Java Tutorial/Class Definition/Abstract Class
Abstract Classes
- Provide a contract between a service provider and its clients.
- An abstract class can provide implementation.
- To make an abstract method, use the abstract modifier in front of the method declaration.
<source lang="java">
public abstract class DefaultPrinter {
public String toString() {
return "Use this to print documents.";
}
public abstract void print(Object document);
}</source>
A demonstration of abstract.
<source lang="java">
abstract class A {
abstract void callme();
void callmetoo() {
System.out.println("This is a concrete method.");
}
} class B extends A {
void callme() {
System.out.println("B"s implementation of callme.");
}
} class AbstractDemo {
public static void main(String args[]) {
B b = new B();
b.callme();
b.callmetoo();
}
}</source>
Using abstract methods and classes.
<source lang="java">
abstract class Figure {
double dim1;
double dim2;
Figure(double a, double b) {
dim1 = a;
dim2 = b;
}
abstract double area();
} class Rectangle extends Figure {
Rectangle(double a, double b) {
super(a, b);
}
double area() {
System.out.println("Inside Area for Rectangle.");
return dim1 * dim2;
}
} class Triangle extends Figure {
Triangle(double a, double b) {
super(a, b);
}
double area() {
System.out.println("Inside Area for Triangle.");
return dim1 * dim2 / 2;
}
} class AbstractAreas {
public static void main(String args[]) {
Rectangle r = new Rectangle(9, 5);
Triangle t = new Triangle(10, 8);
Figure figref;
figref = r;
System.out.println("Area is " + figref.area());
figref = t;
System.out.println("Area is " + figref.area());
}
}</source>
