Inheritance Recitation - 02/22/2008 CS 180 Department of Computer - - PowerPoint PPT Presentation
Inheritance Recitation - 02/22/2008 CS 180 Department of Computer Science, Purdue University Announcements Project 3 grades are out. Collect at the end of the recitation. Project 5 has been posted. 2-week project.
2-week project.
Milestone due 27th Feb. Final submission due 5th March.
You are seriously recommended to start it early.
A reminder regarding the course policy for CS180 -
Inheritance allows us to define a general class
A more specialized class inherits the properties of
An inherit class (or a sub-class) can inherit
Example
General class Vehicle might have instance
More specialized class Automobile might
General class Vehicle might also be used
class Person { private String name; public Person() { name = “no name”; } public Person(String _name) { name = _name; } //setName(String) and getName() methods //sameName(String) method //writeOutput() method }
Even though your program may not need any Person or Student objects, these classes can be
For example, all students, faculty, and staff
is a sub-class of class Person and class Person is called the base class.
public class Student extends Person{ private int studentNumber; public Student(String _name, int _num){ super(_name); studentNumber = _num; } //more methods not in Person class //writeOutput() overwrites the method in Person class
}
When you override a method, the new
When the name is the same, but the number
You can prevent a method definition from being
example public final void someMethod() { … } The method someMethod() will not be
A base class has its own constructors.
Their purpose typically is to initialize the
A derived class has its own constructors
Their purpose typically is to call a
The call to the constructor in the super class
When no call to the constructor in the super
super(initialName);
equivalent definitions: public Student() { super(); studentNumber= 0; }
public Student() { //java automatically includes a call to the //default constructor of the super-class studentNumber= 0; }
Recall that within the definition of one
Example – Student() { this (“Default Name”, 0); } Student(String name, int _num){ ….. }
Any use of this must be the first action in the
Thus, a constructor definition cannot contain a
To use both super and this, include a call using
Student() { this (“Default Name”, 0); } Student(String name, int _num){ super(); ….. }
Example
You cannot use super to invoke a method in
Class Undergraduate can be derived from class Student which is derived from class Person. Class Undergraduate will have all the instance
Person.
class Undergraduate extends Student { private int level; public Undergraduate(){ super(); level = 1; } //more methods specific to an undergraduate void writeOutput(){ //override writeOutput() …. } }
If class Undergraduate is derived from class Student and class Student is derived from
Undergraduate is also an object of class Student
A reference to an object of a sub-class can
However, a reference to an object of an
In Java, every class descends from (and inherits
Therefore, every object of every class is of type
Unless a class is declared explicitly to be a
An object of any class can substituted when a
Every class inherits some methods from the class
Method toString
An abstract class is not intended to be used
Abstract method – By declaring one or more
example public abstract void drawHere(); A class that has at least one abstract method
Public abstract class Figure{ private int offset; public abstract void drawHere(); public void drawAt(int num){ int count; for(count = 0; count < num; count++){ System.out.println(); } drawHere(); } }
An interface specifies the headings for methods
Example -
A class that implements an interface must
To implement an interface, a class must
include the phrase
implements Interface_Name
class CS180 implements Writable {…}
implement all the method headings listed in
Different objects can invoke different method
For example, if Square and Triangle are sub-
Handling the invocation of a method that may be
The type of object being referenced at the time of
different methods.
To use a method name in the class Student
Example
Recall the Person class with sub-classes
Person[] x = new Person[100]; x[0] = new Student(); x[1] = new Graduate(); x[2] = new Staff(); ..
A or a subclass of A
A
instance of class A or a subclass of A
public class Animal { protected int age; public void makeSound() { SOP(“Make sound!”); } } public class Ferret extends Animal { public Ferret() { super(); } public void eatSweets() { SOP(“Yummy!”); } public void makeSound() { SOP(“Squeal!”); } } public class Platypus extends Animal { public Platypus() { super(); } public void swim() { SOP(“Swim!”); } public void makeSound() { SOP(“Quack!”); } } Animal a = new Ferret(); if (a instanceof Ferret) ((Ferret)a).eatSweets(); a = new Platypus(); if (a.getClass() == Platypus.class) ((Platypus)a).swim(); if (Platypus.class.isAssignableFrom(a)) ((Platypus)a).swim(); ________________________________ Animal a = new Ferret(); ((Platypus)a).swim(); // compiles, but runtime error!
Dynamic binding refers to the process carried out
Polymorphism can be thought of as something
Polymorphism, encapsulation, and inheritance
You have become acquainted with
You have learned how to define and use
You have learned about dynamic binding
abstract public class Animal {…} abstract public class Bird extends Animal {…} abstract public class Mammal extends Animal {…} abstract public class Fish extends Animal {…} public class Owl extends Bird {…} public class Bat extends Mammal {…} public class Whale extends Mammal {…} public class Shark extends Fish {…}
Which instantiations are valid? Animal a = new Animal(); Animal b = new Fish(); Animal c = new Shark(); Mammal d = new Bat(); Fish e = new Mammal(); Bird b = new Owl();