ECE 242 Spring 2003 Data Structures in Java - PowerPoint PPT Presentation

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ECE 242 Spring 2003 Data Structures in Java

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  1. ECE 242 Spring 2003Data Structures in Java http://rio.ecs.umass.edu/ece242 Queue Prof. Lixin Gao Data Structures in Java, Prof. Gao

  2. Today’s Topics • Queue • Implementation of Queue • Usage of Queue Data Structures in Java, Prof. Gao

  3. D C B A Queue • Queue: First In First Out (FIFO) • Toll Station • Car comes, pays, leaves • Check-out in Big Y market • Customer comes, checks out and leaves • More examples: Printer, Office Hours, … Output Input Data Structures in Java, Prof. Gao

  4. More Examples of Queue • In our daily life • Airport Security Check • Cinema Ticket Office • Bank, ATM • Anything else ? Data Structures in Java, Prof. Gao

  5. What Is Queue • Queue is an abstract data type • Adding an entry at the rear • Deleting an entry at the front Adding Deleting C B A front rear Data Structures in Java, Prof. Gao

  6. Abstract Data Types • Queue • Operating on both ends • Operations: EnQueue(in), DeQueue(out) enqueue dequeue C B A front rear Data Structures in Java, Prof. Gao

  7. Queue Queue is FIFO ( First-In First-Out) A queue is open at two ends. You can only add entry (enqueue) at the rear , and delete entry (dequeue) at the front. Note that you cannot add/extract entry in the middle of the queue. Data Structures in Java, Prof. Gao

  8. Applications of Queue • Printing Job Management • Packet Forwarding in Routers • Message queue in Windows • I/O buffer Data Structures in Java, Prof. Gao

  9. Printing Job Management • Many users send their printing jobs to ECS public printer • Printer will put them into a queue according to the arrival time and print the jobs one by one • These printing documents are A.doc, B.doc, C.doc and D.doc Data Structures in Java, Prof. Gao

  10. Printing Queue • A.doc B.doc C.doc arrive to printer. Now printing A.doc C B A C B A.doc is finished. Now printing B.doc D.doc comes D C B Now still printing B.doc D C B.doc is finished. Now printing C.doc D C.doc is finished. Now printing D.doc Data Structures in Java, Prof. Gao

  11. First-in First-out (FIFO) A, B, C come in The first one enqueued is the first one dequeued. (FIFO) C B A C When we enqueue entries in the queue and then dequeue them one by one, we will get the items in the same order. B A A, B, C come out C B A Data Structures in Java, Prof. Gao

  12. Question • Queue is an abstract data structure • Item can be Integer, Double, String, Employee, Faculty… • How to implement a general queue for all those types? Data Structures in Java, Prof. Gao

  13. Abstract Data Type • Same as Stack, we use Object data type instead of int or double or String or other data type • Use an array in queue, which stores all items come in. • Object Queue[ ]; • Other implementations are possible Data Structures in Java, Prof. Gao

  14. Array Implementation of Queue n-1 3 2 1 0 D C B A rear front Max_Size After A leaves, n-1 3 2 1 0 D C B rear front Max_Size Data Structures in Java, Prof. Gao

  15. Operations • enqueue • add a new item at the rear • dequeue • remove a item from the front • isEmpty • check whether the queue is empty or not • isFull • check whether the queue is full or not • size • return the number of items in the queue • peek • return the front item Data Structures in Java, Prof. Gao

  16. Problem • An array has limited size, once rear is at the end of this array, and there is new item coming in, what can we do? n-1 3 2 1 0 Y X … … front rear Data Structures in Java, Prof. Gao

  17. Two Solutions • Shifting all items to front in the array when dequeue operation. ( Too Costly… ) • Wrapped around array ---- Circular Array A leaves n-1 3 2 1 0 n-1 3 2 1 0 … … C B A … … C B rear=3 front=0 rear=2 front=0 Data Structures in Java, Prof. Gao

  18. Circular Array rear=3 • Wrapped around array 3 2 C front=0 1 n-1 3 2 1 0 B … … C B A 0 A n-1 rear=3 front=0 Data Structures in Java, Prof. Gao

  19. EnQueue & DeQueue In Circular Array • DeQueue • front = (front + 1) MOD n • EnQueue • rear = (rear + 1) MOD n rear=3 front=1 3 3 2 2 C C 1 1 B B 0 A 0 n-1 n-1 Data Structures in Java, Prof. Gao

  20. Empty/Full In Circular Array • When rear equals front, Queue is empty • When (rear + 1) MOD n equals front, Queue is full • Circular array with capacity n at most can hold n-1 items. Data Structures in Java, Prof. Gao

  21. Implementation for Queue public class ArrayQueuePT { private final static int DEFAULT_CAPACITY = 100; // suppose the default capacity for this queue is 100. private Object queue[ ]; // The array that holds the items private int rear, front; // Index of rear, front item in the queue; } Data Structures in Java, Prof. Gao

  22. ArrayQueuePT Constructor // Creates a queue with the default capacity public ArrayQueuePT () { this(DEFAULT_CAPACITY); } // Creates a queue with a user-specified capacity public ArrayQueuePT (int capacity) { if (capacity < 2) throw new IllegalArgumentException ("Capacity must be > 1"); queue = new Object[capacity]; rear = front = 0; } Data Structures in Java, Prof. Gao

  23. ArrayQueuePT --- Size() • How many items currently in the queue? public int size() { if( rear >= front ) return rear – front; else return queue.length + rear – front; } Data Structures in Java, Prof. Gao

  24. ArrayQueuePT --- isEmpty/isFull // check whether the queue is empty public boolean isEmpty() { return rear == front; } // check whether the queue is full public boolean isFull() { return size() == queue.length-1; } Data Structures in Java, Prof. Gao

  25. ArrayQueuePT --- enqueue() public void enqueue(Object item) { if (item == null) throw new IllegalArgumentException ("Item is null"); if (isFull()) throw new IllegalStateException (“Queue is full"); queue[rear] = item; rear = (rear + 1)%queue.length; } Data Structures in Java, Prof. Gao

  26. ArrayQueuePT --- dequeue() public Object pop( ) { if (isEmpty()) throw new IllegalStateException (“Queue is empty"); Object frontItem = queue[front]; queue[front] = null; front = (front + 1)%queue.length; return frontItem; } Data Structures in Java, Prof. Gao

  27. ArrayQueuePT peek() Method public Object peek() { if (isEmpty()) throw new IllegalStateException (“Queue is empty"); return queue[front]; } Data Structures in Java, Prof. Gao

  28. Interface • Users don’t need to know how Queue is implemented. • User only need to know how they can operate the Queue. • There are many ways to implement Queue, but all of them have the same interfaces • enqueue, dequeue, peek, isFull, isEmpty Data Structures in Java, Prof. Gao

  29. Interface and Implementation Class • Interface contains a skeleton for public operations • No real code for each method • Like function prototype • A class implements interface • Implements every method • Statement: public class myclass implements interface { … … } Data Structures in Java, Prof. Gao

  30. Interface for Queue public interface QueuePT { public boolean isEmpty(); public boolean isFull(); public Object peek(); public Object dequeue(); public void enqueue(Object item); public int size(); } Data Structures in Java, Prof. Gao

  31. Implementation of ArrayQueuePT Class • Example: QueuePT.java, ArrayQueuePT.java public class ArrayQueuePT implements QueuePT { private final static int DEFAULT_CAPACITY = 100; private Object queue[];// The array holds the queue private int rear, front; // Index of rear, front items in queue Data Structures in Java, Prof. Gao

  32. Create an object of ArrayQueuePT // create a queue with default capacity QueuePT myqueue = new ArrayQueuePT(); // create a queue with 1024 elements QueuePT registry = new ArrayQueuePT(1024); Data Structures in Java, Prof. Gao

  33. Using ArrayQueuePT • enqueue and dequeue characters in Queue myque • myque.enqueue( new Character(‘a’) ) • char ch = (Character)myque.dequeue()).charValue() • enqueue and dequeue Employee in Queue registry • registry.enqueue( new Employee(“Lixin”, “5456”) ) • Employee emp = (Employee) registry.dequeue(); Data Structures in Java, Prof. Gao

  34. Printing Job Management • 4 documents are ready to print • Print the printer status • new document comes • which document is finished • Using ArrayQueuePT • Complete Example: • PrintingJobQueue.java Data Structures in Java, Prof. Gao

  35. New Printing Jobs come public void printstatus() { QueuePT que = new ArrayQueuePT( ); //Create a newqueue System.out.println("Printing job: Null "); //print the printer status // new printing jobs come, a.doc, b.doc, c.doc, d.doc System.out.println("New printing job: a.doc"); que.enqueue( "a.doc" ); System.out.println("New printing job: b.doc"); que.enqueue( "b.doc" ); System.out.println("New printing job: c.doc"); que.enqueue( "c.doc" ); System.out.println("New printing job: d.doc"); que.enqueue( "d.doc" ); Data Structures in Java, Prof. Gao

  36. Finishing Printing Jobs // print the printer status System.out.println("\nPrinting job: there are " + que.size() + " jobs in the queue"); System.out.println(" " + que.dequeue() + " is Finished"); System.out.println(" " + que.dequeue() + " is Finished"); System.out.println(" " + que.dequeue() + " is Finished"); System.out.println(" " + que.dequeue() + " is Finished"); } Data Structures in Java, Prof. Gao

  37. Customer Service In Fleet Bank • Suppose there is only one customer service available in Fleet Bank in Saturday morning • In every 3 minutes, a new customer arrives at the end of waiting line • Each customer will need 5 minutes for the service • Print out the information after the first 30 minutes • The time of arriving and leaving for each customers • How many customers are in the line? • Who is the current serving customer? Data Structures in Java, Prof. Gao

  38. Customer Service Queue • Complete example: BankServiceQueue.java public void run( ) { // Create a new queue QueuePT que = new ArrayQueuePT(100); int time = 0;// in minutes int incustomer = 0;// secquence of customers int servicetime = 0;// service times Data Structures in Java, Prof. Gao

  39. Customer In Service // what's going on in 30 minutes while ( time <= 30 ) { // if queue is not empty, one customer service is working if( que.size()!=0 ) { servicetime = servicetime + 1; // customer leaves when finishing the service, the service time is 5 minutes if( servicetime == 5 ) { String name = (String)que.dequeue(); System.out.println("<< " + name + " leaves at time = " + time); // start to service time for next customer servicetime = 0; } } Data Structures in Java, Prof. Gao

  40. New Customer Comes // in every 3 minutes, there is a new customer coming. if( time%3==0 ) { incustomer = incustomer + 1; String name = "CUSTOMER " + incustomer; que.enqueue( name ); System.out.println(">> " + name + " arrives at time = " + time); } time = time + 1; } Data Structures in Java, Prof. Gao

  41. Print Status After 30 Minutes // print the status after 30 minutes System.out.println("\n==========================" ); if( que.size()!=0 ) { System.out.println("There are " + que.size() + " customers in the line" ); System.out.println("The current serving customer is " + que.peek() ); } else { System.out.println("There are no customers in the line" ); } } Data Structures in Java, Prof. Gao

  42. Priority Queue --- Air Travel • Only one check-in service in United Airline at airport • Two waiting lines for passengers • one is First class service • the other is Economy class service • Passengers in the first-class waiting line have higher priority to check in than those in the economy-class waiting line. Data Structures in Java, Prof. Gao

  43. Priority Queue • Two queues • one is high priority queue • the other is low priority queue • Service rules: • First serve the people in high priority queue • If no passengers are in high priority queue, serve the passengers in low priority queue Data Structures in Java, Prof. Gao

  44. Two Queues • High Priority Queue, will come in hpQue • Low Priority Queue, will come in lpQue High Priority Queue Check In Customers coming in H D C G F E B A Low Priority Queue Data Structures in Java, Prof. Gao

  45. Pseudocode For Arrival Passengers Arrival: if( new Passenger comes ) { if( is First Class) hpQue.enqueue( new Passenger ); else lpQue.enqueue( new Passenger ); } Data Structures in Java, Prof. Gao

  46. Pseudocode For Service Check-In Service: if( hpQue is not empty ) { serve the passenger from high priority queue, hpQue.dequeue(); } else { serve the passenger from low priority queue, lpQue.dequeue(); } Data Structures in Java, Prof. Gao