1. Classes & Objects: Examples15
Classes & Objects: Examples

2. Previously
Methods
Arguments
Return
Types
Constructors

3. Review Classes consist of: Instances of classes are called objects
Data
Variables / members / properties
Functionality
Methods - which optionally accept arguments and return values
Instances of classes are called objects
Instance Variables / Methods
The default
One copy for each instance of the class
Class Variables / Methods
static
One copy shared by all instances of the class

4. Overview Data Hiding Access Control Class Methods Abstract Classes
Understanding Hello World

5. Data Hiding
Class and instance variables can be accessed from anywhere providing the full object or class name is used.
Variables declared in a method are “hidden”
Local to that method
(scope and scoping)

6. Access Control
Methods, classes and variables may be given modifiers to indicate how they may be used.
static - used to indicate a class variable
public
private
Public Methods
This is the default. Public methods may be used anywhere (providing the full class/object name is used).
Private Methods
Access is restricted to the class that contains the method.

7. Class Methods
Class methods (like class variables) have the static modifier.
This means that they can be used without instantiation.
static int squared(int i) {
return i*i; }
Classes that are not allowed to be instantiated are abstract classes.

8. Abstract Classes
Abstract classes are declared with the abstract modifier.
Abstract classes may not be instantiated.
Abstract classes serve to hold static variables and methods.
This is often useful for libraries of predefined functionality.

9. Understanding Hello World
This class is called Hello
The main method is a class method
(i.e. it can be accessed without
creating an instance of Hello)
The main method is public (i.e.
accessible outside of the class Hello)
The main method does not return any data
The Hello class contains a
method called main
The main method has a parameter
called args (an array of String)
class Hello {
public static void main (String[] args)
System.out.println(“Hello World!”); }