Test 3 Solutions

1. (20%) Implement a method called isFileThere(String fileName) which returns true if either fileName exists (is the name of a file in the filesystem) or filename.png exists. For example, if the file is "apple" then it should return true if either "apple" or "apple.png" refer to a file in the filesystem. Return false otherwise.
   Answer:

    public static boolean isFileThere(String fileName) {
     File f = new File(fileName);
     if (f.exists()) {
      return true;
     }
     f = new File(fileName + ".png");
     if (f.exists()) {
      return true;
     }
     return false;
    }
    

2. (20%) What does the following program print out?

   public class Exceptional {
   
    public void happyDays(int x, int y) throws Exception {
     int a[] = {4}; //array with one item.
     try {
      int d = a[x];
      if (y == 0) {
       throw new Exception("Heeeeyyyy");
      }
      System.out.println("Hello Fonzie");
     }
     catch (IndexOutOfBoundsException e) {
      System.out.println("Hello Richie");
     }
    }
   
    public static void main(String[] args) {
     Exceptional e = new Exceptional();
     try {
      e.happyDays(0,1);
      e.happyDays(1,1);
      e.happyDays(0,0);
     } catch (Exception e1) {
      System.out.println("Hello Potsie");
     }
    }
   
   }

   Answer: There where three different tests, each with a different answer:

   public static void main(String[] args) {
    Exceptional e = new Exceptional();
    try {
     e.happyDays(0,1);
     e.happyDays(1,1);
     e.happyDays(0,0);
    } catch (Exception e1) {
     System.out.println("Hellow Potsie");
    }
    System.out.println("-------");
    try {
     e.happyDays(1,1);
     e.happyDays(0,1);
     e.happyDays(0,0);
    } catch (Exception e1) {
     System.out.println("Hellow Potsie");
    }
    System.out.println("--------");
    try {
     e.happyDays(1,1);
     e.happyDays(0,0);
     e.happyDays(1,0);
    } catch (Exception e1) {
     System.out.println("Hellow Potsie");
    }
   }
   Hello Fonzie
   Hello Richie
   Hellow Potsie
   -------
   Hello Richie
   Hello Fonzie
   Hellow Potsie
   --------
   Hello Richie
   Hellow Potsie
     
     

3. (30%) Implement a recursive method that returns the mutant power of a Person. The mutant power of a Person depends on whether or not that person has the X-Gene, and how many of her ancestors have the X-Gene, with maternal lineage being doubly important.
   Specifically, you are given the class

   public class Person {
    public Person mother;
    public Person father;
    /** True if person had the X-Gene */
    public boolean xgene;
   } 

   where either mother or father could be null, signifying that no one up that lineage has the X-Gene (ignore them). The mutantPower of a Person Y is given by the formula:
   • mutant-power(Y) = (1 if Y has X-Gene, 0 otherwise) + 2 * mutant-power(Y's mom) + mutant-power(Y's dad).
   Implement int Person.calculateMutantPower(). For example, the code below prints out xavier's mutant power which is 3.

   public static void main(String[] args) {
    Person xavier = new Person();
    xavier.xgene = true;
    Person mom = new Person();
    mom.xgene = true;
    xavier.mother = mom;
    System.out.println(xavier.calculateMutantPower()); // 3
   }

   Answer:

   public int calculateMutantPower () {
    int power = (xgene) ? 1 : 0;
    int motherPower = 0;
    int fatherPower = 0;
    //The base case is when both mother and father are null, in which case we just return 1.
    if (mother != null) {
     motherPower = mother.calculateMutantPower();
    }
    if (father != null) {
     fatherPower = father.calculateMutantPower();
    }
    return power + 2*motherPower + fatherPower;
   }
     

4. (30%) Implement a class called Set which implements a mathematical set, which we define as a bunch of elements of the same type where no element appears more than once (no repeats allowed). You will use an ArrayList to implement your Set.
   The Set will implement 3 methods:
   1. a no-argument constructor (Tip: see Listing 12.10 in your textbook)
   2. an add(...) method which adds a new element to the set, if its not already there,
   3. a toString() method which turns the whole set into a string, with a dash '-' between each element.
   The following is an example usage of the Set class you will write:

   public static void main(String[] args) {
    Set<String> s = new Set<String>();
    s.add("Hi");
    s.add("Hello");
    s.add("Hi");
    s.add("Hi");
    s.add("how are you?");
    System.out.println(s); //prints Hi-Hello-how are you?-
   }
        

   Answer:

   import java.util.ArrayList;
   
   public class Set<T> {
   
    private ArrayList<T> list;
    
    public Set() {
     list = new ArrayList<T>(10); 
    }
    
    public void add(T element) {
     if (!list.contains(element)) {
      list.add(element);
     }
    }
    
    public String toString() {
     String result = "";
     for (int i=0; i < list.size();i++) {
      result += list.get(i) + "-";
     }
     return result;
    }
   }