BM214E Object Oriented Programming Lecture 11

Transcription

1 BM214E Oriented Programming Lecture 11

2 Interfaces & Polymorphism (continued)

3 Mini-review References: Pointers to objects. Include many of the classic confusing things about pointers! Inheritance: defined with keyword "extends". Subclasses inherit all of the fields and methods of the parent class Define subclasses in their own file Arrays are objects which require a reference Watch[] watcharray = new Watch[10]; For the most part avoid static. Creating a (reference variable vs creating an instance) TetrisBlock tb; tb = new TetrisBlock();

4 We ve already seen one use of interfaces public class P1Constants { public static final int FEETPERMILE = 5280; public class SomeOtherClass {... public int convertfeet2miles(int feet) { return feet * P1Constants.FEETPERMILE;...

5 We ve already seen one use of interfaces public interface P1Constants { final int FEETPERMILE = 5280; public class SomeOtherClass implements P1Constants {... public int convertfeet2miles(int feet) { return feet * FEETPERMILE;...

6 But the key feature... I have this structure Aquatic LandBased Goldfish Shark Dog Cat Giraffe But I want some of the animals to be Pets?

7 Pets will have names! public interface Pet { public String getname(); public void setname(string name); Note: We don't define the variable name

8 So now if I want a dog to be a pet: class Dog implements Pet { private String name; /* Plus other fields */ public String getname() { return name; public void setname(string name) { this.name = name; /* Plus other methods */

9 More Polymorphism

10 In Our Last Episode... Recall last lecture s discussion of polymorphism. This concept is a KEY feature of any object oriented language, not just Java. It is imperative that you understand how it works. Let us review.

11 Recall class { public void move ( ) { ( I am an animal and I am moving. ); // of class extends { public void move( ) { ( Swim, swim, swim, swim! ); // of class Dog extends { public void move ( ) { ( Run, scratch, walk, run ); public void bark ( ){ ( Arf Arf ); // of Dog class Bird extends { public void move( ) { ( Flap, flap, flap again ); // of Bird

12 Recall class Driver { public static void main (String[ ] argv) { [ ] animalarray = new [3]; int iindex; animalarray[0] = new Bird( ); animalarray[1] = new Dog( ); animalarray[2] = new ( ); for (iindex=0; iindex < animalarray.length; iindex++) { animalarray[iindex].move( ); // of for // of main // of Driver Output: Flap, flap, flap again Run, scratch, walk, run Swim, swim, swim, swim! We also noted that we could use polymorphism to hold an array of animals, and employ dynamic binding to invoke specific behaviors.

13 And so... Let s review the creation of an object, using a slightly more complex class example class { public int numlegs; public static int total = 0; public ; This may not be the best design. Don t forget this is an academic exercise. We re doing this for a reason. public (int numlegs){ this.numlegs = numlegs; strtype = ; total++; public void move ( ) { ( I am an animal and am moving. ); public String { return strtype + with + numlegs + legs. ; // of This time, we have instance and class variables. The int total is a population counter. The int numlegs records the number of legs for the animal. The String strtype describes the animal.

14 And so... Let s review the creation of an object, using a slightly more complex class example class { public int numlegs; public static int total = 0; private ; The move() method is the same as before. We ve added a constructor and a simple. public (int numlegs){ this.numlegs = numlegs; strtype = ; total++; public void move ( ) { ( I am an animal and am moving. ); public String { return strtype + with + numlegs + legs. ; // of In a perfect world, we d have room on the slides for the accessors and modifiers.

15 And also... class extends { public (int numlegs){ super(numlegs); strtype = ; public void move( ) { ( Swim, swim, swim, swim! ); // of The class is very much the same. We just add a new constructor, which invokes the parent constructor, and then corrects the strtype assignment.

16 And also... class Bird extends { public Bird(int numlegs){ super (numlegs); strtype = Bird ; public void move( ) { ( Flap, flap, flap again ); // of Bird Same story with the Bird class. We added a new constructor, and set the type appropriately.

17 And also... class Dog extends { public Dog(int numlegs){ super (numlegs); strtype = Dog ; public void move ( ) { ( Run, scratch, walk, run ); public void bark ( ){ ( Arf Arf ); // of Dog Same story with the Dog class. We added a new constructor, and set the type appropriately.

18 And Finally... class Driver { public static void main (String[ ] argv) { [ ] animalarray = new [3]; int iindex; animalarray[0] = new Bird(2); animalarray[1] = new (0); animalarray[2] = new Dog(3); for (i=0; i < animalarray.length; i++) { (animalarray[i].); animalarray[i].move( ); // of for ( Total Count: +.total); // of main // of Driver We also revise our Driver class, passing in appropriate values to the constructors. (We happen to have a three-legged dog.)

19 class { public int numlegs; public static int total = 0; private ; public (int numlegs){ this.numlegs = numlegs; strtype = ; total++; public void move ( ) { ( I am an animal and am moving. ); public String { return strtype + with + numlegs + legs. ; // of class Driver { public static void main (String[ ] argv) { [ ] animalarray = new [3]; int iindex; animalarray[0] = new Bird(2); animalarray[1] = new (0); animalarray[2] = new Dog(3); for (i=0; i < animalarray.length; i++) { (animalarray[i].); animalarray[i].move( ); // of for // of main // of Driver What Happens? class extends { public (int numlegs){ super(numlegs); strtype = ; public void move( ) { ( Swim, swim, swim, swim! ); // of class Dog extends { public Dog(int numlegs){ super (numlegs); strtype = Dog ; public void move ( ) { ( Run, scratch, walk, run ); public void bark ( ){ ( Arf Arf ); // of Dog class Bird extends { public Bird(int numlegs){ super (numlegs); strtype = Bird ; public void move( ) { ( Flap, flap, flap again ); // of Bird If we run the Driver, what happens? Moreover, WHY does it happen?

20 A Look At Memory We intend to take a close look at these objects in memory. But before we do that, we have to condense what we ve written. We ll need to agree on some symbols and a notation system. class { public int numlegs; public static int total; private ; public (int numlegs){ this.numlegs = numlegs; strtype = ; public void move ( ) { ( I am an animal and am moving. ); public String { return strtype + with + numlegs + legs. ; // of int numlegs We ll represent instances in memory as a box. We ll have a little room for the variables, and a few key methods.

21 A Look At Memory We intend to take a close look at these objects in memory. But before we do that, we have to condense what we ve written. We ll need to agree on some symbols and a notation system. class { public int numlegs; public static int total; private ; public (int numlegs){ this.numlegs = numlegs; strtype = ; public void move ( ) { ( I am an animal and am moving. ); public String { return strtype + with + numlegs + legs. ; // of int numlegs In future classes, you ll learn about UML diagrams. But for now, it s all boxes and arrows. Since an inherits from (remember IS-A relationships are specified by inheritance), we also draw an box.

22 A Look At Memory class { public int numlegs; public static int total; private ; Now, a is more complex. It inherits from which extends. public (int numlegs){ this.numlegs = numlegs; strtype = ; public void move ( ) { ( I am an animal and am moving. ); public String { return strtype + with + numlegs + legs. ; // of class extends { public (int numlegs){ super(numlegs); strtype = ; public void move( ) { ( Swim, swim, swim, swim! ); // of int numlegs This whole area is a object in memory. Since a IS-A, and an IS-A, we draw all three boxes to express one object

23 A Look At Memory Thus, if we make a instance, we have a reference pointing to this block of memory that we ve drawn. ftemp; ftemp = new (0); ftemp int numlegs = 0 By convention, we will have the reference point to a particular portion of the block of memory, even though the entire block is one object. Since our reference is a type, our arrow enters at the portion of the object in memory.

24 Recall Our Creation Timeline Remember that a call to the (int) constructor involves an explicit (int) constructor call. Even if we did not have this explicit call, Java would implicitly call the parent class constructor, just as it implicitly makes an when the (int) constructor is invoked from the (int) constructor int numlegs = 0 int numlegs = 0 int numlegs = 0 ftemp = new (0); To make a, Java must first make an. To make an, Java must first make an. ftemp

25 Wow. Notice something else. Since all are types, we can have an reference pointing to this block of memory. otemp; otemp = new (0); int numlegs = 0 A single instance otemp We adjust where the reference enters the block of memory, that that it reflects the reference type.

26 Ahh. So we can just make new reference types and have them point into our block at a different locations. By twiddling the reference into our block of memory, we can polymorph the object. atemp; atemp = new (0); atemp int numlegs = 0 We adjust where reference points, so that it reflects its type.

27 Hey, Cool int numlegs = 3 ref That s a simple example of polymorphism.

28 Hey, Cool int numlegs = 3 ref That s a simple example of polymorphism.

29 Hey, Cool It s the same object in memory. We just change the reference type pointing to it. int numlegs = 3 ref That s a simple example of polymorphism.

30 Polymorphism Many forms int numlegs = 3 One object <type> ref Literally, polymorphism means Many forms So for us we take it to imply, "One object; many forms

31 Thus So we have three box groupings that represent arbitrary instances of, Dog and Bird, pointed to by any appropriate reference type. int numlegs = 0 int numlegs = 3 Dog bark(); int numlegs = 2 Bird ftemp dtemp btemp ftemp = new (0); Dog dtemp = new Dog(3); Bird btemp = new Bird(2);

32 class { public int numlegs; public static int total = 0; private ; public (int numlegs){ this.numlegs = numlegs; strtype = ; total++; public void move ( ) { ( I am an animal and am moving. ); public String { return strtype + with + numlegs + legs. ; // of class Bird extends { public Bird(int numlegs){ super (numlegs); strtype = Bird ; public void move( ) { ( Flap, flap, flap again ); // of Bird class extends { public (int numlegs){ super(numlegs); strtype = ; public void move( ) { ( Swim, swim, swim, swim! ); // of Recall class Dog extends { public Dog(int numlegs){ super (numlegs); strtype = Dog ; public void move ( ) { ( Run, scratch, walk, run ); public void bark ( ){ ( Arf Arf ); // of Dog So we figured out how to represent inheritance in memory. Now recall that we also had our classes point to a String literal describing the type. A String literal is, well, literally a String. It s something in quotes in your source code. Literally what s there.

33 The Constant Pool atemp; atemp = new Bird(2); atemp int numlegs = 2 Bird Constant Pool Bird Dog There s an area of the Java Virtual Machine called the Constant Pool. It s a special area to hold declared constants (final variables), since they cannot be changed. As it turns out, this is where String literals are placed.

34 The Constant Pool atemp; atemp = new Bird(2); atemp int numlegs = 2 Bird Constant Pool Bird Dog Since Strings are s, and are addressed through references, the strtype variable in the class points to one of these literals in the Constant Pool.

35 The Constant Pool atemp; atemp = new Bird(2); atemp int numlegs = 2 Bird Constant Pool Bird Dog Note that this is different for primitive instance variables. Primitives are not references. So we just write the value right next to the field

36 In Fact... atemp; atemp = new Bird(2); atemp int numlegs = 2 Bird Constant Pool String String Bird String String Dog In fact, the also inherits from. So we could even draw the Constant Pool as a collection of objects with inheritance. But this is getting too complicated.

37 int numlegs = 2 Bird All of the objects created by our Driver class have unique values (numlegs) and unique references. So, we ll abstract away most of the details, and just note that objects in Java are handled through references. Constant Pool Bird Dog int numlegs = 0 int numlegs = 3 Dog bark();

38 int numlegs = 2 Bird Note also that if we assign the to point to something else, it might not reference the Constant Pool. Constant Pool String <whatever IOHelper returns> Operations like new String(); substring(); and IOHelper.readLine(); cause the allocation of new String objects Bird Dog Bird btemp = new Bird(0); btemp.settype (IOHelper.readLine());

39 What About Non-Instance Data? int numlegs = 0 int numlegs = 3 Dog bark(); The int numlegs was an instance variable of the class, and each instance had its own copy. But the variable int total-- the population counter--was declared static. Thus, it was a class variable. How can we account for this in our diagrams?

40 What About Non-Instance Data? int numlegs = 3 int numlegs = 0 Dog bark(); Class Pool class { public int numlegs; public static int total = 0; private ; public (int numlegs){ this.numlegs = numlegs; strtype = ; total++; public void move ( ) { ( I am an animal and am moving. ); public String { return strtype + with + numlegs + legs. ; // of It just so happens there is something called the Class Pool in Java, which holds information about each class used in your program. You can think of this as the text area where your code is kept. The other parts of memory are the data area where objects are kept.

41 As it turns out, the Class Pool is enormous. It holds not only your classes, but all the classes used by Java--String,, Integer, etc. Every class used to run your program gets loaded here. Java uses the Class Pool to keep track of the working code and methods in each class. Since static variables are of the class and not of the instance, they are kept as part of this storage device. class extends { public (int numlegs){ super(numlegs); strtype = ; public void move( ) { ( Swim, swim, swim, swim! ); // of class Dog extends { public Dog(int numlegs){ super (numlegs); strtype = Dog ; public void move ( ) { ( Run, scratch, walk, run ); public void bark ( ){ ( Arf Arf ); // of Dog (In class ) static int total: 3 Class Pool class Bird extends { public Bird(int numlegs){ super (numlegs); strtype = Bird ; public void move( ) { ( Flap, flap, flap again ); // of Bird class Driver { public static void main (String[ ] argv) { [ ] animalarray = new [3]; int iindex; animalarray[0] = new Bird(2); animalarray[1] = new (0); animalarray[2] = new Dog(3); for (i=0; i < animalarray.length; i++) { (animalarray[i].); animalarray[i].move( ); // of for // of main // of Driver class { public int numlegs; public static int total = 0; private ; public (int numlegs){ this.numlegs = numlegs; strtype = ; total++; public void move ( ) { ( I am an animal and am moving. ); public String { return strtype + with + numlegs + legs. ; // of

42 That s why we can use the class name to invoke class methods, or access class variables. When we access instance variables, however, we must work through an instance of the class. int x =.total; int leg = mydog.getlegcount(); (This assumes we have an accessor; there s not enough room on these slides.) class extends { public (int numlegs){ super(numlegs); strtype = ; public void move( ) { ( Swim, swim, swim, swim! ); // of class Dog extends { public Dog(int numlegs){ super (numlegs); strtype = Dog ; public void move ( ) { ( Run, scratch, walk, run ); public void bark ( ){ ( Arf Arf ); // of Dog (In class ) static int total: 3 Class Pool class Bird extends { public Bird(int numlegs){ super (numlegs); strtype = Bird ; public void move( ) { ( Flap, flap, flap again ); // of Bird class Driver { public static void main (String[ ] argv) { [ ] animalarray = new [3]; int iindex; animalarray[0] = new Bird(2); animalarray[1] = new (0); animalarray[2] = new Dog(3); for (i=0; i < animalarray.length; i++) { (animalarray[i].); animalarray[i].move( ); // of for // of main // of Driver class { public int numlegs; public static int total = 0; private ; public (int numlegs){ this.numlegs = numlegs; strtype = ; total++; public void move ( ) { ( I am an animal and am moving. ); public String { return strtype + with + numlegs + legs. ; // of

43 Style Point: You can also access class members (static stuff) using instance references. Integer itemp = new Integer(3); String strnumber = 42 ; int x = itemp.parseint(strnumber); // OK int y = Integer.parseInt (strnumber); // better There is, however, a convention, of using class names to access class methods/fields. Using an instance of the class also works; however, it gives the impression that it must be done through an instance. So, (1) Use what works. (2) But use what s clear.

44 Back to Polymorphism So, we have these three blocks, representing objects in memory, each different, holding unique references and primitive values. How can these disparate objects be held in an array, which must be homogeneous? int numlegs = 2 Bird int numlegs = 0 int numlegs = 3 Dog bark(); The organizing principle is the shared inherited relationship with. Since, Dog and Bird all extend from, we can make an array to hold different expressions of this class.

45 Back to Polymorphism So, we have these three blocks, representing objects in memory, each different, holding unique references and primitive values. How can these disparate objects be held in an array, which must be homogeneous? int numlegs = 2 Bird int numlegs = 0 int numlegs = 3 Dog bark(); We could also have made an array of types. But we then will have problems invoking methods on members of this array, since lacks key methods--like move().

46 Back to Polymorphism So, we have these three blocks, representing objects in memory, each different, holding unique references and primitive values. How can these disparate objects be held in an array, which must be homogeneous? int numlegs = 2 Bird int numlegs = 0 int numlegs = 3 Dog bark(); So we select as our common type. It s the most specific type, and yet is still held in common by all the members we plan to hold in the array.

47 Back to Polymorphism Bird btemp = new Bird(); otemp = () btemp; atemp = () btemp; Here, the casting is not needed since we are upcasting. It s shown to be explicit. otemp atemp btemp int numlegs = 2 Bird Recall that we can have many reference types pointing to the same or Dog or Bird instance in memory. Thus, since arrays must be of a single type, we just polymorph any references we have into references.

48 Thus, when we have many objects in memory, we polymorph our references to the instances. This gives us an array of types, even though each instance is a different subclass. [ ] final int length = 3 int numlegs = 2 Bird [ 0 ] [ 1 ] [ 2 ] int numlegs = 0 int numlegs = 3 Dog bark(); News Flash: Arrays are objects as well, and extend from. Notice that the int length field of an array is final. We can t resize arrays; we can only make new larger ones and copy the data over. Let s invoke some methods on these objects.

49 Is our picture correct? [ ] final int length = 3 int numlegs = 2 Bird int numlegs = 0 What if we executed this code: otemp; otemp = animalarray[1]; otemp int numlegs = 3 Dog bark(); No. The code works. But our drawing is pointing to the wrong part of our object

50 Is our picture correct? [ ] final int length = 3 int numlegs = 2 Bird int numlegs = 0 What if we executed this code: otemp; otemp = animalarray[1]; otemp int numlegs = 3 Dog bark(); Much better. This fussiness will pay off shortly.

51 Does this work? [ ] final int length = 3 int numlegs = 2 Bird int numlegs = 0 Now, let s add another line of code: otemp; otemp = animalarray[1]; otemp. otemp int numlegs = 3 Dog bark(); NO. The class has no method called move()

52 [ ] final int length = 3 int numlegs = 2 Bird int numlegs = 0 The fix is in: otemp; otemp = animalarray[1]; atemp = () otemp; otemp int numlegs = 3 Dog bark(); atemp Note the explicit down casting was necessary.

53 [ ] final int length = 3 int numlegs = 2 Bird int numlegs = 0 The fix is in: otemp; otemp = animalarray[1]; atemp = () otemp; atemp. otemp int numlegs = 3 Dog bark(); atemp Hmm... Let s look at this closely.

54 otemp; otemp = animalarray[1]; atemp = () otemp; atemp. [ ] final int length = 3 It looks like both and have move() methods. Which one gets called when the atemp.move() line executes? int numlegs = 2 Bird int numlegs = 0 int numlegs = 3 Dog bark(); otemp atemp int numlegs = 0 The reference type we are using is an type. Would that determine whether or has their method called?

55 Dynamic Binding otemp; otemp = animalarray[1]; atemp = () otemp; atemp. Understand this term. Understand what is does. It is a CORE feature of any Oriented language. int numlegs = 0 atemp Here, the principle of dynamic binding will ensure that at run time, the most specific behavior will be invoked. Here, the move() method is more specific than its parent method. So, the s move() method gets called with the atemp.move() line.

56 Sanity Check otemp; otemp otemp = animalarray[1]; atemp = () otemp; (otemp.); What Happens Here? int numlegs = 0 Does casting somehow overpower dynamic binding? What about: ( (()otemp). );

57 Sanity Check otemp; otemp otemp = animalarray[1]; atemp = () otemp; (otemp.); What Happens Here? int numlegs = 0 No matter how you cast things, dynamic binding takes hold. It s like the law of gravity. What about: ( (()otemp). );

58 Sanity Check otemp; otemp otemp = animalarray[1]; atemp = () otemp; (otemp.); What if had its own? int numlegs = 3 No matter how you cast things, dynamic binding takes hold. It s like the law of gravity. Dynamic binding will always resolve, at run time, to the most specific version of the method. ALWAYS.

59 Always? otemp; otemp otemp = animalarray[1]; otemp. // WRONG! Does dynamic binding also work miracles? That is, does it let you find methods in extending classes, if the present class does not have such a method? int numlegs = 3 No such method move() in NO. This would cause a compile time error. Java is strongly typed, meaning that each time you invoke a method, the method MUST be present in the class--even if dynamic binding would later find a more specific version. So: no, dynamic binding does not trump type safety in Java.

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