Answers to Questions and Exercises: I/O: Reading and Writing - Java Tutorial 5.0 英文版

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Trail: Essential Java Classes
Lesson: I/O

Answers to Questions and Exercises: I/O: Reading and Writing

Questions

Question 1. What class would you use to read a few pieces of data that are at known positions near the end of a large file?
Answer 1. RandomAccessFile.
Question 2. What class in the java.util.zip package gives you access to the entries in a ZIP archive and allows you to read those entries through a stream?
Answer 2. ZipInputStream. This class implements an input stream filter for reading files in the ZIP file format.

Question 3. How would you append data to the end of a file? Show the constructor for the class you would use and explain your answer.
Answer 3. Here's a quick answer: Use the FileWriter and BufferedWriter classes to append data to the end of the text file. Here is the FileWriter constructor, you pass in true to write to the file in append mode:
FileWriter writer = new FileWriter (String filename, boolean append);
An alternate answer is to use RandomAccessFile and skip to the end of the file and start writing:
...
RandomAccessFile file = new RandomAccessFile(datafile, "rw");
file.skipBytes((int)file.length()); //skip to the end of the file
file.writeBytes("Add this text to the end of datafile"); //write at the end of the file
file.close();
...
Question 4. How can you improve the performance of the following code? Explain your answer and show the new line(s) of code.
int i;
URL url = new URL("http://java.sun.com/");
URLConnection javaSite = url.openConnection();
InputStream input = javaSite.getInputStream();
InputStreamReader reader = new InputStreamReader(input);
while ((i = reader.read()) != -1) {
    System.out.print(i);
}
Answer 4. Buffer! To improve the performance of your code, you should buffer all your input and output where possible (remember you can't buffer a RandomAccessFile). In this exercise, you should add two buffers: the InputStream is wrapped within a BufferedInputStream and the InputStreamReader is wrapped within a BufferedReader. Here is the revised code (changes shown in bold):
int i;
URL url = new URL("http://java.sun.com/");
URLConnection javaSite = url.openConnection();
InputStream input = javaSite.getInputStream();
BufferedInputStream in = new BufferedInputStream(input);
BufferedReader reader = new BufferedReader(new InputStreamReader(in));
while ((i = reader.read()) != -1) {
    System.out.print(i);
}

Exercises

Exercise 1. Modify the program discussed in the section How to Use Pipe Streams so that it uses input streams and output streams in place of readers and writers.
Answer 1. See RhymingWords.java.
Exercise 2. Implement a pair of classes, one Reader and one Writer, that count the number of times a particular character, such as e, is read or written. The character can be specified when the stream is created. Write a program to test your classes. You can use farrago.txt as the input file.
Answer 2. See the following three files: CounterDemo.java, CountReader.java, and CountWriter.java.
Exercise 3. The file datafile begins with a single long that tells you the offset of a single int piece of data within the same file. Using the RandomAccessFile class, write a program that gets the int piece of data. What is the int data?
Answer 3. 123. See FileReader.java for the solution. If you're interested in seeing how the file was written, see FileWriter.java.

Exercise 4. In this exercise, you'll implement object serialization for the Card2 class.

Exercise 4a. Rename the class Card3 and make it serializable.
Answer 4a. See Card3.java.

Exercise 4b. Create a program named CardWriter that creates a Card3 instance, displays its value, and serializes it into a file named card.out. Here is an example of what CardWriter might display:

Card to write is: Ace of Spades
Answer 4b. See CardWriter.java.

Exercise 4c. Create a program named CardReader that reads the Card3 object from card.out and displays its value. For example:

Card read is: Ace of Spades
Answer 4c. See CardReader.java.

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