1. 1 Chapter 10 l Vector l Linked Data Structures l interface Dynamic Data Structures

2. 2 Overview l This chapter is about data structures that are dynamic: »They can grow and shrink while your program is running l Vectors are similar to arrays but are moreflexible. l Linked lists are a dynamic data structure commonly used in many programming languages.

3. 3 Vectors "Well, I'll eat it," said Alice, "and if it makes me grow larger, I can reach the key; and if it makes me grow smaller, I can creep under the door; so either way I'll get into the garden…" Lewis Carroll, Alice's Adventures in Wonderland VECTORS Think of them as arrays that can get larger or smaller when a program is running.

4. 4 Arrays Versus Vectors Arrays Bad: l Size is fixed when declared l Inefficient storage: can use a partially full array, but space has been allocated for the full size l If one more value needs to be added past the maximum size the array needs to be redeclared Good: l More efficient (faster) execution l Allows primitive type elements Vectors Good : l Size is not fixed l Better storage efficiency: a partially full vector may be allocated just the space it needs l If one more value needs to be added past the maximum size the vector size increases automatically Bad: l Less efficient (slower) execution l Elements must be class types (primitive types not allowed)

5. 5 Using Vectors l Vectors are not automatically part of Java »they are in the util library »you must import java.util.* Create a vector with an initial size of 20 elements: Vector v = new Vector(20);

6. 6 Initial Capacity and Efficiency: a Classic Engineering Tradeoff l Engineering involves making difficult tradeoffs »"There's no such thing as a free lunch." –an American saying »Usually, if you gain something you lose something somewhere else l Choosing the initial size of a vector is an example of a tradeoff »making it too large wastes allocated memory space »making it too small slows execution –it takes time to resize vectors dynamically l Solution? »optimize one at the expense of the other »or make good compromises –choose a size that is not too big and not too small

7. 7 Vector Syntax Array: a is a String array a[i] = "Hi, Mom!"; String temp = a[i]; Vector: v is a vector v.setElementAt("Hi, Mom!", i); String temp = (String)v.elementAt(i); Instead of the index in brackets and = for assignment, use vector method setElementAt with two arguments, the value and the index Use vector method elementAt(int index) to retrieve the value of an element Note: the cast to String is required because the base type of vector elements is Object l The idea is the same as for arrays, but the syntax is different l As with arrays, the index must be in the range 0 to size-of-the- vector

8. 8 Vector Methods l The vector class includes many useful methods: »constructors »array-like methods, e.g. setElementAt & elementAt »methods to add elements »methods to remove elements »search methods »methods to work with the vector's size and capacity, e.g. to find its size and check if it is empty »a clone method to copy a vector l See the text for more information

9. 9 A little Detail about setElementAt l Vectors put values in successive indexes »addElement is used to put initial values in a vector »new values can be added only at the next higher index You cannot use setElemetAt to put a value at any index »setElemetAt can be used to assign the value of an indexed variable only if it has been previously assigned a value with addElement

10. 10 Base Type of Vectors l The base type of an array is specified when the array is declared »all elements of arrays must be of the same type The base type of a vector is Object »elements of a vector can be of any class type »in fact, elements of a vector can be of different class types! »to store primitive types in a vector they must be converted to a corresponding wrapper class Good Programming Practice Although vectors allow elements in the same vector to be of different class types, it is best not to have a mix of classes in the same vector - –it is best to have all elements in a vector be the same class type.

11. 11 The following code looks very reasonable but will produce an error saying that the class Object does not have a method named length : Vector v = new Vector() String greeting = "Hi, Mom!"; v.addElement(greeting); System.out.println("Length is " + (v.elementAt(0)).length()); String, of course, does have a length method, but Java sees the type of v.elementAt(0) as Object, not String Solution? Cast v.elementAt(0) to String : System.out.println ("Length is " + ((String)v.elementAt(0)).length()); Detail: One Consequence of the Base Type of Vectors Being Object

12. 12 One More Detail: Size Versus Capacity l Be sure to understand the difference between capacity and size of a vector: »capacity is the declared size of the vector –the current maximum number of elements »size is the actual number of elements being used -- the number of elements that contain valid values, not garbage -- remember that vectors add values only in successive indexes Loops that read vector elements should be limited by the value of size, not capacity, to avoid reading garbage values

13. 13 Programming Tip: Increasing Storage Efficiency of Vectors l A vector automatically increases its size if elements beyond its current capacity are added l But a vector does not automatically decrease its size if elements are deleted The method trimSize shrinks the capacity of a vector to its current size so there is no extra, wasted space »the allocated space is reduced to whatever is currently being used To use storage more efficiently, use trimSize when a vector will not need its extra capacity later

14. 14 The method clone is used to make a copy of a vector but its return type is Object, not Vector »of course you want it to be Vector, not Object l So, what do you do? »Cast it to Vector Vector v = new Vector(10); Vector otherV; otherV = vector; Vector otherV = (Vector)v.clone(); And Another Detail: Correcting the Return Type of clone This just makes otherV another name for the vector v (there is only one copy of the vector and it now has two names) This creates a second copy of v with a different name, otherV

15. 15 And Yet Another Detail: Protecting Private Variables l Just as with arrays, be careful not to return addresses of private vector variables, otherwise calling methods can access them directly »"Information Hiding" is compromised l To protect against it, return a copy of the vector »use clone as described in the previous slide l But that's not all: »if the elements of the vector are class (and not primitive) types, they may not have been written to pass a copy »they may pass the address »so additional work may be required to fix the accessor methods

16. 16 Linked Lists “Math" "and" “Psy" head “CS" One node in the list Data in a node A link in a node Null link signifying the end of the list null l Linked lists consists of objects known as nodes l Each node has a place for data and a link to another node l Links are shown as arrows l Each node is an object of a class that has two instance variables: one for the data and one for the link The head of the list is not a node.

17. 17 ListNode Class: Instance Variables and Constructor public class ListNode { private String data; private ListNode link; public ListNode(String newData, ListNode linkValue) { data = newData; link = linkValue; } Two parameters for the constructor: l data value for the new node l Link value for the new node “CS"

18. 18 Start at beginning of list Stepping through a List “Math" "and" “Psy" head “CS" When position is at this last node, position.getLink() is null and the loop will terminate. null ListNode position; position = head; while (position != null) {... position = position.getLink(); } position This reference is position.getLink(). Moves to next node in the list. Excerpt from showList in StringLinkedList

19. 19 Adding a Node To add a node at the beginning of the list: public void addANodeToStart(String addData) { head = new ListNode(addData, head); } The new node will point to the old start of the list, which is what head pointed to. The value of head is changed to point to the new node, which is now the first node in the list.

20. 20 Deleting a Node To delete a node from the beginning of the list: public void deleteHeadNode() { if (head != null) { head = head.getLink(); } else // prints an error message and exits... l Doesn't try to delete from an empty list. l Removes first element and sets head to point to the node that was second but is now first.

21. 21 Node Inner Classes Using an inner class makes StringLinkedList self- contained because it doesn't depend on a separate file l Making the inner class private makes it safer from the point of view of information hiding public class StringLinkedList { private ListNode head; private class ListNode { }

22. 22 l An object that allows a program to step through a collection of objects and do some action on each one is called an iterator. l For arrays, an index variable can be used as an iterator, with the action of going to the next thing in the list being something like: index++; l In a linked list, a reference to the first node can be used as an iterator StringLinkedListSelfContained has an instance variable called current that is used keep track of where the iteration is. l The goToNext method moves to the next node in the list by using the statement: current = current.next; Iterators

23. 23 goToNext “Math" "and" “Psy" head “CS" null previous current “Math" "and" “Psy" head “CS" null previous current current.link current = current.link gives current a reference to this node BeforeAfter

24. 24 Other Methods in the Linked List with Iterator getDataAtCurrent —returns the data part of the node that the iterator ( current ) is at moreToIterate— returns a boolean value that will be true if the iterator is not at the end of the list resetIteration— moves the iterator to the beginning of the list l Can write methods to add and delete nodes at the iterator instead of only at the head of the list. »Following slides show diagrams illustrating the add and delete methods.

25. 25 Adding a Node Step 1 “Math" "and" head “CS" “Psy" null newNode current Before "Cheatem" "and" head "Duey" "Howe" null newNode current After Create the node with reference newNode Add data to the node newNode.link = current.link null

26. 26 Adding a Node Step 2 “Math" "and" head “CS" “Psy" null newNode current After current.link = newNode.link The node has been added to the list although it might appear out of place in this diagram. “Math" "and" head “CS" “Psy" null newNode current Before

27. 27 l After creating the node, the two statements used to add the node to the list are: newNode.link = current.link; current.link = newNode; l What would happen if these two steps were done in reverse order? Adding a Node

28. 28 Deleting a Node Step 1 “Math" previous “CS" current Before “Math" previous “CS" “Psy" null “Psy" null newNode current After previous.link = current.link What should be done next? "and" This node will be removed from the list.

29. 29 Deleting a Node Step 2 current = current.link The node has been deleted from the list although it is still shown in this picture. “Math" previous “CS" “Psy" null newNode current After "and" “Math" previous “CS" “Psy" null newNode current Before "and" This node is not accessible from the head of the list.

30. 30 l A doubly linked list allows the program to move backward as well as forward in the list. l The beginning of the node class for a doubly-linked list would look something like this: private class ListNode { private Object data private ListNode next; private ListNode previous; A Doubly Linked List null Declaring the data reference as class Object allows any kind of data to be stored in the list.

31. 31 Interfaces l A Java interface is a collection of abstract methods and constants l An abstract method is a method header without a method body (i.e., no implementation) An abstract method in an interface can be declared using the modifier abstract, but because all methods in an interface are abstract, it is usually left off. »cf: abstract methods in an abstract class must be declared explicitly using the abstract modifier. l An interface is used to formally define a set of methods that a class will implement

32. 32 Interfaces public interface Doable { public void doThis(); public int doThat(); public void doThis2 (float value, char ch); public boolean doTheOther (int num); } interface is a reserved word None of the methods in an interface are given a definition (body) A semicolon immediately follows each method header

33. 33 Interfaces l An interface cannot be instantiated »Doable d = new Doable(); // error l Like a class, a user-defined interface can be used as the type of variables. »Doable a, b; l Methods in an interface have public visibility by default l A class formally implements an interface by »stating so in the class header »providing implementations for each abstract method in the interface l If a class asserts that it implements an interface, it must define all methods in the interface or the compiler will produce errors.

34. 34 Interfaces public class CanDo implements Doable { public void doThis () { // whatever } public void doThat () { // whatever } // etc. } implements is a reserved word Each method listed in Doable is given a definition

35. 35 Interfaces l A class can implement more than one interfaces l The interfaces are listed in the implements clause, separated by commas l The class must implement all methods in all interfaces listed in the header

36. 36 9 Interfaces l An interface can be implemented by multiple classes l Each implementing class can provide their own unique version of the method definitions l An interface is not part of the class hierarchy l A class can be derived from a base class and implement one or more interfaces

37. 37 10 Interface constants l Unlike interface methods, interface constants require nothing special of the implementing class l Constants in an interface can be used in the implementing class as if they were declared locally l This feature provides a convenient technique for distributing common constant values among multiple classes

38. 38 11 Extending Interfaces An interface can be derived from another interface, using the extends reserved word l The child interface inherits the constants and abstract methods of the parent l Note that the interface hierarchy and the class hierarchy are distinct l Unlike class hierarchy, an interface can extend more than one interfaces. »public interface Transformable extends Scable, Translatable, Rotatable { } l A class that implements an interface must define also all methods in all ancestors of the interface.

39. 39 interface Printable { public String name(); public String print(); // public can be omitted } // interface Printable class PrintLogger { public void log (Printable file) { System.out.println (file.name() + " : " + file.print()); } // method log } // class PrintLogger An interface Example

40. 40 class File { protected String id; protected int size; public File (String id, int size) { this.id = id; this.size = size; } // constructor File public String name() { return id; } // method name } // class File class TextFile extends File implements Printable { protected String text; public TextFile (String id, int size, String contents) { super(id, size); text = contents; } // constructor TextFile public String print() { return text; } } // class TextFile

41. 41 class BinaryFile extends File { protected byte[] data; public BinaryFile (String id, int size, byte[] data) { super(id, size); this.data = data; } // constructor BinaryFile } // class BinaryFile class ImageFile extends BinaryFile implements Printable { public ImageFile (String id, int size, byte[] data) { super(id, size, data); } // constructor ImageFile public String print() { return new String (data); } } // class Image_File

42. 42 public class Printer { public static void main (String[] args) { byte[] logoData = {41, 42, 49, 44 }; TextFile report = new TextFile (“Reprot 1", 1024, "One two three …"); ImageFile logo = new ImageFile(“Picture 1", 4, logoData); PrintLogger daily = new PrintLogger(); daily.log (report); daily.log (logo); }

43. 43 Marker interface l An interface without including any method. » useful for providing additional information about an object. »EX: » java.lang.Serializable » java.lang.Cloneable »java.rmi.Remote Ex: Object obj; Object copy; copy = o.clone() // may raise CloneNotSupportedExceptionexception if(obj instanceof Cloneable) copy = o.clone(); else copy = null;

44. 44 Polymorphism via Interfaces l An interface name can be used as the type of an object reference variable Doable obj; The obj reference can be used to point to any object of any class that implements the Doable interface The version of doThis that the following line invokes depends on the type of object that obj is referring to: obj.doThis();

45. 45 Summary l Vectors can be thought of as arrays that can grow in length as needed during run time. The base type of all vectors is Object. l Thus, vector elements can be of any class type, but not primitive types. l A linked list is a data structure consisting of objects known as nodes, such that each node can contain data, and each node has a reference to the next node in the list. l You can make a linked list self-contained by making the node class an inner class of the linked list class. l You can use an iterator to step through the elements of a collection.