Inheritance, Superclass, Subclass

Inheritance: 9.1.1

• inheritance is a feature of Object-Oriented Programming
  • All classes can inherit instance variables or methods from another class
    • The class being inhereted from is the parent class or superclass
    • This class becomes the child class or subclass of the parent
    • A class can only have one parent
• A child class is the same kind of thing as a parent
  • A car is a kind of vehicle
    • Vehicle is the parent class, Car is the child
• All classes inherit from java.lang.Object

UML

• A UML (Unified Modeling Language) class diagram shows classes and relationshis between classes
• An open triangle points to the parent class

Subclass extends Superclass: 9.1.2

• To make a subclass inheret from a superclass, use the keyword extends

public class Car extends Vehicle
public class Motorcycle extends Vehicle

Why Use Inheritance?: 9.1.3

• Inheritance allows you to reuse data and behavior from a parent class
  • Generalization
    • Generalize many classes into the same “umbrella”
• It is useful for specialization
  • You want most of the parent behavior
    • You want to do a few things differently

Test for inheritance

• You can test for inheritance by using the instanceof operator

public class Person {}  // stub code

public class Student extends Person {} // stub code

Person p = new Person("John");
Student s = new Student("James");

if (s instanceof Person) {
    // this will run; is true!
}

if (p instanceof Student) {
    // this will not run; is false!
}

is-a vs. has-a: 9.1.4

• Another type of relationship is the has-a relationship
  • association relationship
  • This is when a class has a reference to another class

UML

• The 1 near Course means there is 1 course object associated with the number on the other end of the line
  • This is an asterisk: 0 to many
  • There is one Course associated with 0-many CoursePeriods

Java

• The Course class has an array/list of CoursePeriod objects as an attribute
• A CoursePeriod has a Course attribute to hold the information about the course

import java.util.ArrayList;

public class CoursePeriod {
    private Course courseInfo;
    private int period;
}

public class Course {
    private ArrayList<CoursePeriod> periodList;
}

is-a Substitution Test: 9.1.5

• If you aren’t sure if a class should inherit another, try to see if you can substitute the subclass type for the superclass
  • If it doesn’t make sense, use association

Summary: 9.1.7

• A class heirarchy can be developed by putting common attributes and behaviors of related classes into a superclass
• Classes that extend a superclass, called subclasses, can use existing attributes in a superclass without repeating these in code
• The keyword extends is used to establish inheritance between a subclass and a superclass
  • A class can only extend one superclass
• Extending a subclass from a superclass creates an is-a relationship from the subclass to the superclass