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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 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.access
ibility
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.
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
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.
