Object-Oriented Programming:Session 7

Software Reusability

• A new class (subclass) can be created from an existing class (superclass) to inherit its data fields and behaviors.
• The subclass can enhance the inherited behaviors and attributes with new functionalities.
• Behaviors from the superclass can be customized by the child class.
• The relationship between the subclass and superclass is an "is-a" relationship, where the child class is a more specific version of the parent class.
• An object of the subclass is also an object of the superclass.

Direct vs Indirect Inheritance

• In a class hierarchy, a direct superclass is inherited explicitly, one level down.
• An indirect superclass is inherited two or more levels down the hierarchy.
• Java supports both direct and indirect inheritance.

Single vs Multi Inheritance

• Single inheritance occurs when a subclass inherits from only one superclass.
• Multi inheritance occurs when a subclass inherits from two or more superclasses.
• Java does not support multi inheritance.
• However, Java supports the implementation of multiple interfaces.

Inheritance Design Level

Employee Class Hierarchy

• Employee class has attributes: First Name (FN), Second Name (SN), Tax Number.
• CommissionEmployee is a subclass of Employee, with additional attributes: Gross Sale, Commission Rate.
• HourlyEmployee is a subclass of Employee, with additional attributes: Wage, Hours.
• SalariedEmployee is a subclass of Employee, with additional attributes: Monthly Salary.
• BasedPlusCommissionEmployee is a subclass of Employee, with additional attributes: Base Salary.

Constructors

• A default constructor is automatically provided by the Java compiler for any class that contains no constructors.
• The default constructor has no arguments and sets fields to default values.
• If some initialization is needed, a constructor must be written explicitly.
• Constructor overloading allows multiple constructors with different signatures (argument lists).
• The specific constructor called is based on the parameters specified when "new" is executed.
• A constructor can call another constructor using the "this" keyword, which refers to the current object.
• The parent class for all Java classes is the Object class, providing automatic inheritance of methods like toString().

Java Object Life Cycle

• Objects are created using the "new" keyword and a constructor.
• Instance variables and methods are accessed using dot notation.
• An object has characteristics (state) that exist as long as the object exists.
• Object state characteristics can change in value during the object's lifetime.
• Unused objects are automatically cleaned up by the garbage collector if the program holds no more reference.
• A reference can be explicitly dropped by setting the variable holding the reference to null.

Packages

• A package is a collection of related classes and interfaces providing access protection and namespace management.
• Packages can have no modifier (default) or the "package" keyword.
• Advantages of packages include determining related types, ease of programming, reusability, and ease of finding.
• A package creates a new namespace, avoiding naming conflicts, but restricts access within the package and outside.

Creating and Using Packages

• A package should have a right name and a package statement at the top of each class belonging to the package.
• The package statement maps to the file system.
• Package names should be all lower case.
• Package members can be referred to by their fully qualified name or imported using the "import" statement.
• Importing the entire package can be done using "import" with a wildcard (*), but be careful not to import a package within a package.

Enumerated Data Type

• An enum type is a special data type that enables a variable to be a set of predefined constants.
• Enums are declared with the "enum" keyword, followed by a comma-separated list of enum constants.
• Enums extend java.lang.Enum and are implicitly public, static, and final.
• Enums can have fields, constructors, and methods, but constructors are only available within the enum itself.
• The static "values()" method returns an array of enum constants in the order they were declared.
• The static "valueOf(String str)" method returns the enumerated object whose name is str.
• The instance "name()" method returns the enum constant as a string.