Question of the Day  There are two escalators at each subway stop. One going up & one down.  Whenever an escalator needs to be fixed, they almost always make the working escalator go up. Why?

Sharing Among Classes

Composition has-a  Used when there is “has-a” relationship has a  Student has a name has a  Full name has a first name has an  Car has an engine has an  Rectangle has an upper right vertex  Use a field to compose classes  So we would add name field to Student class  firstName field in FullName class  If must use data externally, add getters & setters

Composition Example public class FullName { private String firstName; private String lastName; // constructor, getters & setters also here } public class Student { private FullName name; // Brevity is the soul of wit- Shakespeare public String getFirstName() { return name.getFirstName(); } public void setFirstName(String newName) { name.setFirstName(newName); }

Composition Example public class FullName { private String firstName; private String lastName; // constructor, getters & setters also here } public class Student { private FullName name; // Brevity is the soul of wit- Shakespeare public String getFirstName() { return name.getFirstName(); } public void setFirstName(String newName) { name.setFirstName(newName); }

Inheritance Is-a  “Is-a” relationships implemented via inheritance is a  Automobile is a Vehicle is a  Car is a Vehicle is an  Truck is an Automobile is an  Car is an Automobile extends  Starts with superclass which subclass extends extends  Automobile extends Vehicle extends  Truck extends Automobile extends  Car extends Automobile (& Vehicle ?)

extends extends Example extends extends extends public class Vehicle {…} public class Automobile extends Vehicle {…} public class Car extends Automobile {…} public class Truck extends Automobile {…} VehicleAutomobileCarTruck

extends extends Example  Each class extends exactly one class  extends  extends explicitly specifies superclass  If not stated, class defaults to subclass of Object VehicleAutomobileCarTruck

extends extends Example  Each class extends exactly one class  extends  extends explicitly specifies superclass  If not stated, class defaults to subclass of Object ObjectVehicleAutomobileCarTruck

extends extends Example  Extended by as many classes as heart desires  Automobile superclass of Car, Truck  Vehicle superclass of Automobile, Car, Truck ObjectVehicleAutomobileCarTruck

Superclass (& Subclass)  Every class is subclass of:  superclass  superclass’s superclass  superclass’s superclass’s superclass  superclass’s superclass’s superclass’s superclass, …  Object is-ais-a  Truck is-a Automobile is-a Vehicle is-a  So Truck is-a Object also

What Gets Inherited And How?  Class inherits all members from superclass  Subclass can use them directly unless they are private  Use as if they were copied into class w/o copying

Inheritance Example public class Sailor { private String str = "Aye, aye"; public String getMyString() { return "Capt."; } } public class Pirate extends Sailor { public String getFullString() { return “not ” + getMyString(); } public Pirate() { str = "Yarr"; } public static void main(String[] args) { Pirate sub = new Pirate(); Sailor ship = new Sailor(); System.out.println(sub.getMyString()); System.out.println(ship.getMyString()); System.out.println(sub.getFullString()); System.out.println(sub.str); System.out.println(ship.str);

Inheritance Example public class Sailor { private String str = "Aye, aye"; public String getMyString() { return "Capt."; } } public class Pirate extends Sailor { public String getFullString() { return “not ” + getMyString(); } public Pirate() { str = "Yarr"; } public static void main(String[] args) { Pirate sub = new Pirate(); Sailor ship = new Sailor(); System.out.println(sub.getMyString()); System.out.println(ship.getMyString()); System.out.println(sub.getFullString()); System.out.println(sub.str); System.out.println(ship.str);

Inheritance Example public class Sailor { protected String str = "Aye, aye"; public String getMyString() { return "Capt."; } } public class Pirate extends Sailor { public String getFullString() { return “not ” + getMyString(); } public Pirate() { str = "Yarr"; } public static void main(String[] args) { Pirate sub = new Pirate(); Sailor ship = new Sailor(); System.out.println(sub.getMyString()); System.out.println(ship.getMyString()); System.out.println(sub.getFullString()); System.out.println(sub.str); System.out.println(ship.str);

What Gets Inherited And How?

Overriding Methods  Subclass can redeclare inherited method  Overloaded when different signature used  Method is called overridden if signatures identical  Specialize method execution in subclass w/this  Changed only for subclass & its subclasses  Original used by superclass & other classes  Actual method called determined by instance’s type

Overriding Methods  Call overriden method as defined in superclass  Only in subclass overriding the method  If method is not overriden then this is not needed  super  super.methodName(…) used to call method supersuper  No multiples: super.super.methodName(…)  Cannot make less accessible when overriding  However, method can make more accessible

Overriding Methods  Call overriden method as defined in superclass  Only in subclass overriding the method  If method is not overriden then this is not needed  super  super.methodName(…) used to call method supersuper  No multiples: super.super.methodName(…)  Cannot make less accessible when overriding  However, method can make more accessible

Inheritance Example public class SuperClass { public String getMyString() { return “SUPER”; } } public class SubClass extends SuperClass { public String getMyString() { return “sub ”; } public static void main(String[] args) { SubClass sub = new SubClass(); SuperClass super1 = new SuperClass(); System.out.println(sub.getMyString()); System.out.println(super1.getMyString()); System.out.println(sub.getMyString());

Your Turn  Get into your groups and complete activity

For Next Lecture  1 st quiz in class on Monday  Covers everything from first 3 weeks of school  Focus will be on objects, fields, methods, types,…  Problems like on activities, weekly assignments, more  There is weekly assignment problem on Angel  Due by 5PM Tuesday (via Assignment Submitter)  Both problems graded using provided JUnit tests