1. Object Based Programming
Chapter 8

2. In This Chapter We will learn about classes Garbage Collection
Data Abstraction and encapsulation

3. Contrast Procedural Languages Object Oriented Languages
Action oriented
Concentrate on writing functions
Data supports the actions
Object Oriented Languages
Concentrate on creating reference types
Look for nouns which indicate classes
Fields are data members
Methods support manipulation of the objects

4. Find the Objects … the Verbs
What are verbs that represent things that the objects do?
In this game, what are nouns that are potential classes?

5. Abstract Data Types (ADTs)
Classes in Java make creation of ADTs easier
Implementation details hidden from users of the class
Client code not dependent on implementation
Example of a class
Figure 8.1
Time class – keeps track of time in 24 hour format

6. Declaring Classes Usually data fields are specified as private
Methods are declared as public
Any class member (data or method) which does not need to be accessed outside the class should be private
It does not matter which comes first, data fields or methods
The author prefers data fields first

7. Constructors Methods which have the same name as the class
Included in a class to ensure that instance variables contain valid values when objects are created
The constructor is called when a class object is created with the new
Note:
Do not have the constructor return a value

8. Using Classes View Figure 8.2 Note Advantages of classes
Creation of a time object with new
Call of the constructor
Use of public methods to get and set the private data values
Use of toString functions
Advantages of classes
Simplifies client perception of classes
Reduces number of parameters

9. Class Scope Variables and methods belong to the class's scope
Inside the class's scope
All class members are available, accessible
Outside the class's scope
Client code that uses the class
Only public members are available, accessible
Access modifiers public and private control this availability

10. Controlling Access to Members
The following would not be allowed – why?
The data members are private.
public class TimeTest2 {
public static void main( String args[] ) {
Time1 t = new Time1();
t.hour = 7; }

11. Using the this Reference
Every object can access a reference to itself
Use the keyword this
If a method has a local variable with same name as class variable, use the this to access the class variable
Explicit use of this can increase program clarity for human readers
Note example, Figure 8.4

12. Using Overloaded Constructors
Multiple constructors for the same class are allowed
The signature of the constructors must be different
Different numbers and/or types of parameters
See example in Figure 8.5 and 8.6

13. Using Set and Get Methods
Private data fields manipulated only by provided public methods
Some methods used to set the data values
Others used to get the values
Return the value directly
Print the value
Note: this is not the same as making the data values public
The set methods are written to ensure valid data values

14. Composition
A class can have references to objects of other classes as members
Example:
An alarm clock class contains a Time object as one of its members
Employee class has Date object as one of its members
See
Date Class, Figure 8.7
Employee Class Figure 8.8
Employee Test program Figure 8.9

15. Enumerations – enum types
Declared with an enum declaration
A comma-separated list of enum constants
Declares an enum class with the following restrictions:
enum types are implicitly final
enum constants are implicitly static
Attempting to create an object of an enum type with new is a compilation error

16. Enumerations – enum types
enum constants can be used anywhere constants can
enum constructor
Like class constructors, can specify parameters and be overloaded
Note use from chapter 6, Figure 6.9
See example, Figure 8.10
Note test program, Figure 8.11

17. Enumerations – enum types
static method values
Generated by the compiler for every enum
Returns an array of the enum’s constants in the order in which they were declared
static method range of class EnumSet
Takes two parameters, the first and last enum constants in the desired range
Returns an EnumSet containing the constants in that range, inclusive
An enhanced for statement can iterate over an EnumSet as it can over an array

18. Enumeration as a Class When an enumeration is defined
A class is created
Default methods include
toString
equals
ordinal
valueOf
Also possible to provide additional methods

19. Enumeration as a Class
Consider the following example of an enumeration of card suits
View definition of class Suit
Note constructor, getColor
View demonstration program

20. Enumeration as a Class View class LetterGrade
An enumeration class
Note class elements
Consider the following code LetterGrade myGrade = LetterGrade.B_PLUS;
Discuss the results of calls to various LetterGrade methods

21. Garbage Collection Garbage collection finalize method
JVM marks an object for garbage collection when there are no more references to that object
JVM’s garbage collector will retrieve those objects memory so it can be used for other objects
See lines 27 – 32 of Figure 8.13
finalize method
All classes in Java have the finalize method
Inherited from the Object class
finalize is called by the garbage collector when it performs termination housekeeping
finalize takes no parameters and has return type void

22. static Class Members
Normally each class object has its own copy of the instance variables
Possible to specify a variable that is shared by all existing objects of the class
Called a static variable
Also called a class variable – class wide information
Also possible to have a static method
Can be called even if no object of the class has been instantiated
Use class name, then dot, then static method name
Note examples in Fig. 8.12, test pgm Fig. 8.13

23. static Class Members String objects are immutable
String concatenation operations actually result in the creation of a new String object
static methods cannot access non-static class members
Also cannot use the this reference

24. static Class Members static method gc of class System
Indicates that the garbage collector should make a best-effort attempt to reclaim objects eligible for garbage collection
It is possible that no objects or only a subset of eligible objects will be collected
Note example call in Fig. 8.13

25. static Import Recall static fields and methods of class Math
J2SE 5.0 enables importing static members as if declared in class where used
Note Figure 8.14 import static java.lang.Math.*

26. Final Instance Variables
Principle of least privilege
Code should have only the privilege and access it needs to accomplish its task, but no more
final instance variables
Keyword final
Specifies that a variable is not modifiable (is a constant)
final instance variables can be initialized at their declaration
If they are not initialized in their declarations, they must be initialized in all constructors – Fig. 8.15

27. Packages Packages enable grouping together multiple related classes
Specify a class to be part of a package with first line package myStuff;
Place all classes in same directory which is named with the name of the package
In your program which uses the package import myStuff.*;

28. Software Reusability Many classes exist in the Java API
Avoid "reinventing the wheel"
Study capabilities of Java API
If it has a class that fits your needs, use it rather than creating your own

29. Data Abstraction & Encapsulation
Information hiding
Classes hide details of implementation from client
Example:
Stack can be implemented with array or with linked list
Client program which depends on one implementation will have problems if new version with different implementation comes out
Only use the methods provided