CS 2102 Exam 2 D-Term 2013

Transcription

1 NAME: CS 2102 Exam 2 D-Term 2013 Questions 1-5: (20) Question 6: (45) Question 7: (35) TOTAL: (100) 1

2 1. (4 points) Which of the following statements are true? Circle all that apply. (a) A subclass object inherits private fields from its superclass (b) A subclass object is not allowed to access private fields in its superclass by name (c) A subclass object is not allowed to activate a private method defined in its superclass (d) If a method is public in a superclass, it may not be declared private in any of its subclasses 2. (4 points) Assume we defined the MenuItem class to appear as follows: class MenuItem { private String name; static protected Integer price; // rest of class definition omitted... Which of the following statements are true? Circle all that apply. (a) the code will not compile (b) subclasses of MenuItem will not be able to access the price field by name (c) all MenuItem objects will have the same price (d) we will not be able to write methods that change the value of the price 3. (4 points)true or false: in Java, if two objects ob1 and ob2 return the same value for hashcode(), then it must be true that ob1.equals(ob2). 2

3 4. (4 points) The following class is defined for use as the key value for a hash table: class Key{ int k1; int k2; Key(int k1, int k2){ this.k1 = k1; this.k2 = k2; boolean equals (Key k){ return this.k1 == k.k1 && this.k2 == k.k2; int hashcode(){ int h1, h2; h1 = k1.hashcode(); h2 = k2.hashcode(); return h1 * 37 + h2; Are instances of this class appropriate to use as keys in a hash table (yes or no)? If no, explain why not in one sentence. 5. (4 points) Which of the following statements best describes memoization? (a) a hashtable that uses function inputs as keys and function results as values (b) a data structure that maps previously used inputs to outputs (c) a transformation on functions that stores previously computed results so as not to process the same inputs more than once (d) a function that never computes the same result twice 3

4 6. (45 points) A Java program is used to generate mailing lists for various subsets of students at a university. For example, sometimes a letter is to be sent to all undergraduates whose year of graduation is Or, a letter inviting all honors students to a banquet requires a mailing list of all students with a GPA over 3.5. Refer to the Mailing list handout provided with this exam as you answer the following questions: (a) (20 points) Write a method in the MasterList class called mailinglist that returns a mailing list (a LinkedList) of all those students in the master list who satisfy a given IChoose function. (b) (5 points) A programmer working on the mailing list program notices that the implementation of the accept method in both the Ugrad and Grad classes is exactly the same. Since good programming practice dictates that shared data and methods be defined in the abstract class, he decides to remove the implementation from each of the subclasses and put it in the AbsStudent class instead. Explain why this is not a good idea for this particular program. 4

5 (c) (20 points) The MasterList class fixes the representation of the master list information in a linked list. It is desired to make the program more flexible so that any data structure can be used to store the information in the master list. Outline the steps required to do this (i.e. explain, using any combination of prose and Java code, how you would modify the given program to accomplish this). 5

6 7. (35 points) Software for a social networking site uses the following class definitions: class MyFace{ private LinkedList<Person> persons; public MyFace(){ this.persons = new LinkedList<Person> (); // adds a new Person to the social network and returns that Person public Person makeperson(string name, int age){ Person p = new Person(name, age); this.persons.add(p); return p; // make the given people friends of each other public void makefriends (Person a, Person b){ a.friend(b); <--- YOU NEED TO WRITE THE friend METHOD class Person{ private String name; private int age; private LinkedList<Person> friends; public Person (String name, int age){ this.name = name; this.age = age; this.friends = new LinkedList<Person>(); (a) (5 points) Write the method friend for the Person class. 6

7 (b) (15 points) It is desired to initialize a MyFace called M to represent the social network pictured here: Hao ---- Denny---Kate / / Brian Complete the definition of the Examples class below so that M is initialized as pictured. class Examples{ MyFace M = new MyFace(); // add any code necessary to initialize M so that it represents the // pictured social network 7

8 (c) (15 points) A method with the following signature is added to the MyFace class: // return a list of the names of everyone who is a direct friend of // the given person public LinkedList<String> namesoffriends(person P){... How would you test the namesoffriends method, if you wanted to test the method on the person Denny in the pictured social network? Either write the Java code for the Examples class, or provide a detailed explanation of how you would test. 8