Question of the Day  Thieves guild states it will sell to members: lock picking kits  $0.67 each 40’ rope  $2.12 each Wire cutters  $4.49 each How much would one pay for 4 lock picking kits, 8 ropes, and 5 wire cutters?

Question of the Day  Thieves guild  Thieves guild states it will sell to members: lock picking kits  $0.67 each 40’ rope  $2.12 each Wire cutters  $4.49 each How much would one pay for 4 lock picking kits, 8 ropes, and 5 wire cutters?  Nothing, they’d steal it.

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

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  Subclass extends superclass for inheritance extends  Automobile extends Vehicle extends  Truck extends Automobile extends  Car extends Automobile

Example of an Object Hierarchy extends extends extends public class Vehicle {…} public class Automobile extends Vehicle {…} public class Car extends Automobile {…} public class Truck extends Automobile {…} ObjectVehicleAutomobileCarTruck

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

What Gets Inherited And How?

What About Constructors? "is-a"  Fields and methods inherited since have "is-a"  Car is NOT a Vehicle without same data & actions  Adding members allowed, since Car more specific  But we (almost) always have access to fields & methods  Only during instantiation are constructors used  Cannot use elsewhere so would they help subclass?  Name is specific & tied to superclass; how to use?

Inherit Such a Course Word  Superclass constructor still needed by subclass… is-a  Remember, subclass instance is-a superclass instance  Instantiating superclass, too, when creating subclass  Somehow need to initialize instances superclass part  … but constructors not inherited (in same way)

Constructors Chains  Java requires reusing superclass’s constructor  Already initializes fields in superclass  Provides us with way to maximize laziness  Allows sharing constructor code within class, too  Still want to avoid copying code  Often do identical work initializing fields  Constructors increase laziness through chaining

Constructors Chains  Java requires reusing superclass’s constructor  Already initializes fields in superclass  Provides us with way to maximize laziness  Allows sharing constructor code within class, too  Still want to avoid copying code  Often do identical work initializing fields  Constructors increase laziness through chaining

Chains Explained

About To Get a Little Bumpy

Chaining Constructors Required  Every Java constructor must be chained…  this(…) or super(…) can be used to do chaining  Class loops forever or will call superclass constructor  But why is requirement new to you this lecture?

Chaining Constructors Required  Every Java constructor must be chained…  this(…) or super(…) can be used to do chaining  Class loops forever or will call superclass constructor  But why is requirement new to you this lecture?

Implicit Chaining Sounds Dirty  If chaining not explicit made at constructor start  Java adds implicit call to super() automatically  Superclass needs constructor with no parameters…  … which exists in Object (default superclass)  But this empty constructor may not be in your classes

Constructors Not Inherited  Constructors not inherited directly by subclass  Calling superclass constructor relies upon chaining  Only via chaining is constructor accessible in subclass  Trying to use superclass constructor for new illegal  Not all classes define constructor of their own  If not defined, automatically get one with no params  Implied chain used by this implicit constructor must  To do this superclass must include usable constructor

Stickers public class Sticker { protected String text; public Sticker(String words) { text = words; } public void printMe() { System.out.println(text); } } public class CSticker extends Sticker { private String color; public CSticker(String clr, String type) { text = type; color = clr; } public static void main(String[] args) { Sticker s = new Sticker(“boo”); CSticker cs = new CSticker(“hoo”); s.printMe(); cs.printMe(); } }

Stickers public class Sticker { protected String text; public Sticker(String words) { text = words; } public void printMe() { System.out.println(text); } } public class CSticker extends Sticker { private String color; public CSticker(String clr, String type) { text = type; color = clr; } public static void main(String[] args) { Sticker s = new Sticker(“boo”); CSticker cs = new CSticker(“hoo”); s.printMe(); cs.printMe(); } }

Stickers public class Sticker { protected String text; public Sticker(String words) { text = words; } public void printMe() { System.out.println(text); } } public class CSticker extends Sticker { private String color; public CSticker(String clr, String type) { text = type; color = clr; } public CSticker(String type) { this("blue", type); } public static void main(String[] args) { Sticker s = new Sticker(“boo”); CSticker cs = new CSticker(“hoo”); s.printMe(); cs.printMe(); } }

Stickers public class Sticker { protected String text; public Sticker(String words) { text = words; } public void printMe() { System.out.println(text); } } public class CSticker extends Sticker { private String color; public CSticker(String clr, String type) { text = type; color = clr; } public CSticker(String type) { this("blue", type); } public static void main(String[] args) { Sticker s = new Sticker(“boo”); CSticker cs = new CSticker(“hoo”); s.printMe(); cs.printMe(); } }

Stickers public class Sticker { protected String text; public Sticker(String words) { text = words; } public void printMe() { System.out.println(text); } } public class CSticker extends Sticker { private String color; public CSticker(String clr, String type) { text = type; color = clr; } public CSticker(String type) { this("blue", type); } public static void main(String[] args) { Sticker s = new Sticker(“boo”); CSticker cs = new CSticker(“hoo”); s.printMe(); cs.printMe(); } }

Stickers public class Sticker { protected String text; public Sticker(String words) { text = words; } public void printMe() { System.out.println(text); } } public class CSticker extends Sticker { private String color; public CSticker(String clr, String type) { super(type); color = clr; } public CSticker(String type) { this("blue", type); } public static void main(String[] args) { Sticker s = new Sticker(“boo”); CSticker cs = new CSticker(“hoo”); s.printMe(); cs.printMe(); } }

Stickers public class Sticker { protected String text; public Sticker(String words) { text = words; } public void printMe() { System.out.println(text); } } public class CSticker extends Sticker { private String color; public CSticker(String clr, String type) { super(type); color = clr; } public CSticker(String type) { this("blue", type); } public static void main(String[] args) { Sticker s = new Sticker(“boo”); CSticker cs = new CSticker(“hoo”); s.printMe(); cs.printMe(); } }

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

Polymorphism And Assignment

Types in Java  In Java, use reference variables like silhouettes  Compiler uses variable's type to see what is legal  But (may) lose detail: this ignores instances type  Important distinction between these two types  Variable's type determines if code can compile  Call method based upon instance's type

Inheritance Example public class SuperClass { public String getMyString() { return “SUPER”; } } public class SubClass extends SuperClass { public String getMyString() { return “sub ” + super.getMyString(); } public String otherMethod() { return "other"; } 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.otherMethod()); super1 = sub; System.out.println(super1.getMyString()); System.out.println(super1.otherMethod()); SubClass sub2 = super1;

Inheritance Example public class SuperClass { public String getMyString() { return “SUPER”; } } public class SubClass extends SuperClass { public String getMyString() { return “sub ” + super.getMyString(); } public String otherMethod() { return "other"; } 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.otherMethod()); super1 = sub; System.out.println(super1.getMyString()); System.out.println(super1.otherMethod()); SubClass sub2 = super1;

Your Turn  Get into your groups and complete activity

For Next Lecture  Read GT2.4 for Friday  What is an abstract class?  What is an abstract method?  What is an interface ?  Why would anyone use any of these?  There is weekly assignment problem on Angel  Due by 5PM Tuesday (via )  Problem #2 graded using the provided JUnit tests