Java. This means that the word ‘new’ has a special meaning in the Java language. ‘new’ is the name of an operator whose job it is to create an instance of a given class 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. 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 a constructor call. 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)

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 – 1

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

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 class EcoSystem { public EcoSystem() { }... the reserved word class...

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

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

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

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

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

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

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

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

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

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

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

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

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

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