Object Based Programming

Object Based Programming Chapter 8

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

Contrast • Procedural 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

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?

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

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

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

Using Classes • View Figure 8.2 • Note • 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

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

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; } }

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

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

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

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

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

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

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

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

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

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

Garbage Collection • Garbage collection • 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

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

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

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

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 8.14import static java.lang.Math.*

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

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

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

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

Object Based Programming