1. David Evans http://www.cs.virginia.edu/evans CS201j: Engineering Software University of Virginia Computer Science Lecture 12: Subtyping and Inheritance

2. 9 October 2003CS 201J Fall 20032 Menu Subtyping Inheritance What is Object-Oriented Programming

3. 9 October 2003CS 201J Fall 20033 Subtyping Cell ConwayLifeCell ConwayLifeCell is a subtype of Cell Cell is a supertype of ConwayLifeCell ConwayLifeCell ≤ Cell

4. 9 October 2003CS 201J Fall 20034 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 both subtyping and inheritance just subtyping

5. 9 October 2003CS 201J Fall 20035 Inheritance and Subtyping public class ExtremeLifeCell extends Cell { public CellState getNextState () // 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 (countAliveNeighbors () > 0) { return CellState.createAlive (); } else { return getState (); } ExtremeLifeCell is a subtype of Cell - anywhere a Cell is expected, we can use an ExtremeLifeCell ExtremeLifeCell inherits from Cell - the rep of an ExtremeLifeCell includes the rep of Cell - all public methods and constructors of Cell are also available for ExtremeLifeCell

6. 9 October 2003CS 201J Fall 20036 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 () Calls class A’s display method Calls class B’s display method

7. 9 October 2003CS 201J Fall 20037 Dynamic Dispatch Search for the method up the type hierarchy, starting from the actual (dynamic) type of the object A B A a = new A (); B b = new B (); a.display (); b.display (); a apparent type actual type b apparent type actual type

8. 9 October 2003CS 201J Fall 20038 Dynamic Dispatch Search for the method up the type hierarchy, starting from the actual (dynamic) type of the object A B A a = new A (); B b = new B (); a.display (); b.display (); a = b; a apparent type actual type b apparent type actual type Now: apparent type of a is A, actual type of a is B

9. 9 October 2003CS 201J Fall 20039 public class Grid { /*@non_null@*/ Cell [][] cells; … public void step () // MODIFIES: this // EFFECTS: Executes one step for each cell in the grid. { CellState [][] nextStates = new CellState [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++) { nextStates [i][j] = cells[i][j].getNextState (); } } for (int i = 0; i < rows; i++) { for (int j = 0; j < columns; j++) { cells[i][j].setState (nextStates[i][j]); } } } } } apparent type: Cell actual type: any subtype of Cell (could be ConwayLifeCell )

10. 9 October 2003CS 201J Fall 200310 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

11. 9 October 2003CS 201J Fall 200311 Downcasting java.util.Vector: public Object elementAt (int i); public class StringSet { Vector elements; public String choose () { String s = elements.elementAt (0); return s; } String s = (String) elements.elementAt (0); Casting changes the apparent type. The VM must check that the actual type is a subtype of the cast type.

12. 9 October 2003CS 201J Fall 200312 Downcasting java.util.Vector: public Object elementAt (int i); public class StringSet { Vector elements; public String choose () { String s = elements.elementAt (0); return s; } String s = (String) elements.elementAt (0); Casting changes the apparent type. The VM must check that the actual type is a subtype of the cast type.

13. 9 October 2003CS 201J Fall 200313 A Type Hierarchy Shape Quadrangle Triangle Rectangle Parallelogram Rhombus Square Equilateral EquilateralTriangle What are the subtypes of Parallelogram? What are the supertypes of Square?

14. 9 October 2003CS 201J Fall 200314 A Class Hierarchy Shape Quadrangle Triangle Rectangle Parallelogram Rhombus Square Equilateral EquilateralTriangle

15. 9 October 2003CS 201J Fall 200315 Reusing Implementations Shapes should have a setColor method Change Shape, Quadrangle, Parallelogram, Triangle, Equilateral, EquilateralTriangle, Rhombus, Rectangle, Square, etc. Change Shape others inherit new attribute and method automatically

16. 9 October 2003CS 201J Fall 200316 Add isEquilateral method class Shape { public bool isEquilateral () { return false; } } class Equilateral { public bool isEquilateral () { return true; } }

17. 9 October 2003CS 201J Fall 200317 Is a Rhombus equilateral? Shape Quadrangle Parallelogram Rhombus Equilateral isEquilateral? isEquilateral () { return false; } isEquilateral () { return true; } Inheritance can be tricky!

18. 9 October 2003CS 201J Fall 200318 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)

19. 9 October 2003CS 201J Fall 200319 Java’s Solution: Interfaces Define a type with no implementation Classes can implement many interfaces: class B extends A implements I1, I2, I3 { … } means B is a subtype of A, I1, I2, and I3 B inherits the implementation of A

20. 9 October 2003CS 201J Fall 200320 Example Interface public interface Comparable { int compareTo (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. }

21. 9 October 2003CS 201J Fall 200321 Java’s Sorting Routines public class java.util.Arrays { public static void sort (Object[] a, int fromIndex, int toIndex) // REQUIRES: All elements in a between // fromIndex and toIndex must // implement the Comparable interface. // EFFECTS: Sorts the elements of a between // fromIndex and toIndex into ascending // order, according to the natural ordering of // its elements (defined by compareTo).

22. 9 October 2003CS 201J Fall 200322 PS3 sortonce implementation public class WordTally { private TallyRecord [] entries; private boolean isSorted; private /*@spec_public@*/ int numEntries; public String getRankedWord(int n) { if (!isSorted) { java.util.Arrays.sort (entries, 0, numEntries); isSorted = true; } if (n <= numberOfWords()) { return entries[n-1].word; } else { return null; }

23. 9 October 2003CS 201J Fall 200323 Implementing Comparable class TallyRecord implements Comparable { /*@non_null@*/ String word; int tally; //@invariant tally > 0; public TallyRecord(/*@non_null@*/ String p_word, int p_tally) //@requires p_tally > 0; { word = p_word; tally = p_tally; } public int compareTo (Object o) throws RuntimeException { if (o instanceof TallyRecord) { TallyRecord t2 = (TallyRecord) o; if (tally > t2.tally) { return -1; } else if (tally == t2.tally) { return word.compareTo (t2.word); } else { return 1; } } else { throw new RuntimeException ("Comparison to non-TallyRecord: " + o); }

24. 9 October 2003CS 201J Fall 200324 Object- Oriented Programming

25. 9 October 2003CS 201J Fall 200325 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

26. 9 October 2003CS 201J Fall 200326 What is Object-Oriented Programming?

27. 9 October 2003CS 201J Fall 200327 “Object-oriented programming is programming with inheritance. Data abstraction is programming using 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.” Bjarne Stroustrup’s Answer

28. 9 October 2003CS 201J Fall 200328 “I invented the term Object-Oriented and I can tell you I did not have C++ in mind.” Alan Kay

29. 9 October 2003CS 201J Fall 200329 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

30. 9 October 2003CS 201J Fall 200330 Programming Language History Simula (Dahl and Nygaard, 1962-7) –First language with subtyping and inheritance CLU (Liskov et. al., 1970s) –First language with good support for data abstraction (but no subtyping or inheritance) Smalltalk (Kay et. al., 1970s) –First successful language and programming system to support subtyping and inheritance

31. 9 October 2003CS 201J Fall 200331 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.

32. 9 October 2003CS 201J Fall 200332 Who was the first object-oriented programmer?

33. 9 October 2003CS 201J Fall 200333 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

34. 9 October 2003CS 201J Fall 200334 Charge PS4 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?