EXAM: e02: Midterm 2

When working with Java code, we sometimes download .jar files, and sometimes create .jar files.

1. (6 pts) What is a jar file?

2. (6 pts) To make a jar file executable, you have to specify one piece of information that goes into the so-called manifest of the jar file. What is this information?

For this problem, you may find it helpful to consult the reference material regarding interfaces and methods related to sorting on Handout B.

• Below, you will code for the file Student.java. This code compiles cleanly as given.
• However, as a result of something missing in Student.java, the code for StudentMain.java shown on Handout A, does not compile; it gives an error message as shown.

Suppose you were shown this code at a job interview, and asked to explain in plain english, why the code doesn’t compile, and to fix the code. The interviewer gives you two hints:

1. There is nothing wrong with StudentMain.java; rather the problem is that two changes need to be made in Student.java
2. Make sure you pay attention to the comment on line 8 of StudentMain.java

1. (16 pts) Make the two changes needed, directly on the listing below.
   You may cross out code, and/or write new code where it belongs.

   1
   2
   3
   4
   5
   6
   7
   8
   9
   10
   11
   12
   13
   14
   15
   16
   17
   18
   19
   20
   21
   22
   23
   24
   25
   26
   27
   28
   public class Student {                                                
   
   
       private String name;
       private int perm;
       private String major;
   
   
       public Student (String name, int perm, String major) {
   	this.name = name;
   	this.perm = perm;
   	this.major = major;
       }
   
   
       public String getName() { return name; }
       public int getPerm() { return perm; }
       public String getMajor() { return major; }
   
   
   
   
   
   
   
   
   
   }

2. (8 pts) How would you explain your fixes to the interviewer? Write in plain english at an appropriate level of abstraction; so that the interviewer has confidence in your technical knowledge.

Exceptions in Java can be divided into two broad categories:

• One category is the kind that, if there is any chance it can happen in the code has to be “caught or declared to be thrown”
• The other category is the kind that can happen, but doesn’t have to be “caught or declared to be thrown”.

1. Consider the kind that DOES have to be “caught, or declared to be thrown”. Indicate for each statement, whether the statement is true or false, by checking the appropriate box.

   (2 pts)	It is a subclass of RuntimeException	☐ True	☐ False
   (2 pts)	If it is thrown, there is a problem with the program logic	☐ True	☐ False
   (2 pts)	IndexOutOfBoundsException would be an example of this type of exception	☐ True	☐ False
   (2 pts)	FileNotFoundException would be an example of this type of exception	☐ True	☐ False

2. Consider the kind that does NOT have to be “caught, or declared to be thrown”. Indicate for each statement, whether the statement is true or false, by checking the appropriate box.

   (2 pts)	It is a subclass of RuntimeException	☐ True	☐ False
   (2 pts)	If it is thrown, there is a problem with the program logic	☐ True	☐ False
   (2 pts)	UnknownHostException (often a symptom of having no internet connection) would be an example of this type of exception	☐ True	☐ False
   (2 pts)	NullPointerException would be an example of this type of exception	☐ True	☐ False

The class java.lang.String (i.e plain old java String objects) has a method:

Note that perhaps contrary to expectations, String does NOT have a method static int compare(String s1, String s2);

Consider the main program in StringSort.java on Handout A which sorts some strings. Note that the place where the sort is supposed to appear, at lines 14-19 shows where one or more lines of code could go to sort the array.

You are now asked to fill in two different things that could go in that space. In each case, it is just one call to either java.util.Collections.sort or the sort1 method of ArrayList<String> that does the sorting, but you may need extra space for a lambda expression.

For this problem, you may find it helpful to consult the reference material regarding interfaces and methods related to sorting on Handout B.

1. (10 pts) Sort the strings in lexicographic order, i.e. so that the output of the program is:

   [a, an, be, cat, cow, dog, duck, goose, moose, pig, to]
   

2. (10 pts) Sort the first by their length, then in lexicographic (i.e. the order used by the compareTo method of java.lang.String).

   That is, the output of the program should be:

   ["a","an","be","to","cat","cow","dog","pig","duck","goose","moose"]
   

   Your answer should use a lambda expressions for an instance of java.util.Comparator<String>.

Chapter 1 of HFDP covered the strategy pattern, using an example of a Duck class, where you could set the FlyBehavior or a QuackBehavior at runtime. The strategy pattern, according to our textbook: “defines a family of algorithms, encapsulates each one, and makes them interchangeable. Strategy lets the algorithm vary independently from clients that use it.”

Consider the interfaces java.lang.Comparable, and java.util.Comparator and how they are used.

1. (6 pts) Which of these is an example of the Strategy pattern? (check one)

   ☐ Comparable

   ☐ Comparator

   ☐ both

   ☐ neither

2. (6 pts) Explain your answer to the multiple choice question above.

(6 pts) Some Java files start with lines of code such as this one:

package edu.ucsb.cs56.pluto;

What purpose do these lines serve? That is, what problem do they solve, and how do they solve it?