322C EE Data Structures Spring 2009
Quiz 5 Instructor: Ramesh Yerraballi Grader: Vinoth Chandar Due : Wednesday 4/29 in class Total : 100 points Student: Anirudh Pande Date: 4/29/09
1) Collections.sort(List<T>) method takes any list and sorts it according to the type of objects held in the list. For example, Strings are sorted alphabetically, Integers are sorted by value and Date objects chronologically. [15 points ] a) How is this behavior implemented?
Collections.sort is implemented as a modified mergesort, where the merge is omitted if the highest element in the low sublist is less than the lowest element in the high sublist. The method sorts the specified list into ascending order, according to the natural ordering of its elements. The implementation of the method dumps the specified list into an array, sorts the array, and iterates over the list resetting each element from the corresponding position in the array. The best case of the algorithm offers a n log(n) performance. For the method to work, all elements in the list must implement the Comparable interface. Furthermore, all elements in the list must be mutually comparable, meaning, e1.compareTo(e2) must not throw a ClassCastException for any elements e1 and e2 in the list. Finally, the specified list must be modifiable, but not resizeable.
b) List<Integer> lst = new ArrayList<Integer> (); Write java code to provide you a sorted list of integers, lst, in the descending order, using the Collections.sort method. [You cannot use the Collections.reverse method]
class sortList implements Comparator { for(int i=0;i<lst.size();i++) { int smaller = list.compare((lst.get(i)),(lst.get(i+1))); lst.add(i,smaller); public int compare(int i1, int i2) { return new Integer(i1).compareTo(new Integer(i2)); } }
2) Implement a generic queue class , MyQueue<T> using two stacks [Stack class in the Collections API]. The class should have a default constructor, enqueue method that takes an element as argument and adds it to the end of the queue and dequeue method that removes an element from the front of the queue. [30 points ] An example usage is given below.
public class MyQueue { public
static
void main(String[] args){
public MyQueue() { Stack <Integer> inboxStack = new Stack<Integer>; Stack<Integer> outboxStack = new Stack<Integer>; } void Enqueue(int element) { inboxStack.push(element); } int Dequeue() { if(outboxStack.Count > 0) { return outboxStack.Pop(); } else { while(inboxStack.Count > 0) { outboxStack.Push(inboxStack.Pop()); } if(outboxStack.Count > 0) { return outboxStack.Pop(); } } } } }
MyQueue<Integer> queue = new MyQueue<Integer>(); queue.enqueue(2); queue.enqueue(4); System.out.println(queue.dequeue()); System.out.println(queue.dequeue());
3) What is the difference between the Iterator interface and the ListIterator interface? [ 5 points]
ListIterator is a sub-interface of Iterator and so the difference between these interfaces is that ListIterator has a specific usage within lists whereas Iterator does not. ListIterator can perform operations such as remove, set elements within a list whereas Iterator the works on any collection as well as on lists.
4) You have a stream of float values coming to your computer from an external sensor every minute. Design an appropriate data structure such that 1) Given the date, you should be able to retrieve the sensor reading as fast as you can 2) In addition to 1, given any time range, you must be able to retrieve all the values in that interval with minimal programming effort. [10 points]
5) Joe is a typical lazy programmer. He wants to store some extra information about objects in his program [say they are encapsulated in an ExtraInformation class ]. He wants to retrieve the extra information, given the object, as fast as possible. Also, he requires that this extra information be garbage collected from the data store automatically [I.e he does not have to invoke any methods on the data structure representing the extra information store] when the original object is no longer in use, in his program. Suggest a suitable data structure for joe and also explain how it will solve his problem [15 points]
Joe should use a hash table for his program. Hash tables have a big-Oh time of logN so if Joe wants to retrieve information the fastest, the hash table would be ideal for his job. Also hash tables store data without any methods being invoked when the original object is not in use so it would ideal for Joes needs.
6) What is referred to as the fail fast behavior of Iterators in the Collections API? [ 5 Points ]
Fail-fast term is about Java collections and theirs iteration and modification, which are not synchronized. The iterator returned by a class's iterator method is named fail-fast if: the set is modified at any time after the iterator is created, in any way except through the iterator's own remove method. This causes the Iterator to throw a ConcurrentModificationException.
7) The RandomAccess interface has no methods. Why do you think that such an interface is required? [10 points]
The RandomAccess interface is not required but it is useful in allowing generic algorithms to alter their behavior to provide good performance when applied to either random or sequential access lists. This interface is encouraged to be used if a list implementation provides asymptotically linear access times if they get huge, but constant access times in practice. Also, the best algorithms for manipulating random access lists can produce quadratic behavior when applied to sequential access lists so this algorithm is encouraged to check whether the given list is an instanceof this interface before applying an algorithm that would provide poor performance if it were applied to a sequential access list, and to alter their behavior if necessary to guarantee acceptable performance.
8) As a continuation of problem 4, assume that you do not know the exact times of that day that the sensor reported values at. Can you use some Collections API class to perform lookups given anytime of the day, with minimal programming effort? Write code to implement a search such that given a time of the day, we obtain the nearest sensor reading in time. If there are ties, choose the latest reading of the two. [10 points]