Midterm 2 Grading Key

Transcription

1 Midterm 2 Grading Key Question Correct Answer Question Correct Answer 1 C A: LEGAL 2 A B: LEGAL 3 FALSE C: LEGAL 4 B or D D: LEGAL 7 5 E E: ILLEGAL 6 C F: LEGAL G: ILLEGAL H: LEGAL

2 8. Calling Constructors (6 points) Felis constructor Tiger constructor Sabertooth constructor 9. Containment vs. Inheritance (6 points) Containment or composition describes a HAS-A relationship; one class contains an instance of another class as one of its instance variables. Inheritance describes an IS-A relationship; a subclass is based on its superclass (although it is usually more specialized), and can be readily substituted for an instance of the superclass.

3 10. Object Assignment (5 points) This statement is ILLEGAL, because you cannot assign a superclass (or any class that is higher up in the inheritance hierarchy) to a variable whose reference type is its subclass. In this case, there is no guarantee that a Student object has all of the same characteristics as an instance of its GraduateStudent subclass. 11. Principles of Polymorphism (6 points) The variable's REFERENCE TYPE (i.e., the type it is declared as) determines what methods may be called on that variable; we can only call methods that are found in an instance of the reference type. However, the variable's ACTUAL TYPE (determined by the specific class or subclass that is assigned to that reference variable) indicates which version of each method will be called; the most recently defined version of the method (i.e., the overridden version described closest to the actual type) will be invoked. For example, if Student extends Person, and Student overrides (redefines) method x() in Person, we can write: Person p = new Student(); p.x(); We are only allowed to call methods that are defined in the Person class (the reference class). If we do so, we will get the version of the method that is defined by the Student (actual type) class.

4 12. Triple-Cutting An Array (10 points) // 2 points: correctly copying or rearranging each group // 4 points for correctly modifying the array parameter void triplecut(playingcard [ ] deck, int pos1, int pos2) { PlayingCard [ ] temp = new PlayingCard[deck.length]; // Copy the contents of the original array into the temp array, // one section at a time int tempindex = 0; // next index to update in temp array // Group 3 comes first... for (int i = pos2 + 1; i < deck.length; i++) { temp[tempindex] = deck[i]; tempindex++; } // Group 2 for (int i = pos1; i <= pos2; i++) { temp[tempindex] = deck[i]; tempindex++; } // Group 1 comes at the end... for (int i = 0; i < pos1; i++) { temp[tempindex] = deck[i]; tempindex++; } // Now copy everything back into the original array (deck) for (int i = 0; i < temp.length; i++) { deck[i] = temp[i]; } }

5 13. Extracting Characters To The End (10 points) // 4 points: correctly producing the first part of the result // (every character except the one to be extracted). // 4 points: correctly producing the second part of the string // (the correct number of copies of the target character to move to // the end of the string). // 2 points: returning (not just printing) the result string. String extracttoend(string source, char movetoend) { String normalchars = ""; String extractedchars = ""; for (int i = 0; i < source.length(); i++) { char c = source.charat(i); if (c == movetoend) { extractedchars = extractedchars + c; } else { normalchars = normalchars + c; } } return normalchars + extractedchars; }

6 14. The Airplane class (10 points) // Assign 1 point for each correctly-declared instance variable or // method (don't worry whether the class is declared public or not). public class Airplane { private String fleetid; private int seatcapacity; private String manufacturer; private int modelnumber; private int range; public Airplane (String id, String maker, int model, int seats) { } public String getid () { } public void setid (String newid) { } public boolean addpassenger (Passenger p) { } private boolean isfull () { } }

7 CSE 114: Computer Science I Fall The MAMultipleChoice class (20 points) class MAMultipleChoice extends MultipleChoice { private String studentsanswer = null; // can be protected instead } public MAMultipleChoice () { } // worth 2 points public MAMultipleChoice(String text, String answer, int points) { super(text, answer, points); } // worth 2 points public MAMultipleChoice (String t, String a, int p, int s) { super(t, a, p, s); } // worth 2 points public String answer () { return correctanswer; } public String tostring () { return "Multiple-Answer " + super.tostring(); } // worth 2 points // worth 2 points public boolean iscorrect (String response) // worth 4 points { studentsanswer = response; // for part (e) int numcorrect = 0; String correctlower = correctanswer.tolowercase(); String actual = response.tolowercase(); } for (int i = 0; i < actual.length(); i++) { String ans = "" + actual.charat(i); if (correctlower.indexof(ans)!= -1) { numcorrect++; } } return (numcorrect == correctanswer.length()); public int score () // worth 6 points { if (studentsanswer == null) { return pointvalue; } } int partpoints = pointvalue / correctanswer.length(); int score = 0; String correctlower = correctanswer.tolowercase(); String actual = response.tolowercase(); for (int i = 0; i < actual.length(); i++) { String ans = "" + actual.charat(i); if (correctlower.indexof(ans)!= -1) { score += partpoints; } } return score;

8 16. The answerkey() method (10 points) // 3 points: making the question numbers 1-based, not 0-based. // 5 points: putting each question's info on a separate line. // 2 points: returning the result instead of printing it. public String answerkey() { String key = ""; for (int i = 1; i <= numquestions; i++) { Question next = quiz[i-1]; key = key + i + ". " + next.answer() + "\n"; } return key; }