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Inheritance and Polymorphism. This section is not required material!!!!. Since we are ahead in lecture, I’m delving into it If you feel confused, don’t worry It will be gone over in more detail in CS 201 Ask questions if you are confused about inheritance You aren’t the only one!.
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This section is not required material!!!! • Since we are ahead in lecture, I’m delving into it • If you feel confused, don’t worry • It will be gone over in more detail in CS 201 • Ask questions if you are confused about inheritance • You aren’t the only one!
Motivation • Consider a transportation computer game • Different types of vehicles: • Planes • Jets, helicopters, space shuttle • Automobiles • Cars, trucks, motorcycles • Trains • Diesel, electric, monorail • Ships • … • Let’s assume a class is written for each type of vehicle
Motivation • Sample code for the types of planes: • fly() • takeOff() • land() • setAltitude() • setPitch() • Note that a lot of this code is common to all types of planes • They have a lot in common! • It would be a waste to have to write separate fly() methods for each plane type • What if you then have to change one – you would then have to change dozens of methods
Motivation • Indeed, all vehicles will have similar methods: • move() • getLocation() • setSpeed() • isBroken() • Again, a lot of this code is common to all types of vehicles • It would be a waste to have to write separate move() methods for each vehicle type • What if you then have to change one – you would then have to change dozens of methods • What we want is a means to specify one move() method, and have each vehicle type inherit that code • Then, if we have to change it, we only have to change one copy
Motivation • Provides: • move() • getLocation() • setSpeed() • isBroken() • Provides: • fly() • takeOff() • land() • setAltitude() • setPitch() • Provides: • oilChange() • isInTraffic() • Provides: • derail() • getStation()
Motivation • What we will do is create a “parent” class and a “child” class • The “child” class (or subclass) will inherit the methods (etc.) from the “parent” class (or superclass) • Note that some classes (such as Train) are both subclasses and superclasses
Another example • Consider shapes in a graphics program • Shape class • Circle class • Cube class • Dodecahedron class
Inheritance • Organizes objects in a top-down fashion from most general to least general • Inheritance defines a “is-a” relationship • A mountain bike “is a” kind of bicycle • A SUV “is a” kind of automobile • A border collie “is a” kind of dog • A laptop “is a” kind of computer
Musical instrument hierarchy • The hierarchy helps us understand the relationships and similarities of musical instruments • A clarinet “is a” kind of reeded instrument • Reeded instruments “are a” kind of aerophone • The “is-a” relationship is transitive • A clarinet “is a” kind of reeded instrument • A reeded instrument “is a” kind of aerophone • A clarinet “is a” kind of aerophone
Reeded Musical Clarinet Aerophone Instrument Instrument Object-oriented terminology • In object-oriented programming languages, a class created by extending another class is called a subclass • The class used for the basis is called the superclass • Alternative terminology • The superclass is also referred to as the base class • The subclass is also referred to as the derived class
z-axis y-axis (x, y, z) x-axis ThreeDimensionalPoint • Build a new class ThreeDimensionalPoint using inheritance • ThreeDimensionalPoint extends the awt class Point • Point is the superclass (base class) • ThreeDimensionalPoint is the subclass (derived class) • ThreedimensionalPoint extends Point by adding a new property to Point—a z-coordinate
Class ThreeDimensionalPoint See next slide package geometry; import java.awt.*; public class ThreeDimensionalPoint extends Point { // private class constant private final static int DEFAULT_Z = 0; // private instance variable public int z = DEFAULT_Z; • Note that ThreeDimensionalPoint inherits the variables in the Point class • Thus, it has an x and y variables (inherited from Point) • And it has a z variable (defined above) Keyword extends indicatesthat ThreeDimensionalPointis a subclass of Point New instance variable
Bizarre links du jour • An eBay auction: • http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&category=19270&item=5535890757 • A new form of entertainment: • http://www.cnn.com/2004/US/11/16/life.hunting.reut/
Packages • Allow definitions to be collected together into a single entity—a package • ThreeDimensionalPoint will be added to the geometry package • Classes and names in the same package are stored in the same folder • Classes in a package go into their own namespace and therefore the names in a particular package do not conflict with other names in other packages • For example, a package called Graph might have a different definition of ThreeDimensionalPoint • When defining members of a class or interface, Java does not require an explicit access specification. The implicit specification is known as default access. Members of a class with default access can be accessed only by members of the package.
About extends • If class A extends class B • Then class A is the subclass of B • Class B is the superclass of class A • A “is a” B • A has (almost) all the methods and variables that B has • If class Train extends class Vehicle • Then class Train is the subclass of Vehicle • Class Vehicle is the superclass of class Train • Train “is a” Vehicle • Train has (almost) all the methods and variables that Vehicle has
Thus, everything extends Object • Either directly or indirectly • So what does that give us? • Object contains the following methods: • clone() • equals() • toString() • and others… • Thus, every class has those methods
A note about equals() • Why does the equals() method always have to have the following prototype: • boolean equals(Object obj) • Many other class in the Java SDK require the user of equals() • Such as the Vector class • Those classes need to know how the equals() method will work in order for them to work properly
ThreeDimensionalPoint • Methods toString(), equals() , and clone() should not have different signatures from the Point versions ThreeDimensionalPoint c = new ThreeDImensionalPoint(1, 4, 9); ThreeDimensionalPoint d = (ThreeDimensionalPoint) c.clone(); String s = c.toString(); boolean b = c.equals(d); Cast is necessary as return type of subclass methodclone() is Object Invocation of subclasstoString() method Invocation of subclassequals() method
ThreeDimensionalPoint • Constructors // ThreeDimensionalPoint(): default constructor public ThreeDimensionalPoint() { super(); } // ThreeDimensionalPoint(): specific constructor public ThreeDimensionalPoint(int a, int b, int c) { super(a, b); setZ(c); }
ThreeDimensionalPoint • Accessors and mutators // getZ(): z-coordinate accessor publicdouble getZ() { return z; } // setZ(): y-coordinate mutator publicvoid setZ(int value) { z = value; }
ThreeDimensionalPoint • Facilitators // translate(): shifting facilitator publicvoid translate(int dx, int dy, int dz) { translate(dx, dy); int zValue = (int) getZ(); setZ(zValue + dz); } calls the inherited translate method in Point
ThreeDimensionalPoint ThreeDimensionalPoint a =new ThreeDimensionalPoint(6, 21, 54); a.translate(1, 1); // invocation of superclass translate() a.translate(2, 2, 2); // invocation of 3DPoint’s translate() • Java determines which method to use based on the number of parameters in the invocation • After the first call to translate, what is the value of a? • After the second call to translate, what is the value of a? • Note that this is still overloading!
ThreeDimensionalPoint • Facilitators // toString(): conversion facilitator public String toString() { int a = (int) getX(); int b = (int) getY(); int c = (int) getZ(); return getClass() + "[" + a + ", " + b + ", " + c + "]"; } • What’s getClass()?
ThreeDimensionalPoint • Facilitators // equals(): equality facilitator publicboolean equals(Object v) { if (v instanceof ThreeDimensionalPoint) { ThreeDimensionalPoint p = (ThreeDimensionalPoint) v; int z1 = (int) getZ(); int z2 = (int) p.getZ(); returnsuper.equals(p) && (z1 == z2); } else { returnfalse; } } calls the inherited equals method in Point
ThreeDimensionalPoint • Facilitators // clone(): clone facilitator public Object clone() { int a = (int) getX(); int b = (int) getY(); int c = (int) getZ(); returnnew ThreeDimensionalPoint(a, b, c); }
ColoredPoint • Suppose an application calls for the use of colored points. • We can naturally extend class Point to create ColoredPoint • Class ColoredPoint will be added to packagegeometry package geometry; import java.awt.*; public class ColoredPoint extends Point { // instance variable Color color;…
Class hierarchy Object Point ThreeDimPoint ColoredPoint
ColoredPoint • Constructors // ColoredPoint(): default constructor public ColoredPoint() { super(); setColor(Color.blue); } // ColoredPoint(): specific constructor public ColoredPoint(int x, int y, Color c) { super(x, y); setColor(c); }
ColoredPoint • Accessors and mutators // getColor(): color property accessor public Color getColor() { return color; } // setColor(): color property mutator publicvoid setColor(Color c) { color = c; }
ColoredPoint • Facilitators // clone(): clone facilitator public Object clone() { int a = (int) getX(); int b = (int) getY(); Color c = getColor(); returnnew ColoredPoint(a, b, c); }
ColoredPoint • Facilitators // toString(): string representation facilitator public String toString() { int a = (int) getX(); int b = (int) getY(); Color c = getColor(); return getClass() + "[" + a + ", " + b + ", " + c + "]"; }
ColoredPoint • Facilitators // equals(): equal facilitator publicboolean equals(Object v) { if (v instanceof ColoredPoint) { Color c1 = getColor(); Color c2 = ((ColoredPoint) v).getColor(); returnsuper.equals(v) && c1.equals(c2); } else { returnfalse; }
Colored3DPoint • Suppose an application needs a colored, three-dimensional point. • Can we create such a class by extending both ThreeDimensionalPoint and ColoredPoint?
Class hierarchy Object Point ThreeDimPoint ColoredPoint Colored3DPoint
Colored3DPoint • Java does not support multiple inheritance • Java only supports single inheritance • C++ supports multiple inheritance package Geometry; import java.awt.*; public class Colored3DPoint extends ThreeDimensionalPoint { // instance variable Color color;
Class hierarchy Object Point ThreeDimPoint ColoredPoint Colored3DPoint
Colored3DPoint • Constructors // Colored3DPoint(): default constructor public Colored3DPoint() { setColor(Color.blue); } // Colored3DPoint(): specific constructor public Colored3DPoint(int a, int b, int c, Color d) { super(a, b, c); setColor(d); }
Colored3DPoint • Accessors and mutators // getColor(): color property accessor public Color getColor() { return color; } // setColor(): color property mutator public void setColor(Color c) { color = c; }
Colored3DPoint • Facilitators // clone(): clone facilitator public Object clone() { int a = (int) getX(); int b = (int) getY(); int c = (int) getZ(); Color d = getColor(); returnnew Colored3DPoint(a, b, c, d); }
Colored3DPoint • Facilitators // toString(): string representation facilitator public String toString() { int a = (int) getX(); int b = (int) getY(); int c = (int) getZ(); Color d = getColor(); return getClass() + "[" + a + ", " + b + ", " + c + ", " + d + "]"; }
Colored3DPoint • Facilitators // equals(): equal facilitator publicboolean equals(Object v) { if (v instanceof Colored3DPoint) { Color c1 = getColor(); Color c2 = ((Colored3DPoint) v).getColor(); returnsuper.equals(v) && c1.equals(c2); } else { returnfalse; }
Biggest software errors • Ariane 5 rocket explosion (1996) • Due to loss of precision converting 64-bit double to 16-bit int • Pentium division error (1994) • Due to incomplete look-up table (like an array) • Patriot-Scud missile error (1991) • Rounding error on the time • The missile did not intercept an incoming Scud missile, leaving 28 dead and 98 wounded • Mars Climate Orbiter (1999) • Onboard used metric units; ground computer used English units • AT&T long distance (1990) • Wrong break statement in C code • Therac-25, X-ray (1975-1987) • Badly designed software led to radiation overdose in chemotherapy patients • NE US power blackout (2003) • Flaw in GE software contributed to it • References: http://www5.in.tum.de/~huckle/bugse.html, http://en.wikipedia.org/wiki/Computer_bug, http://www.cs.tau.ac.il/~nachumd/verify/horror.html
Overriding • Consider the following code: class Foo { // automatically extends Object public String toString () { return “Foo”; } } ... Foo f = new Foo(); System.out.println (f); • Now there are two toString() method defined • One inherited from class Object • One defined in class Foo • And they both have the same prototype! • Which one does Java call?
Overriding • Java will call the most specific overriden method it can • toString() in Foo is more specific than toString() in Object • Consider our transportation hierarchy: • Assume each class has its own toString() method • Car extends Automobile extends Vehicle (extends Object) • Assume each defines a toString() methods • The toString() method in Vehicle is more specific (to vehicles) than the one in Object • The toString() method in Automobiles is more specific than the ones in Vehicle or Object • The toString() method in Car is more specific than the ones in Automobile, Vehicle, or Object • Thus, for a Car object, the Car toString() will be called • There are ways to call the other toString() methods • This has to be specifically requested
Overriding • This is called overriding, because the toString() in Foo “overrides” the toString() in Object • Note that the prototype must be EXACTLY the same • With overloading, the parameter list must be DIFFERENT • Overriding only works with inheritance • In particular, you can only override a method already defined in a parent (or grandparent, etc.) class
Polymorphism • Consider toString() again • Although defined in Object, most classes define their own version • When an object is printed, which toString() method is called? • Consider overloading multiple constructors • Which is called – a specific constructor or a default constructor? • That depends on the parameter list supplied • The fact that Java can call different methods of the same name is called polymorphism • It may not be clear which method to call because of either overriding or overloading (or both!)
Polymorphism • A code expression can invoke different methods depending on the types of objects being manipulated • Example: function overloading like method min() from java.lang.Math • The method invoked depends on the types of the actual arguments Example int a, b, c; double x, y, z; … c = min(a, b); // invokes integer min() z = min(x, y); // invokes double min() • This polymorphism is dealing with overloadingmethods