Lecture 12 Subtyping and Inheritance CS 201 j

Lecture 12: Subtyping and Inheritance CS 201 j: Engineering Software University of Virginia 9 October 2003 Computer Science CS 201 J Fall 2003 David Evans http: //www. cs. virginia. edu/evans

Menu • Subtyping • Inheritance • What is Object-Oriented Programming 9 October 2003 CS 201 J Fall 2003 2

Subtyping Cell Conway. Life. Cell is a subtype of Cell is a supertype of Conway. Life. Cell ≤ Cell Conway. Life. Cell 9 October 2003 CS 201 J Fall 2003 3

Inheritance • To implement a subtype, it is often useful to use the implementation of its supertype • This is also called “subclassing” • In Java: class B extends A B is a subtype of A B inherits from A class C implements F C is a subtype of F 9 October 2003 CS 201 J Fall 2003 both subtyping and inheritance just subtyping 4

Inheritance and Subtyping public class Extreme. Life. Cell extends Cell { public Cell. State get. Next. State () // EFFECTS: Returns the next state for this cell. // The next state will be alive if this cell or any of its neighbors // is currently alive. { if (count. Alive. Neighbors () > 0) { return Cell. State. create. Alive (); } else { Extreme. Life. Cell is a subtype of Cell return get. State (); - anywhere a Cell is expected, we can } use an Extreme. Life. Cell } Extreme. Life. Cell inherits from Cell } - the rep of an Extreme. Life. Cell includes the rep of Cell - all public methods and constructors of Cell are also available for Extreme. Life. Cell 9 October 2003 CS 201 J Fall 2003 5

Method Dispatch • B is a subtype of A • If both A and B have a method display which method should be called? A a = new A (); B b = new B (); a. display (); b. display (); a = b; a. display () 9 October 2003 Calls class A’s display method Calls class B’s display method CS 201 J Fall 2003 6

Dynamic Dispatch • Search for the method up the type hierarchy, starting from the actual (dynamic) type of the object apparent type A A a = new A (); B b = new B (); a actual type B b apparent type a. display (); b. display (); actual type 9 October 2003 CS 201 J Fall 2003 7

Dynamic Dispatch • Search for the method up the type hierarchy, starting from the actual (dynamic) type of the object apparent type A A a = new A (); B b = new B (); a actual type B b apparent type Now: apparent type of a is A, actual type of a is B actual type 9 October 2003 a. display (); b. display (); a = b; CS 201 J Fall 2003 8

public class Grid { /*@non_null@*/ Cell [][] cells; … public void step () // MODIFIES: this // EFFECTS: Executes one step for each cell in the grid. { Cell. State [][] next. States = new Cell. State [rows][columns]; // Since we need to update all cells synchronously, we first calculate the next // state for each cell, and store it in a temporary array. Then, we update all cells. for (int i = 0; i < rows; i++) { for (int j = 0; j < columns; j++) { next. States [i][j] = cells[i][j]. get. Next. State (); } } for (int i = 0; i < rows; i++) { for (int j = 0; j < columns; j++) { cells[i][j]. set. State (next. States[i][j]); } } } 9 October 2003 CS 201 J Fall 2003 apparent type: Cell actual type: any subtype of Cell (could be Conway. Life. Cell) 9

Apparent and Actual Types • Apparent types are associated with declarations: they never change • Actual types are associated with object: they are always a subtype of the apparent type • Compiler does type checking using apparent type • Virtual Machine does method dispatch using actual type 9 October 2003 CS 201 J Fall 2003 10

Downcasting java. util. Vector: public Object element. At (int i); public class String. Set { Vector elements; public String choose () { String s = elements. element. At (0); String s = (String) elements. element. At (0); Casting changes the apparent type. return s; } 9 October 2003 The VM must check that the actual type is a subtype of the cast type. CS 201 J Fall 2003 11

Downcasting java. util. Vector: public Object element. At (int i); public class String. Set { Vector elements; public String choose () { String s = elements. element. At (0); String s = (String) elements. element. At (0); Casting changes the apparent type. return s; } 9 October 2003 The VM must check that the actual type is a subtype of the cast type. CS 201 J Fall 2003 12

A Type Hierarchy Shape Quadrangle Triangle Equilateral Parallelogram Rhombus Rectangle Equilateral. Triangle What are the supertypes of Square? Square 9 October 2003 What are the subtypes of Parallelogram? CS 201 J Fall 2003 13

A Class Hierarchy Shape Quadrangle Triangle Equilateral Parallelogram Rhombus Rectangle Equilateral. Triangle Square 9 October 2003 CS 201 J Fall 2003 14

Reusing Implementations • Shapes should have a set. Color method • Change Shape, Quadrangle, Parallelogram, Triangle, Equilateral. Triangle, Rhombus, Rectangle, Square, etc. • Change Shape others inherit new attribute and method automatically 9 October 2003 CS 201 J Fall 2003 15

Add is. Equilateral method class Shape { public bool is. Equilateral () { return false; } } class Equilateral { public bool is. Equilateral () { return true; } } 9 October 2003 CS 201 J Fall 2003 16

Is a Rhombus equilateral? is. Equilateral () { return false; } Shape is. Equilateral () { Equilateral return true; } Quadrangle Parallelogram Inheritance can be tricky! Rhombus is. Equilateral? 9 October 2003 CS 201 J Fall 2003 17

Solutions • Java – Allow multiple supertypes using interfaces, but only one implementation – Pro: Safe and Simple, Con: Limits Reuse • C++ – Allow it, let programmers shoot themselves if they want • Eiffel – Explicit renaming or hiding (error if not done) 9 October 2003 CS 201 J Fall 2003 18

Java’s Solution: Interfaces • Define a type with no implementation • Classes can implement many interfaces: class B extends A implements I 1, I 2, I 3 { … } means B is a subtype of A, I 1, I 2, and I 3 B inherits the implementation of A 9 October 2003 CS 201 J Fall 2003 19

Example Interface public interface Comparable { int compare. To (Object o) { // EFFECTS: Compares this object with the specified // object for order. Returns a negative integer, zero, // or a positive integer as this object is less than, // equal to, or greater than the specified object. } 9 October 2003 CS 201 J Fall 2003 20

Java’s Sorting Routines public class java. util. Arrays { public static void sort (Object[] a, int from. Index, int to. Index) // REQUIRES: All elements in a between // from. Index and to. Index must // implement the Comparable interface. // EFFECTS: Sorts the elements of a between // from. Index and to. Index into ascending // order, according to the natural ordering of // its elements (defined by compare. To). 9 October 2003 CS 201 J Fall 2003 21

PS 3 sortonce implementation public class Word. Tally { private Tally. Record [] entries; private boolean is. Sorted; private /*@spec_public@*/ int num. Entries; public String get. Ranked. Word(int n) { if (!is. Sorted) { java. util. Arrays. sort (entries, 0, num. Entries); is. Sorted = true; } if (n <= number. Of. Words()) { return entries[n-1]. word; } else { return null; } } 9 October 2003 CS 201 J Fall 2003 22

Implementing Comparable class Tally. Record implements Comparable { /*@non_null@*/ String word; int tally; //@invariant tally > 0; public Tally. Record(/*@non_null@*/ String p_word, int p_tally) //@requires p_tally > 0; { word = p_word; tally = p_tally; } public int compare. To (Object o) throws Runtime. Exception { if (o instanceof Tally. Record) { Tally. Record t 2 = (Tally. Record) o; if (tally > t 2. tally) { return -1; } else if (tally == t 2. tally) { return word. compare. To (t 2. word); } else { return 1; } } else { throw new Runtime. Exception ("Comparison to non-Tally. Record: " + o); } } 9 October 2003 CS 201 J Fall 2003 23

Object. Oriented Programming 9 October 2003 CS 201 J Fall 2003 24

What is an Object? • Packaging state and procedures – state: the rep • What a thing is – procedures: methods and constructors • What you can do with it 9 October 2003 CS 201 J Fall 2003 25

What is Object-Oriented Programming? 9 October 2003 CS 201 J Fall 2003 26

Bjarne Stroustrup’s Answer “Object-oriented programming is programming with inheritance. Data abstraction is programming user-defined types. With few exceptions, object-oriented programming can and ought to be a superset of data abstraction. These techniques need proper support to be effective. Data abstraction primarily needs support in the form of language features and object-oriented programming needs further support from a programming environment. To be general purpose, a language supporting data abstraction or object-oriented programming must enable effective use of traditional hardware. ” 9 October 2003 CS 201 J Fall 2003 27

“I invented the term Object-Oriented and I can tell you I did not have C++ in mind. ” Alan Kay 9 October 2003 CS 201 J Fall 2003 28

Programming Language History • Before 1954: twidling knobs, machine code, assembly code • FORTRAN (John Backus, UVa dropout, 1954) – Formula Translation • Algol (Peter Naur, Alan Perlis, et. al. , 1958 -1960) – Most influential programming language – Many of the things Algol did first (types, while, blocks) are in Java 9 October 2003 CS 201 J Fall 2003 29

Programming Language History • Simula (Dahl and Nygaard, 1962 -7) – First language with subtyping and inheritance • CLU (Liskov et. al. , 1970 s) – First language with good support for data abstraction (but no subtyping or inheritance) • Smalltalk (Kay et. al. , 1970 s) – First successful language and programming system to support subtyping and inheritance 9 October 2003 CS 201 J Fall 2003 30

Object-Oriented Programming • Object-Oriented Programming is a state of mind where you program by thinking about objects • It is difficult to reach that state of mind if your language doesn’t have: – Mechanisms for packaging state and procedures • Java has class – Subtyping • Java has extends (subtype and subclass) and implements (subtype) • Other things can help: dynamic dispatch, implementation inheritance, automatic memory management, mixins, good Indian food, Krispy Kremes, etc. 9 October 2003 CS 201 J Fall 2003 31

Who was the first object-oriented programmer? 9 October 2003 CS 201 J Fall 2003 32

By the word operation, we mean any process which alters the mutual relation of two or more things, be this relation of what kind it may. This is the most general definition, and would include all subjects in the universe. Again, it might act upon other things besides number, were objects found whose mutual fundamental relations could be expressed by those of the abstract science of operations, and which should be also susceptible of adaptations to the action of the operating notation and mechanism of the engine. . . Supposing, for instance, that the fundamental relations of pitched sounds in the science of harmony and of musical composition were susceptible of such expression and adaptations, the engine might compose elaborate and scientific pieces of music of any degree of complexity or extent. Ada, Countess of Lovelace, around 1830 9 October 2003 CS 201 J Fall 2003 33

Charge • PS 4 due Thursday, Oct 16 • Exam 1 out Thursday, Oct 16 • Exam review in section tomorrow – Bring your questions • Next week: – When is it safe to say B is a subtype of A? 9 October 2003 CS 201 J Fall 2003 34