Object-Oriented Programming:Session 7
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Questions and Answers

What is the 'is-a' relationship in inheritance?

  • A child class is a more general version of a parent class.
  • A child class is a combination of multiple parent classes.
  • A child class is unrelated to a parent class.
  • A child class is a more specific version of a parent class. (correct)

    • Which of the following is NOT supported by Java?

  • Single inheritance
  • Multi inheritance (correct)
  • Implementation of many interfaces
  • Extension of abstract classes

    • What is the purpose of inheritance design at the level of Employee entity?

  • To define the attributes of an Employee
  • To optimize the memory usage of Employee objects
  • To implement the functionality of different Employee types
  • To model the relationships between different types of Employees (correct)

    • What is an indirect superclass?

    <p>A superclass that is inherited two or more levels down the hierarchy</p> Signup and view all the answers

    What is the benefit of software reusability using inheritance?

    <p>Reducing code duplication</p> Signup and view all the answers

    What is the relationship between an object of a subclass and an object of a superclass?

    <p>An object of a subclass is a subtype of an object of a superclass</p> Signup and view all the answers

    What happens to an object when all references to it are dropped?

    <p>It becomes eligible for garbage collection</p> Signup and view all the answers

    What is the primary purpose of a package in Java?

    <p>To provide a namespace for classes and interfaces</p> Signup and view all the answers

    What is the default access modifier for a package in Java?

    <p>No modifier (default)</p> Signup and view all the answers

    What is the significance of the enum keyword in Java?

    <p>It declares a special data type with predefined constants</p> Signup and view all the answers

    What is the return type of the values() method in an enum type?

    <p>An array of enum constants</p> Signup and view all the answers

    What is the purpose of the default constructor in Java?

    <p>To set default values for instance variables</p> Signup and view all the answers

    What is the significance of the 'this' keyword in constructor overloading?

    <p>It is used to refer to the current object</p> Signup and view all the answers

    What happens to an object's instance variables when it is garbage collected?

    <p>They are removed from memory</p> Signup and view all the answers

    What is the role of the Object class in Java inheritance?

    <p>It is the parent class for all Java classes</p> Signup and view all the answers

    What is the term for the characteristics of an object that exist throughout its lifetime?

    <p>Object state</p> Signup and view all the answers

    Study Notes

    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.

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    Test your understanding of software reusability concepts, including creating subclasses, inheritance, and class hierarchies in object-oriented programming.

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