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Inheritance. Agenda. Inheritance Creating a Subclass Overriding and overloading Final keyword. Classes and Inheritance. Inheritance. A class can extend another class, inheriting all its data members and methods while redefining some of them and/or adding its own.
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Agenda • Inheritance • Creating a Subclass • Overriding and overloading • Final keyword
Inheritance • A class can extend another class, inheriting all its data members and methods while redefining some of them and/or adding its own. • Inheritance represents the is a relationship, an object of a subclass also can be treated as an object of its superclass.
Whyinheritance • To take advantage code reuse. • To use polymorphism.
Example // Circle.java: Class definition for describing Circle public class Circle { private double radius; public Circle() {} public Circle(double radius) { this.radius = radius; } public double getRadius() { return radius; } /** Set a new radius */ public void setRadius(double radius) { this.radius = radius; } /** Return area */ public double findArea() { return radius * radius * Math.PI; } }
Example, cont. // Cylinder.java: Class definition for describing Cylinder public class Cylinder extends Circle { private double length = 1; /** Return length */ public double getLength() { return length; } /** Set length */ public void setLength(double length) { this.length = length; } /** Return the volume of this cylinder */ public double findVolume() { return findArea() * length; } } UML Diagram
Example, cont. Cylinder cylinder = new Cylinder(); System.out.println("The length is " + cylinder.getLength()); System.out.println("The radius is " + cylinder.getRadius()); System.out.println("The volume of the cylinder is " + cylinder.findVolume()); System.out.println("The area of the circle is " + cylinder.findArea()); The output is: The length is 1.0 The radius is 1.0 The volume of the cylinder is 3.14159 The area of the circle is 3.14159
InheritanceHierarchy • A class may have several subclasses and each subclass may have subclasses of its own. • The collection of all subclasses descended from a common ancestor is called an inheritance hierarchy.
Creating a Subclass Creating a subclass extends properties and methods from the superclass. You can also: • Add new properties • Add new methods • Override the methods of the superclass.
UsingtheKeywordsuper • The keyword super refers to the superclass of the class in which super appears. • This keyword can be used in two ways: • To call a superclass constructor • To call a superclass method
Aresuperclass’s Constructor Inherited? No. They are not inherited. They are invoked explicitly or implicitly from subclass. Explicitly using the super keyword. Implicityif super is not used.
CAUTION • You must use the keyword super to call the superclass constructor. • Invoking a superclass constructor’s name in a subclass causes a syntax error. • Java requires that the statement that uses the keyword super appear first in the constructor.
Overriding Methods • A subclass inherits methods from a superclass. • Sometimes it is necessary to modify the method defined in the superclass. • This is referred to as method overriding.
Overriding Methods public class Circle { private double radius; public Circle() {} public Circle(double radius) { this.radius = radius; } public double getRadius() { return radius; } /** Set a new radius */ public void setRadius(double radius) { this.radius = radius; } /** Return area */ public double findArea() { return radius * radius * Math.PI; } }
Overriding Methods // Cylinder.java: New cylinder class that overrides the findArea() // method defined in the circle class. public class Cylinder extends Circle { /** Return the surface area of this cylinder. The formula is * 2 * circle area + cylinder body area */ public double findArea() { return 2 * super.findArea() + 2 * getRadius() * Math.PI * length; } // Other methods are omitted }
Overriding a method rules • The argument list must exactly match.(If they don't match, call it overloaded). • The return type must be the same. • The access level can't be more restrictive. • The access level can be less restrictive.
Overriding a method rules, cont. • Instance methods can be overridden only if they are inherited. • A subclass within the same package can override any superclass method that is not marked private or final. • A subclass in a different package can override only those non-final methods marked public or protected. • You cannot override a method marked final.
MethodOverloading • Method overloading: having multiple methods with the same name but different signatures in a class. • You can overload superclass method public class Animal { public void eat() { System.out.println("Generic Animal Eating Generically"); } } public class Horse extends Animal { public void eat() { System.out.println("Horse eating hay "); } public void eat(String s) { System.out.println("Horse eating " + s); } }
The final Modifier • Thefinalclass cannot be extended: final class Math { ... } • The final variable is a constant: final static double PI = 3.14159; • The final method cannot be overridden by its subclasses.
