1. Lecture 6
Inheritance
CSE 1322
4/26/2018

2. OBJECTIVES How inheritance promotes software reusability.2
OBJECTIVES
How inheritance promotes software reusability.
The concepts of base classes and derived classes.
To create a derived class that inherits attributes and behaviors from a base class.
To use access modifier protected to give derived-class methods access to base-class members.
To access base-class members with base. Or super.
How constructors are used in inheritance hierarchies.
The methods of class object, the direct or indirect base class of all classes.
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3. Introduction – Main Idea3
Introduction – Main Idea
Inheritance allows a class to absorb an existing class’s members.
Terminology:
base/derived (C#, C++)
super/sub (Java)
parent/child (generic terms)
A derived class normally adds more attributes and methods to represent a more specialized group of objects.
Inheritance saves time by reusing proven and debugged high-quality software.
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4. Introduction – Powerful!4
Introduction – Powerful!
Copying/pasting code from one class to another can spread errors. Instead, use inheritance.
When designing classes, push all the common ”stuff” up into a base class.
Benefit? A simple change to the base class propagates all the way down the class hierarchy!
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5. class BaseClass { // BaseClass Members } // end of base class5
Inheritance Concept
class BaseClass { // BaseClass Members } // end of base class
Java Syntax
class DerivedClass extends BaseClass { // DerivedClass own Members } // end of derived class
C# Syntax
class DerivedClass : BaseClass { // DerivedClass own Members } // end of derived class
Note! The direction of the arrows matters and is often wrong!
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6. Inheritance Block Diagram6
Inheritance Concept
class Student {
private int Roll;
protected String Name;
public void setData(); }
class Result extends Student { //Inheriting from Student
private float Mark;
protected void setMark();
public void display(); }
Result Class
private Section
float Mark;
protected Section
void setMarks();
public Section
void display();
void setData();
Inherited from Student Class
Inheritance Block Diagram
String Name;
Question: how many attributes
does Result have? Answer: 3! It inherited 2.
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7. 7
Introduction
The direct base class is the base class which the derived class explicitly inherits.
An indirect base class is any class above the direct base class in the class hierarchy.
In C# the class hierarchy begins with class object. All classes inherit from it!
In Java, the class hierarchy begins with the class Object. All classes inherit from it!
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8. 8
 Examples
The figure lists several simple examples of base classes and derived classes.
Note that base classes are “more general,” and derived classes are “more specific.”
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9. The is-a relationship represents inheritance. 9
Introduction
The is-a relationship represents inheritance.
For example, a car is a vehicle, and a truck is a vehicle.
New classes can inherit from thousands of pre-built classes in class libraries.
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10. Base Classes and Derived Classes10
Base Classes and Derived Classes
The UML class diagram shows an inheritance hierarchy representing a university community.
Each arrow represents an is-a relationship.
Animal
Mammal
Reptile
Dog
Platypus
Lizard
Professor
A Lizard is a reptile which is an Animal
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11. 11
Multiple Inheritance
Is it possible to inherit from more than one class?
class Man { int IQ; void walk(); void wasteTime(); }
class Wolf { int ; void walk(); void howl(); }
class Werewolf { // Inherited stuff }
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12. Multiple Inheritance C++ allows for multiple inheritance
Java and C# do not allow multiple inheritance. Why?
What happens when you call the following code: Werewolf w1 = new Werewolf(); w1.walk();
Does w1 walk like a man or a wolf?
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13. private and protected members13
 private and protected members
A base class’s private members are inherited by derived classes, but are not directly accessible by derived-class methods and properties.
A base class’s protected members can be accessed by members of that base class and by members of its derived classes.
C# A base class’s protected internal members can be accessed by members of a base class, the derived classes and by any class in the same assembly.
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14. Access Modifiers in Inheritance14
Access Modifiers in Inheritance
Access Location
Access Modifiers
public
protected
default
private
Same Class
Yes
Sub-Class of the Same Package NO
Another Class of the Same Package
Sub-Class of Another Package
Sub-Class/ Class of Another Package
Figure: Base Class Member’s Visibility Modes
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15. C# Base Classes and Derived Classes15
C# Base Classes and Derived Classes
A colon (:) followed by a class name at the end of the class declaration header indicates that the class extends the class to the right of the colon.
public class Student : Person
Every class directly or indirectly inherits object’s methods.
If a class does not specify that it inherits from another class, it implicitly inherits from object.
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16. Java Base Classes and Derived Classes16
Java Base Classes and Derived Classes
The key word extends followed a class name at the end of the class declaration header indicates that the class extends the class to the right of the word extends.
public class Student extends Person
Every class directly or indirectly inherits Object’s methods.
If a class does not specify that it inherits from another class, it implicitly inherits from Object.
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17. 17
Things to Note
Constructors are not inherited and we still have default constructors
Class object’s default (empty) constructor does nothing.
Either explicitly or implicitly, a call to the base-class constructor is made.
Can’t call a base constructor with arguments that do not match the number and types of parameters.
Note that even if a class does not have constructors, the default constructor will call the base class’s default or parameterless constructor.
Let’s see an example
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18. Example of “invisible code”
class MyObject extends Object { MyObject() { super(); } } // Same thing as ↑ class MyObject { }
class MyObject : object {
MyObject() { base(); } }
// Same thing as ↑ class MyObject { }
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19. class Base { // Example in Java Base() { System. out
class Base { // Example in Java Base() { System.out.println("Base Constructor"); } } class Child extends Base { Child() { // super call is here, but you can't see it! System.out.println ("Child constructor"); class Main { public static void main(String[] args) { Child c = new Child(); // Output: Base Constructor Child Constructor
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20. A Quick Review of toString() Method20
A Quick Review of toString() Method
We include toString() in a class to return a string representing an object.
Can’t call toString() on a primitive variable (e.g. int, float..)
It is recommended that all subclasses override this method.
Question: What happens if the Derived/sub class doesn’t override toString()?
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21. C# ToString() Is designed to be overridden!21
C# ToString()
ToString() is special - it is one of the methods that every class inherits directly or indirectly from class object. This returns a string representing an object.
Is designed to be overridden!
To override a base-class method, a derived class must declare a method with keyword override.
public override string ToString() { }
The method must have the same signature (method name, number of parameters and parameter types) and return type as the base-class method.
Base method must be marked virtual (can be overridden)
Note: virtual and override go together like PB&J
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22. Java toString() Same as C#, however…22
Java toString()
Same as C#, however…
@Override is optional but it forces the compiler to check that the override is correct
@Override
public String ToString() { }
Can make a method “non-overridable” using the final keyword
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23. More on toString() Method23
More on toString() Method
The toString() returns a string representing an object.
What happens if the Derived class does not override the toString()?
package methodOverriding;
public class MyFriend {
String name;
String ;
MyFriend (String name, String ) {
this.name = name;
this. = ; }
// Without Overriding the toString() Method
public static void main(String[] args) {
MyFriend f1 = new MyFriend("John",
System.out.println(f1); //
System.out.println(f1.toString()); //
Output:
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24. More on toString() Method24
More on toString() Method
What happens if we override the toString()?
package methodOverriding;
public class MyFriend {
String name;
String ;
MyFriend (String name, String ) {
this.name = name;
this. = ; }
@Override
public String toString() // With Overriding the toString() Method
{ return "My friend: "+ name + “\t" + ; }
public static void main(String[] args) {
MyFriend f1 = new MyFriend("John",
System.out.println(f1); //
System.out.println(f1.toString()); //
Output:
My friend: John
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25. public class MyFriend { string name, ; public MyFriend(string name, string ) { this.name = name; this. = ; } public override string ToString() { return "My friend: "+name+"\t"+ ; } class MainClass { public static void Main (string[] args) { Console.WriteLine(new MyFriend("Bob", Output: My friend: Bob
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26. Rules about Overriding26
Rules about Overriding
It is a compilation error to override a method with a different access modifier.
If a public method could be overridden as a protected or private method, the derived-class objects would not respond to the same method calls as base-class objects.
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27. C# Base Classes and Derived Classes27
C# Base Classes and Derived Classes
virtual indicates that a base-class method can be overridden in derived classes. It may/not do that.
There’s another keyword (abstract) that requires a method to be overridden. Later.
A subclass may not override methods that are not marked as abstract, virtual or override
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28. Java Base Classes and Derived Classes28
Java Base Classes and Derived Classes
Similarly, the abstract keyword indicate that a base-class method must be overridden in derived classes.
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29. Problems with Protected29
Problems with Protected
Using protected instance variables creates several potential problems.
The derived-class can set an inherited variable’s value directly without validity checking.
Derived-class methods depend on the base class’s data implementation.
You should be able to change the base-class implementation while still providing the same services to the derived classes.
Declaring base-class instance variables private enables the base-class implementation of these instance variables to change without affecting derived-class implementations.
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30. Calls to a Base Class’s methods30
Calls to a Base Class’s methods
When a base/super-class method is overridden, the derived-class version often calls the base/super-class version to do a portion of the work.
Use either base or super to call those methods
The use of “chained” base references to refer to a member several levels up the hierarchy—as in base.base.Earnings() or super.super.Earnings()—is a compilation error.
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31. class Base { void doSomething(){ System. out
class Base { void doSomething(){ System.out.println("Base doSomething"); } } class Child extends Base { void doSomething() { super.doSomething(); System.out.println("Child doSomething"); class Main { public static void main(String[] args) { Child c = new Child(); c.doSomething(); Output: Base doSomething Child doSomething
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32. class Base { public virtual void doSomething(){ Console
class Base { public virtual void doSomething(){ Console.WriteLine("Base doSomething"); } } class Child : Base { public override void doSomething() { base.doSomething(); Console.WriteLine("Child doSomething"); class MainClass { public static void Main (string[] args) { Child c = new Child(); c.doSomething(); Output: Base doSomething Child doSomething
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33. 33
Commercially
Independent software vendors (ISVs) can develop and sell proprietary classes.
• Users then can derive new classes from these library classes without accessing the source code.
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34. Inherited Methods from object34
Inherited Methods from object
Method
Description
Equals(Object)
Determines whether the specified object is equal to the current object.
Equals(Object, Object)
Determines whether the specified object instances are considered equal.
Finalize()
Allows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection.
GetHashCode()
Serves as the default hash function.
GetType()
Gets the Type of the current instance.
MemberwiseClone()
Creates a shallow copy of the current Object.
ReferenceEquals(Object, Object)
Determines whether the specified Object instances are the same instance.
ToString()
Returns a string that represents the current object.
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35. Inherited Methods from Object35
Inherited Methods from Object
Method
Description
clone​()
Creates and returns a copy of this object.
equals​(Object obj)
Indicates whether some other object is "equal to" this one.
getClass​()
Returns the runtime class of this Object.
hashCode​()
Returns a hash code value for the object.
notify​()
Wakes up a single thread that is waiting on this object's monitor.
notifyAll​()
Wakes up all threads that are waiting on this object's monitor.
toString​()
Returns a string representation of the object.
wait​()
Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object.
wait​(long timeout)
Causes the current thread to wait until either another thread invokes the notify() method or the notifyAll() method for this object, or a specified amount of time has elapsed.
wait​(long timeout, int nanos)
Causes the current thread to wait until another thread invokes the notify() method or the notifyAll() method for this object, or some other thread interrupts the current thread, or a certain amount of real time has elapsed.
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