Final and Abstract Classes - Back to Basics :-)

Restricting Inheritance

Final Members: A way for Preventing Overriding of Members in Subclasses

All methods and variables can be overridden by default in subclasses.

This can be prevented by declaring them as final using the keyword “final” as a modifier. For example:

final int marks = 100;

final void display();

This ensures that functionality defined in this method cannot be altered any. Similarly, the value of a final variable cannot be altered.

Final Classes: A way for Preventing Classes being extended

• We can prevent an inheritance of classes by other classes by declaring them as final classes.
• This is achieved in Java by using the keyword final as follows:

final class Marks 

{ // members 

} 

final class Student extends Person 

{ // members 

}

• Any attempt to inherit these classes will cause an error.

Abstract Classes

• When we define a class to be “final”, it cannot be extended. In certain situation, we want to properties of classes to be always extended and used. Such classes are called Abstract Classes.



• An Abstract class is a conceptual class.



• An Abstract class cannot be instantiated – objects cannot be created.



• Abstract classes provides a common root for a group of classes, nicely tied together in a package:

abstract class ClassName
{
...
…
abstract Type MethodName1();
…
…
Type Method2() 
{
    // method body
}
}

• When a class contains one or more abstract methods, it should be declared as abstract class.



• The abstract methods of an abstract class must be defined in its subclass.



• We cannot declare abstract constructors or abstract static methods.

Abstract Class -Example

Shape is a abstract class.

• The Shape Abstract Class

public abstract class Shape {
        public abstract double area(); 
        public void move() { // non-abstract method
            // implementation
       }
}

• Is the following statement valid?

Shape s = new Shape();

• No. It is illegal because the Shape class is an abstract class, which cannot be instantiated to create its objects.

Abstract Classes

public Circle extends Shape {
    protected double r;
    protected static final double PI =3.1415926535;
    public Circle() { r = 1.0; )
    public double area() { return PI * r * r; }
…
}
public Rectangle extends Shape {
    protected double w, h;
    public Rectangle() { w = 0.0; h=0.0; }
    public double area() { return w * h; }
}

Abstract Classes Properties

• A class with one or more abstract methods is automatically abstract and it cannot be instantiated.



• A class declared abstract, even with no abstract methods can not be instantiated.



• A subclass of an abstract class can be instantiated if it overrides all abstract methods by implementation them.



• A subclass that does not implement all of the superclass abstract methods is itself abstract; and it cannot be instantiated.

Summary

• If you do not want (properties of) your class to be extended or inherited by other classes, define it as a final class.



• Java supports this is through the keyword “final”.



• This is applied to classes.



• You can also apply the final to only methods if you do not want anyone to override them.



• If you want your class (properties/methods) to be extended by all those who want to use, then define it as an abstract class or define one or more of its methods as abstract methods.



• Java supports this is through the keyword “abstract”.



• This is applied to methods only.



• Subclasses should implement abstract methods; otherwise, they cannot be instantiated.

Interfaces

Design Abstraction and a way for loosing realizing Multiple Inheritance

• Interface is a conceptual entity similar to a Abstract class.



• Can contain only constants (final variables) and abstract method (no implementation) - Different from Abstract classes.



• Use when a number of classes share a common interface.



• Each class should implement the interface.

Interfaces: An informal way of realising multiple inheritance

• An interface is basically a kind of class—it contains methods and variables, but they have to be only abstract classes and final fields/variables.



• Therefore, it is the responsibility of the class that implements an interface to supply the code for methods.



• A class can implement any number of interfaces, but cannot extend more than one class at a time.



• Therefore, interfaces are considered as an informal way of realising multiple inheritance in Java.

Interface - Example

Interfaces Definition

Syntax (appears like abstract class):

interface InterfaceName {
    // Constant/Final Variable Declaration
    // Methods Declaration – only method body
}

Example:

interface Speaker {
    public void speak( );
}

Implementing Interfaces

Interfaces are used like super-classes who properties are inherited by classes. This is achieved by creating a class that implements the given interface as follows:

class ClassName implements InterfaceName [, InterfaceName2, …]
{
    // Body of Class
}

Implementing Interfaces Example

class  Politician implements Speaker {
        public void speak(){
            System.out.println(“Talk politics”);
        }
}

----------------------------------------------
class  Priest implements Speaker {
        public void speak(){
            System.out.println(“Religious Talks”);
        }
}
----------------------------------------------
class  Lecturer implements Speaker {
        public void speak(){
            System.out.println(“Talks Object Oriented Design and Programming!”);
        }
}

Extending Interfaces

Like classes, interfaces can also be extended. The new sub-interface will inherit all the members of the superinterface in the manner similar to classes. This is achieved by using the keyword extends as follows: 

interface InterfaceName2 extends InterfaceName1 {
    // Body of InterfaceName2
}

Inheritance and Interface Implementation

A general form of interface implementation:

class ClassName extends SuperClass implements InterfaceName [, InterfaceName2, …]
{
    // Body of Class
}

This shows a class can extended another class while implementing one or more interfaces. It appears like a multiple inheritance (if we consider interfaces as special kind of classes with certain restrictions or special features).

Student Assessment Example

nConsider a university where students who participate in the national games or Olympics are given some grace marks. Therefore, the final marks awarded = Exam_Marks + Sports_Grace_Marks. A class diagram representing this scenario is as follow:

class Student
{
    // student no and access methods
}
interface Sport
{
    // sports grace marks (say 5 marks) and abstract methods
}
class Exam extends Student
{
    // example marks (test1 and test 2 marks) and access methods
}
class Results extends Exam implements Sport
{
    // implementation of abstract methods of Sport interface
    // other methods to compute total marks = test1+test2+sports_grace_marks;
    // other display or final results access methods
}

Interfaces and Software Engineering

• Interfaces, like abstract classes and methods, provide templates of behaviour that other classes are expected to implement.



• Separates out a design hierarchy from implementation hierarchy. This allows software designers to enforce/pass common/standard syntax for programmers implementing different classes.



• Pass method descriptions, not implementation



• Java allows for inheritance from only a single superclass. Interfaces allow for class mixing.



• Classes implement interfaces.

Summary

• Class is a collection of data and methods that operate on that data



• An object is a particular instance of a class



• Object members accessed with the ‘dot’ (Class.v)



• Instance variables occur in each instance of a class



• Class variables associated with a class



• Objects created with the new keyword



• Objects are not explicitly ‘freed’ or destroyed. Java automatically reclaims unused objects.



• Java provides a default constructor if none defined.



• A class may inherit the non-private methods and variables of another class by subclassing, declaring that class in its extends clause.



• java.lang.Object is the default superclass for a class. It is the root of the Java hierarchy.



• Method overloading is the practice of defining multiple methods which have the same name, but different argument lists



• Method overriding occurs when a class redefines a method inherited from its superclass



• static, private, and final methods cannot be overridden



• From a subclass, you can explicitly invoke an overridden method of the superclass with the super keyword. 



• Data and methods may be hidden or encapsulated within a class by specifying the private or protected visibility modifiers.



• An abstract method has no method body. An abstract class contains abstract methods.



• An interface is a collection of abstract methods and constants. A class implements an interface by declaring it in its implements clause, and providing a method body for each abstract method.