1. objects CIS 421 Kutztown University
7/23/2018
Objects
CIS 421
Kutztown University
objects

2. What are Objects An aggregation
a heterogeneous collection of data members
a set of behaviors to access and manipulate the data
Objects often contain objects, since all but simple types are objects (including String)

3. What are Objects Objects are declared in a class
An object is an instantiation of the non-static members of a class
Objects model real things
For example an object might be an account (abstract thing), a vehicle (real thing), a purchase (transaction), or a mouse click (event)

4. What are Objects
Real objects and model objects both have attributes and behavior
A model object is an object modeled in Java
The attributes are the data values or references to other objects
The behaviors are the methods

5. What are Objects
The fact that an object contains behavior and data values is called encapsulation
To invoke the behavior, a message is sent to the object reference (invocation)
The message must have the same spelling as the method name

6. Summary of What Objects Are
Objects encapsulate data values and behaviors upon those data
They model real objects
They receive messages to invoke their behaviors

7. The Purpose of a Class
A class in Java is a mechanism used to define an object, among other possibilities
Sometimes they are called templates for objects (Java doesn’t have C++ style templates)
They act as a blue print of an object
When an object is instantiated (created) the template is used

8. The Purpose of a Class class is a reserved word
In addition there is a visibility (access) modifier for each class and members (data and methods) of the class
private, public, or protected

9. The Purpose of a Class
A visibility modifier tells the system how to protect the members from the outside world (other objects)
To obey the OO concept, data members should be private
The class is usually public (it must be designated with an access qualifier)
public methods constitute the user interface
methods for internal use should be private (or protected)

10. Static Declarations Static declarations belong to the class
Some things MUST be static
main()
the System class’ declarations
System.exit() method
System.out stream
InputReader is an example of a class containing only static declarations
static declarations can be accessed via the class name or via an object of the class’ type

11. Objects vs. Simple Types
All object declarations are just a reference. The object must be instantiated
Arrays are objects!
Simple types allocate memory for the data by their declaration
IntegerObject
Example of ridiculously simplistic object

12. Example: Array of 10 int Two ways to declare reference int[] list;
no value inside []
represents a reference to an array of int
size undetermined until instantiated
Instantiate: list=new int[10]
array now has 10 elements

13. Example: Array of 10 int Wait! Not always like this... IntegerObject:
Example of very simple object type
IntegerObject Ilist[]=
new IntegerObject[10]
Array has 10 references, not objects.
Must instantiate each separately
Examples: MinMax

14. Built-in Java Objects Simple types all have object wrapper type;
Integer wraps int
Double wraps double
Character wraps char
All are immutable
Must be constructed with value that can’t be changed

15. Built-in Java Objects (con’t)
String is also immutable
Note: String is special; can be constructed via assignment
String S=“Hello”;
S=“ABCD”
S is constructed twice
First, with “Hello”
Then, with “ABCD”
What happened to the first object S referred to (“pointed” at)?

16. Garbage Collection
String object “Hello” was abandoned when S was reassigned
Memory leak?
No. Java has a built-in garbage collector
All objects carry info on # of references
When #references is 0, memory can be “collected” and placed back onto the heap
e.g String S=“Hello”
String T=S
“Hello” has two references to it
No destructors, or memory deallocator (e.g. C++ delete operator)