Inheritance

Inheritance  New class (derived class) is created from another class (base class).  Ex. EmployeeEmployee  HourlyEmployee  SalariedEmployee Some things like name and hireDate are common to all employees.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?

Inheritance  Employee HourlyEmployeeHourlyEmployee SalariedEmployeeSalariedEmployee  Some things are relevant to only a salaried employee or an hourly employee. Can you think of anything?Can you think of anything?

Employee public class Employee { private StringmName; private DatemHireDate; 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 ) { … } }

Inheritance syntax public class DerivedClassName extends BaseClassName {DeclarationsOfAddedStaticVariablesDeclarationsOfAddedInstanceVariablesDefinitionsOfAddedAndOverriddenMethods}

Inheritance steps 1. Define base class (e.g., Employee) 2. Define derived class public HourlyEmployee extends Employee { …}

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.

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?

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

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

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.If the return type is a class type, it can be changed to a descendent type of the original class type.

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

Super ctor  Subclass ctor may call superclass ctor. Ex. super( a, b, … );Ex. super( a, b, … ); Number of args may differNumber of args may differ Call to super() must be first line in subclass ctor.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?How can we demonstrate this?  Typically, a base class ctor should always be called.

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?How can we demonstrate this? public class Tester7 extends Tester7Superclass { public Tester7 ( String s ) { 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?

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?How can we demonstrate this? public class Tester7 extends Tester7Superclass { public Tester7 ( String s ) { 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!

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

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).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.Object is the base class for classes that don’t extend anything. Object is the ancestor of all classes.

java.lang.Object  Many methods.  See /api/java/lang/Object.html. /api/java/lang/Object.html /api/java/lang/Object.html

Types  instanceof operator Object instanceof ClassNameObject instanceof ClassName True if Object is of type ClassName; false otherwiseTrue if Object is of type ClassName; false otherwise Example:Example: Integer i = new Integer(12); Object o = i; //won’t compile (not possible): //System.out.println( (i instanceof 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