The JavaTM Tutorial

| Start of Tutorial > Start of Trail > Start of Lesson | Search Feedback Form

Trail: Essential Java Classes
Lesson: I/O: Reading and Writing (but no 'rithmetic)

## Answers to Questions and Exercises: I/O: Reading and Writing (but no 'rithmetic)

### 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. Suppose you wanted to write code that reads from a file one word at a time. The code needs to peek ahead to find where the words are separated by whitespace. What input stream could you use to accomplish this?
> Answer 4. Answer 4: You can use the `java.util.StringTokenizer` or `java.io.StreamTokenizer` to parse your input into words. Each class has a default set of delimiters (like white space) that you can specify.

> Question 5. 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 5. 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`.

| Start of Tutorial > Start of Trail > Start of Lesson | Search Feedback Form

Copyright 1995-2001 Sun Microsystems, Inc. All rights reserved.