CSE 143 Au03 Midterm 2 Sample Solution Page 1 of 7

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1 CSE 143 Au03 Midterm 2 Sample Solution Page 1 of 7 Question 1. (4 points) (a) If a precondition is not true when a method is called, two possible ways to detect and handle the situation are to use an assert statement or throw an exception. Which should be used? If the answer is it depends, then describe when it would be appropriate to use assert and when it would be appropriate to use an exception, or if neither is appropriate, describe why not. It depends. If the method is a public one that is intended to be used by client code, then an appropriate exception should be thrown. If the method is private and the failed precondition indicates an internal programming error, this should be checked with an assertion. (b) Java classifies some exceptions as checked and some as unchecked. Methods that might encounter checked exceptions must either handle them with a try-catch block or declare them in a throws clause in the method heading. Given that good exception handling is important in robust programs, it would seem like we should handle every sort of exception this way, yet this isn t required for unchecked exceptions. Why not? In your explanation, give at least one example of an unchecked exception. Unchecked exceptions like NullPointerException generally are the result of programming errors, which could happen almost anywhere. Requiring these to be declared or caught would be pointless, since this would have to be done on almost every method. Question 2. (2 points) Two of the key interfaces in the Java collection classes are List and Set. What are the two major differences between these kinds of collections? 1. Lists are ordered, Sets are not 2. Lists may contain duplicate items, Sets do not

2 CSE 143 Au03 Midterm 2 Sample Solution Page 2 of 7 Question 3. (2 points) When discussing files and streams we said that a file or stream had to be opened before it could be used for input or output. What happens when a file or stream is opened? Opening a stream or file creates a connection between the internal objects the programmer uses to manipulate the stream and the actual stream or file controlled by the operating system. Question 4. (2 points) Two of your colleagues can t agree on the best way to write a method heading and they ve come to you for advice. One colleague wants to write the following: void analyze(list thedata) {... while the other one prefers to use void analyze(arraylist thedata) {... Method analyze only uses methods that are present in both List and ArrayList, so it will work correctly with either type of parameter. Which parameter type is better and why? Or if there s no reason to prefer one over the other, why not? List would be better. That would allow method analyze to work with any kind of list, not just ArrayLists. Even if analyze is currently only used to process ArrayLists, by using List as the parameter type, we have the flexibility to use different kinds of Lists in the future without requiring modifications to analyze.

3 CSE 143 Au03 Midterm 2 Sample Solution Page 3 of 7 Selected reference information about some Java library classes and methods that may be useful for answering the questions on the following pages. Feel free to remove this page from the exam for reference if you wish. class String All of the search methods in class String return -1 if the item is not found int indexof(char ch) first position of ch int indexof(char ch, int start) first position of ch starting from start int indexof(string str) first position of str int indexof(string str, int start) first position of str starting from start int lastindexof(char ch) last position of ch int lastindexof(char ch, int start) last position of ch searching backward from start int lastindexof(string str) last position of str int lastindexof(string str, int start) last position of str searching backward from start String substring(int start) substring of this string from position start to the end String substring(int start, end) substring of this string from position start to end-1 int length() length of this string All Collection classes (List, ArrayList, Set, HashSet, ) boolean add(object obj) boolean addall(collection other) void clear() boolean contains(object obj) Iterator iterator() boolean remove(object obj) int size() Additional methods in List, ArrayList add(int position, Object obj) remove(int position) Map, HashMap Object put(object key, Object value) Object get(object key) Set keyset() int size() Finally, if a is a Java array, a.length is the number of elements in that array.

4 CSE 143 Au03 Midterm 2 Sample Solution Page 4 of 7 Question 5. (8 points) Complete the definition of the following method so it returns the number of distinct words in the ArrayList that is its argument. In other words, if a word appears in the array two or more times, it should only be counted once. Words that differ in capitalization should be considered different, i.e., thing, Thing, and THING are all different. You should assume that every element in the list refers to a valid String object. /** Return the number of distinct words in ArrayList words */ public int nwords(arraylist words) { Set uniquewords = new HashSet(); Iterator it = words.iterator(); while (it.hasnext()) { uniquewords.add(it.next()); return uniquewords.size(); There are obviously many possible solutions. A solution closely related to this one would be to store the words as keys in a HashMap. If would also be possible to keep a list of the words and add a new word to the list if it isn t already contained there. As we ll see later in the quarter when we look at the implementation of HashMaps, the list implementation would be considerably slower, but it would work. A couple of people observed that every Collection class implements a constructor initializes its value using the contents of another Collection. So the entire body of method nwords can be replaced by the following: return new HashSet(words).size();

5 CSE 143 Au03 Midterm 2 Sample Solution Page 5 of 7 Question 6. (10 points) DNA sequences can be represented in a computer as strings containing the letters C, A, T, and G. Much research in computational biology involves searching for patterns in long strings of DNA. For this problem, complete the definition of method ncopies, below, so it returns the number of non-overlapping copies of a given pattern in a given string. Examples: If the pattern is CAT and the string is GTACATGCATAG, then ncopies should return 2. If the pattern is TAGT and the string is ATAGTAGTCATAGTC, then ncopies should return 2. In this later case, the pattern only matches the parts of the string in bold type: ATAGTAGTCATAGTC. The TAGT substring that begins at position 4 does not match, because it overlaps the first matching substring. For full credit, your solution must use string operations like indexof from class String to locate copies of the pattern, and not nested loops that compare characters in the pattern and the string one character at a time or extract individual characters from the strings. Some reference information about class String appears earlier in the test. /** return the number of non-overlapping occurrences of * the given pattern in the given string. */ public int ncopies(string pattern, String str){ int nextpos = 0; int nfound = 0; // position of first unprocessed // character in str // # of copies found // Look for copies of pattern. When we find one, advance // nextpos to point to the first character beyond that copy while (str.indexof(pattern, nextpos) >= 0)) { nfound++; nextpos = str.indexof(pattern,nextpos) + pattern.length(); return nfound; This could be made more efficient by not evaluating indexof twice for each occurrence of the pattern (storing the location in a separate variable), and not reevaluating pattern.length() inside the loop. But this solution does have the nice property of being simple and (we hope) correct.

6 CSE 143 Au03 Midterm 2 Sample Solution Page 6 of 7 Question 7. (9 points) The SimpleArrayList class we developed in class included many, but not all, of the operations in the standard Java ArrayList class. For this question, describe how to test and implement SimpleArrayList method add(position, item), which inserts item in the given position in the list. The item at that position in the list, if any, and all subsequent items are shifted to the right. (a) (3 points) Describe how you would test that method add(position, item) works properly. You do not need to give actual Java or JUnit code. Instead describe what test data you would create, if any, what operations you would do on it, and what you would verify about the results. For example, if we were testing the size() method of SimpleArrayList, one reasonable test might be: create a new, empty SimpleArrayList, add Mickey and Donald to the new list, and verify that the value of size() is 2. Your set of tests should be complete enough to cover typical cases, as well as unusual cases, like insert into an empty list, etc. A reasonable set of test cases would be to check the size and list contents after doing each of the following Add an item at position 0 of an empty list Add items to a non-empty list with at least three elements o At the beginning of the list (position 0) o At the end of the list (position size()) o Somewhere in the middle If one were being really compulsive, it might be worth testing lists that originally contain 1 or 2 elements.

7 CSE 143 Au03 Midterm 2 Sample Solution Page 7 of 7 (6 points) Complete the definition of method add(position, item),below. Recall that the contents of a SimpleArrayList were stored in an array and an associated variable recording the current size of the SimpleArrayList as follows private Object[] items; // items in this list are stored in private int size; // items[0..size-1] /** * Insert the specified item at the specified position in this * SimpleArrayList. The element at that position (if any) and * any subsequent elements are shifted to the right. position the location where the new element is to be * inserted item the element to be inserted IndexOutOfBoundsException if the position is out of * range */ public void add(int position, Object item) { if (position < 0 position > size()) { throw new IndexOutOfBoundsException(); // guarantee there is enough room for the new item ensurecapacity(1); // insert the new item and shift existing items as needed // Shift items in the list to the right // (must be done right to left) for (int k = size; k > position; k--) { items[k] = items[k-1]; // Insert new item in correct position and adjust size items[position] = item; size++;