Inheritance

Last modified Thursday, 18-Apr-2024 11:07:34 UTC.

Inheritance by Extending a Class
- The Big Picture
- Inheritance to Specialize (Slides)
  - Common data and methods of classes can be factored out into a superclass.
  - Example: ColdMedicine.java, ColdSyrup.java, ColdPills.java, UseColdMedicine.java (Class picture)
- Inheritance to Add Functionality
  - New data and methods can be added to an existing class by extending it.
  - Example: Car.java, FastCar.java, UseFastCar.java (Class picture)
  - Example: Student.java, GraduateStudent.java, UndergraduateStudent.java, UseStudent.java (Class picture)
- A Big Example: HouseOccupant.java, Human.java, Pet.java, Family.java, UseFamily.java
Abstract Classes and Methods (Slides)
- Example: H2O.java, Water.java, Ice.java, Steam.java, UseH2O.java (Class picture)
The Object Hierarchy
- Every class that does not explicitly extend another class automatically extends the Object class.
- That means every class inherits from the Object class.
- Example: IOExceptions
- The methods are very general!
- Beware the difference between deep and shallow cloning and equality
  - It is often necessary to override the Object class' clone and equals methods. (Picture - classes, Picture - shallow, Picture - deep)
  - Note the implements Cloneable (Interfaces, coming soon)
  - Example: CloneGun.java, CloneCop.java, UseCloneCop.java
Polymorphism (Slides)
- It is necessary to add a "dummy" method/data in the superclass for every "new" subclass method/data.
- Example: UseStudent2.java

Exercises

What is inheritance used for?
What keyword is used to show that a class is a subclass of another?
What is an abstract class? What is an abstract method?

Exam Style Questions

In terms of inheritance, define the terms "superclass", "subclass", "override".
What feature of Java allows a variable of a superclass to refer to an object of a subclass?

Given the Fruit class below, write a subclass Watermelon that adds a counter of the number of pips (initialized to 0 when a Watermelon object is created) and a mutator method that increments numberOfPips. The subclass does not need to override the toString method.

//=============================================================================
public class Fruit {
//-----------------------------------------------------------------------------
    private String name;
//-----------------------------------------------------------------------------
    public Fruit(String name) {

        this.name = name;
    }
//-----------------------------------------------------------------------------
    public String toString() {

        return(name);
    }
//-----------------------------------------------------------------------------
}
//=============================================================================

What data and methods do objects of the class Senator have.

//=============================================================================
public class Politician {
//-----------------------------------------------------------------------------
    private String name;
    protected double fractionOfVote;
//-----------------------------------------------------------------------------
    public Politician(String name) {

        this.name = name;
        fractionOfVote = 0.0;
    }
//-----------------------------------------------------------------------------
    public String toString() {

        return(name);
    }
//-----------------------------------------------------------------------------
    public void setFractionOfVote(double fraction) {

        fractionOfVote = fraction;
    }
//-----------------------------------------------------------------------------
}
//=============================================================================
//=============================================================================
public class Senator extends Politician {
//-----------------------------------------------------------------------------
    private String district;
    private double moneySpentInElection;
//-----------------------------------------------------------------------------
    public Senator(String name,String district,double moneySpent) {

        super(name);
        this.district = district;
        moneySpentInElection = moneySpent;
    }
//-----------------------------------------------------------------------------
    public String toString() {

        return(super.toString() + " from " + district);
    }
//-----------------------------------------------------------------------------
    public void getFractionOfVote() {

        return(fractionOfVote);
    }
//-----------------------------------------------------------------------------
}
//=============================================================================