Application
Programming
Hend Alkittawi

OOP Concepts
Java Abstract Classes

ABSTRACT CLASSES
-

Sometimes it's useful to declare classes for which you never
intend to create objects. These classes are called abstract
classes.

-

The purpose of an abstract class is to provide an appropriate
superclass from which other classes can inherit and thus share
common design.

-

Abstract classes are used only as superclasses in inheritance
hierarchies.

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Abstract classes are incomplete, and cannot be used to
instantiate objects.

ABSTRACT CLASSES
-

To make a class abstract, declare it with the keyword abstract
public abstract class MyClass { … }

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Abstract classes normally contain one or more abstract methods

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An abstract method is an instance method with the keyword
abstract and its declaration

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An Abstract method do not provide implementations
public abstract myMethod();

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Constructors and static methods cannot be declared as
abstract.

ABSTRACT CLASSES
-

Subclasses must declare the missing pieces to become concrete
classes from which objects can be instantiated.

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Each concrete subclass of an abstract superclass must provide
concrete implementations of each of the superclass abstract
methods.

public abstract class Shape {

public class AbstractClassDemo {

private String color;
public String getColor() {
return color;
}
public void setColor(String color) {
this.color = color;
}
public void display() {
System.out.println("This is a " + getType() + " shape. Its color is: " +
}
public abstract String getType();
public abstract void draw();

public static void main(String[] args) {
Circle circle = new Circle();
circle.setColor("red");
circle.setRadius(5.0);
circle.draw(); // implemented abstract method
circle.display(); // concrete method from
abstract class
}
}
getColor());

}
public class Circle extends Shape{
private double radius;
public double getRadius() {
return radius;
}
public void setRadius(double radius) {
this.radius = radius;
}
public void draw() {
System.out.println("Drawing a " + getColor() + " circle with radius " + getRadius() + ".");
}
@Override
public String getType() {
return "Circle";
}
}

public abstract class Shape {
private String color;
public Shape(String color) { this.color = color; }
public String getColor() { return color; }
public void setColor(String color) { this.color = color; }
public void display(){ System.out.println("This is a " + getType() + " shape. Its color is: " +
public abstract String getType();
public abstract void draw();

getColor()); }

}
public class Circle extends Shape{

public class AbstractClassDemo {

private double radius;
public Circle(String color, double radius) {
super(color);
this.radius = radius; }

public static void main(String[] args) {
//Shape shape = new Shape("blue"); // invalid
Circle circle = new Circle("red", 5.0);
circle.draw(); // implemented abstract method
circle.display(); // concrete method from
abstract class
}

public double getRadius() { return radius; }
public void setRadius(double radius) { this.radius = radius; }
}
@Override
public void draw() {
System.out.println("Drawing a " + getColor() + " circle with radius " + getRadius() + ".");
}
@Override
public String getType() {
return "Circle"; }
}

CODE DEMO
-

Create classes to demo
abstract classes
concepts!

DO YOU HAVE ANY
QUESTIONS?

THANK
YOU!

@

hend.alkittawi@utsa.edu

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