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Lesson: Implementations
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Answers to Questions and Exercises:

### Questions

> 1. Question: You plan to write a program that uses several basic collection interfaces: `Set`, `List`, `Queue`, and `Map`. You're not sure which implementations will work best, so you decide to use general-purpose implementations until you get a better idea how your program will work in the real world. Which implementations are these?

>    Answer:
>    `Set`: `HashSet`
>    `List`: `ArrayList`
>    `Queue`: `LinkedList`
>    `Map`: `HashMap` Question: If you need a `Set` implementation that provides value-ordered iteration, which class should you use?

>    Answer: `TreeSet` guarantees that the sorted set is in ascending element order, sorted according to the natural order of the elements or by the `Comparator` provided.

> 2. Question: Which class do you use to access wrapper implementations?

>    Answer: You use the `Collections` class, which provides static methods that operate on or return collections.

### Exercises

> 1. Exercise: Write a program that reads a text file, specified by the first command line argument, into a `List`. The program should then print random lines from the file, the number of lines printed to be specified by the second command line argument. Write the program so that a correctly-sized collection is allocated all at once, instead of being gradually expanded as the file is read in. Hint: To determine the number of lines in the file, use `java.io.File.length` to obtain the size of the file, then divide by an assumed size of an average line.

>    Answer: Since we're accessing the `List` randomly, we'll use `ArrayList`. We estimate the number of lines by taking the file size and dividing by 50. We then double that figure, since it's more efficient to overestimate than to underestmate.

>    >     import java.util.*;
>    >     import java.io.*;

>    >     public class FileList {
>    >         public static void main(String[] args) {
>    >             final int assumedLineLength = 50;
>    >             File file = new File(args[0]);
>    >             List<String> fileList =
>    >                 new ArrayList<String>((int)(file.length() / assumedLineLength) * 2);
>    >             BufferedReader reader = null;
>    >             int lineCount = 0;
>    >             try {
>    >                 reader = new BufferedReader(new FileReader(file));
>    >                 for (String line = reader.readLine(); line != null;
>    >                         line = reader.readLine()) {
>    >                     fileList.add(line);
>    >                     lineCount++;
>    >                 }
>    >             } catch (IOException e) {
>    >                 System.err.format("Could not read %s: %s%n", file, e);
>    >                 System.exit(1);
>    >             } finally {
>    >                 if (reader != null) {
>    >                     try {
>    >                         reader.close();
>    >                     } catch (IOException e) {}
>    >                 }
>    >             }
>    >             int repeats = Integer.parseInt(args[1]);
>    >             Random random = new Random();
>    >             for (int i = 0; i < repeats; i++) {
>    >                 System.out.format("%d: %s%n", i,
>    >                         fileList.get(random.nextInt(lineCount - 1)));
>    >             }
>    >         }
>    >     }

>    This program actually spends most of its time reading in the file, so pre-allocating the `ArrayList` has little affect on its performance. Specifying an initial capacity in advance is more likely to be useful when your program repeatly creates large `ArrayList` objects without intervening I/O.

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