Java Lecture 6

1. Java Lecture 6 CS 1311X Self-Referential Structures Building a Queue 13X11

2. Self-Referential? • Simply means that a class has a reference to an object of that class • Common applications • Linked list nodes • Binary tree nodes

3. Linked Lists Nodes in Java • Amazingly similar to a cons cell • Stripped down version: class Node { Object data; Node next; }

4. data data data next next next A Node Object Some Object Some Object Node ref

5. More useful? class Node { public Object data; public Node next; public Node(Object data) { this(data, null); } public Node(Object data, Node next) { this.data = data; this.next = next; }

6. Node (continued) public String toString() { return "Node: " + data; }

7. Pop Quiz public String toString() { return "Node: " + data + " Next:\n" + next; } What does it do?

8. Test Main public static void main(String args[]) { Node n1 = new Node("Hello "); Node n2 = new Node("World!")' n1.next = n2; System.out.println("Test1\n" + n1); System.out.println("Test2\n" + n2); Node nz = new Node("and Nod."); Node ny = new Node("Blynken ", nz); Node nx = new Node("Wynken, ", ny); System.out.println("Test3\n" + nx); ny = null; nz = null; System.out.println("Test4\n" + nx);

9. Test Main Output Test1 Node: Hello Next: Node: World! Next: null Test2 Node: World! Next: null Test3 Node: Wynken, Next: Node: Blynken Next: Node: and Nod. Next: null Test4 Node: Wynken, Next: Node: Blynken Next: Node: and Nod. Next: null

10. Test Main Node head = new Node("Bob,", new Node("Carol, ", new Node("Ted, ", new Node("and Alice.")))); System.out.println("Test5\n" + head); Scheme like construction

11. Test Main Output Test5 Node: Bob, Next: Node: Carol, Next: Node: Ted, Next: Node: and Alice. Next: null

12. Test Main Node list = new Node("Larry,", new Node(null, new Node("Moe, ", new Node("and Curly.")))); System.out.println("Test6\n" + list); } // main } // Node Another Scheme like construction

13. Test Main Output Test6 Node: Larry, Next: Node: *null* Next: Node: Moe, Next: Node: and Curly. Next: null

14. Questions?

15. C'mon, Kids! Let's build a Queue!

16. So What's a Queue? • First-In First-Out Data Structure • British word for line (Queue up for a pint.) • French word for tail (Like a horse's tail). • Multiple ways to implement • Common to use Linked list • etc. • Typical behaviors • isEmpty • enqueue • dequeue • head (or front or top or peek)

17. Linked List Implementation • Can use our Node class • Will need another class with a catchy name like Queue • What's in the Queue class? • head pointer (reference) • tail pointer (reference) • Note: If head == null then tail == null (and vice versa) and the Queue isEmpty! • implementation of behaviors

18. isEmpty • Returns a boolean • Something like: return (head == null);

19. Let's write some code! class Queue { private Node head; private Node tail; public Queue() { head = null; tail = null; } public boolean isEmpty() { return (head == null); }

20. Case: isEmpty() Create a new Node data points to newData Make head and tail point to the new Node Case: ! isEmpty() Create a new Node data points to newData Make the old tail Node point to the new Node Make the tail pointer point to the new Node Enqueue(Object newData)

21. Create a new Node data points to newData Case: isEmpty(); Make head and tail point to the new Node Create a new Node data points to newData Case: ! isEmpty() Make the old tail Node point to the new Node Make the tail pointer point to the new Node Enqueue(Object newData)

22. Enqueue(newData): isEmpty() Queue Object head tail

23. Nota Bene In the case of a Queue we will always make new Nodes with the next reference set equal to null

24. data next Enqueue(newData): isEmpty() newData Queue Object temp head tail

25. data next Enqueue(newData): isEmpty() newData Queue Object temp head tail

26. data next Enqueue(newData): isEmpty() newData Queue Object temp head tail

27. data next Enqueue(newData): isEmpty() newData Queue Object head tail

28. Questions?

29. data next Enqueue(newData): ! isEmpty() Queue Object head tail

30. data data next next Enqueue(newData): ! isEmpty() newData Queue Object temp head tail

31. data data next next Enqueue(newData): ! isEmpty() newData Queue Object temp head tail

32. data data next next Enqueue(newData): ! isEmpty() newData Queue Object temp head tail

33. data data next next Enqueue(newData): ! isEmpty() newData Queue Object head tail

34. Questions?

35. data data next next Enqueue(newData): ! isEmpty() Queue Object head tail

36. data data data next next next Enqueue(newData): ! isEmpty() Queue Object newData head tail temp

37. data data data next next next Enqueue(newData): ! isEmpty() Queue Object newData head tail temp

38. data data data next next next Enqueue(newData): ! isEmpty() Queue Object newData head tail temp

39. data data data next next next Enqueue(newData): ! isEmpty() Queue Object newData head tail

40. Questions?

41. Let's write some more code! public void enqueue(Object o) { Node temp; temp = new Node(o); if(isEmpty()) { head = temp; tail = temp; } else // Queue is not empty... { tail.next = temp; tail = temp; } }

42. Assume ! isEmpty we'll check Save value that head data reference is pointing to (return value) Make head pointer point to whatever first node's next is pointing to... Case: head is not null Assume ! isEmpty we'll check Save value that head data reference is pointing to (return value) Make head pointer point to whatever first node's next is pointing to... Case: head is null Set tail to null Almost done! Now Dequeue

43. data data data next next next Dequeue() Red Green Queue Object Blue head tail

44. data data data next next next Dequeue() retval Red Green Queue Object Blue head tail

45. data data data next next next Dequeue() retval Red Green Queue Object Blue head tail

46. data data next next Dequeue() retval Red Green Queue Object Blue head tail

47. data data next next Dequeue() Return retval Red Green Queue Object Blue head tail

48. data data next next Dequeue() Green Queue Object Blue head tail

49. Questions?

50. data data next next Dequeue() Green Queue Object Blue head tail