An abstract method is a method signature declaration with no body. For instance:
public abstract class Shape {
. . .
public abstract double getArea();
public abstract double getPerimeter();
}
The methods getArea() and getPerimeter() are abstract. Because the Shape class has an abstract method, it must be declared abstract as well. A class may also be declared abstract without any abstract methods. When a class is abstract, an instance of it cannot be created; one can only create instances of (concrete) subclasses. A concrete class is a class that is not declared abstract (and therefore has no abstract methods and implements all inherited abstract methods). For instance:
public class Circle extends Shape {
public double radius;
. . .
public double getArea() {
return Math.PI * radius * radius;
}
public double getPerimeter() {
return 2.0 * Math.PI * radius;
}
}
There are many reasons to do this. One would be to write a method that would be the same for all shapes but that depends on shape-specific behavior that is unknown at the Shape level. For instance, one could write the method:
public abstract class Shape {
. . .
public void printArea(PrintStream out) {
out.println("The area is " + getArea());
}
}
Admittedly, this is a contrived example, but it shows the basic idea: define concrete behavior in terms of unspecified behavior.
Another reason for having an abstract class is so you can partially implement an interface. All methods declared in an interface are inherited as abstract methods by any class that implements the interface. Sometimes you want to provide a partial implementation of an interface in a class and leave the details to subclasses; the partial implementation must be declared abstract.