Inheritance

1. Inheritance

2. Inheritance • New class (derived class) is created from another class (base class). • Ex. • Employee • HourlyEmployee • SalariedEmployee • Some things like name and hireDate are common to all employees. • Belong in a base class • What else can you imagine belongs in the base class?

3. Inheritance • Employee • HourlyEmployee • SalariedEmployee • Some things are relevant to only a salaried employee or an hourly employee. • Can you think of anything?

4. Employee public class Employee { private String mName; private Date mHireDate; public Employee ( ) { … } public Employee ( String name, Date date ) { … } public Employee ( Employee original ) { … } public String getName ( ) { … } public Date getHireDate ( ) { … } public void setName ( String name ) { … } public void setHireDate ( Date date ) { … } public String toString ( ) { … } public boolean equals ( Employee other ) { … } }

5. Inheritance syntax public class DerivedClassName extends BaseClassName { DeclarationsOfAddedStaticVariables DeclarationsOfAddedInstanceVariables DefinitionsOfAddedAndOverriddenMethods }

6. Inheritance steps • Define base class (e.g., Employee) • Define derived class public HourlyEmployee extends Employee { … }

7. Inheritance • Base class = parent class = ancestor = superclass • Derived class = child class = descendent = subclass • Public and protected attributes and methods in the base class are available to (inherited by) the derived class.

8. Overriding methods • Say our HourlyEmployee class adds an hourlyRate attribute. • Say the base class has toString and equals methods. • The derived class inherits these but they aren’t 100% sufficient for the derived class. • What do we do?

9. Overriding methods • What do we do? • Reimplement the equals and toString methods in the derived class. • These reimplemented methods override the methods in the base class.

10. Overriding methods • Note: • What does the keyword final mean when used with instance variables? • What does the keyword final mean when used with methods? • What does the keyword final mean when used with the class definition? • How does overriding differ from overloading?

11. Covariant return type • In general, one cannot override a method and change the return type. • Exception: (Java 5 or greater) • If the return type is a class type, it can be changed to a descendent type of the original class type.

12. Public and private • Derived class can “loosen” access restrictions. • Derived class can change private to public. • Derived class can’t “tighten” them. • Derived class can’t change public to private.

13. Super ctor • Subclass ctor may call superclass ctor. • Ex. super( a, b, … ); • Number of args may differ • Call to super() must be first line in subclass ctor. • If derived class doesn’t call a base class ctor, then the no-arg ctor of the base class is automatically invoked (first). • How can we demonstrate this? • Typically, a base class ctor should always be called.

14. Super ctor • If derived class doesn’t call a base class ctor, then the no-arg ctor of the base class is automatically invoked (first). • How can we demonstrate this? public class Tester7 extends Tester7Superclass { public Tester7 ( String s ) { System.out.println( "Tester7() says hello." ); } public static void main ( String p[] ) { new Tester7( "fred" ); } } class Tester7Superclass { public Tester7Superclass ( ) { System.out.println( "Tester7Superclass() says hello." ); } } How can we make sure that this doesn’t happen?

15. Super ctor • If derived class doesn’t call a base class ctor, then the no-arg ctor of the base class is automatically invoked (first). • How can we demonstrate this? public class Tester7 extends Tester7Superclass { public Tester7 ( String s ) { System.out.println( "Tester7() says hello." ); } public static void main ( String p[] ) { new Tester7( "fred" ); } } class Tester7Superclass { private Tester7Superclass ( ) { System.out.println( "Tester7Superclass() says hello." ); } } How can we make sure that this doesn’t happen? Won’t even compile!

16. The this ctor • We can use the this ctor to call another ctor in the same class. • Must call the this ctor first • Can’t use both the this ctor and super. • Ex. public HourlyEmployee ( ) { this( “no name”, new Date(“January”, 1, 1000), 0, 0 ); }

17. Types • An object of a derived class has more than one type. • Not only its type but also the type of every one of its ancestors (all the way back to Object). • Object is the base class for classes that don’t extend anything. Object is the ancestor of all classes.

18. java.lang.Object • Many methods. • See http://java.sun.com/j2se/1.5.0/docs/api/java/lang/Object.html.

19. Types • instanceof operator • Object instanceofClassName • True if Object is of type ClassName; false otherwise • Example: Integer i = new Integer(12); Object o = i; //won’t compile (not possible): //System.out.println( (iinstanceof String) ); //OK. Compiles; runs; returns false: System.out.println( (o instanceof String) ); //false System.out.println( (o instanceof Object) ); //true System.out.println( (o instanceof Integer) ); //true