1. Miracle Software Systems, Inc.
Abstract Class and methods
By
Mahehs pappala,
Application Developer,
Miracle Software Systems,Inc.
Email:mpappplai@miraclesoft.com
2. 2
Abstract classes
An abstract class is a class that is declared as abstract.
abstract class MyClass {...}
An abstract class may or may not include abstract methods.
Abstract classes cannot be instantiate(crate a new instance of),
but it can be used for inheritance only.
Whenever a class extends an abstract class then the class should
compulsorily override all the abstract methods of super class.
3. abstract class common
{
abstract void accept();
abstract void display();
}
Class student extends common
{
Void accept()
{}
}
Class student extends common
{
Void accept()
{ }
Void dispaly()
{}
}
Example of Abstract classes
4. 4
Abstract classes
You can declare a class to be abstract even if it does not contain any
abstract methods
This prevents the class from being instantiated.
Any class that extends the abstract class has to provide the
implementation to the abstract methods.
5. Abstract Methods
you can declare a method without defining it:
public abstract void draw(int size);
Notice that the body of the method is missing
A method that has been declared but not defined is an abstract
method
6. 6
Why have abstract classes?
Suppose you wanted to create a class Shape, with subclasses Oval,
Rectangle, Triangle, Hexagon, etc.
You don’t want to allow creation of a “Shape”
Only particular shapes make sense, not generic ones
If Shape is abstract, you can’t create a new Shape
You can create a new Oval, a new Rectangle, etc.
Abstract classes are good for defining a general category containing
specific, “concrete” classes
7. 7
An example abstract class
public abstract class Animal {
abstract int eat();
abstract void breathe();
}
This class cannot be instantiated
Any non-abstract subclass of Animal must provide the eat() and
breathe() methods
8. 8
Why have abstract methods?
Separate interface from implementation.
What we are trying to achieve in object-oriented programming.
Allows programmers to isolate type specific details from the main part of the
code.
Client programs only use the method provided by the Shape class in the
shape hierarchy example.
Code is simpler to write and to read.
Can change types (and add new types) with this propagates to existing code.
9. 9
A problem
class Shape { ... }
class Star extends Shape {
void draw() { ... }
...
}
class Crescent extends Shape {
void draw() { ... }
...
}
Shape someShape = new Star();
This is legal, because a Star is a Shape
someShape.draw();
This is a syntax error, because some Shape might not have a draw() method
Remember: A class knows its superclass, but not its subclasses
10. 10
A solution
abstract class Shape {
abstract void draw();
}
class Star extends Shape {
void draw() { ... }
...
}
class Crescent extends Shape {
void draw() { ... }
...
}
Shape someShape = new Star();
This is legal, because a Star is a Shape
However, Shape someShape = new Shape(); is no longer legal
someShape.draw();
This is legal, because every actual instance must have a draw() method
