Working with Text Files and File Name Filters in Java

Review today's material by taking this three-question quiz.

Questions

1. What happens when you create a FileOutputStream using a reference to an existing file?

1. An exception is thrown.

2. The data you write to the stream are appended to the existing file.

3. The existing file is replaced with the data you write to the stream.

1. What two primitive types are interchangeable when you're working with streams?

1. byte and boolean

2. char and int

3. byte and char

1. In Java, what is the maximum value of a byte variable and the maximum value of an unsigned byte in a stream?

1. Both are 255

2. Both are 127

3. 127 for a byte variable and 255 for an unsigned byte

Answers

1. c. That's one of the things to look out for when using output streams; you can easily wipe out existing files.

2. b. Because a char is represented internally by Java as an integer value, you can often use the two interchangeably in method calls and other statements.

3. c. The byte primitive data type has values ranging from -128 to 127, whereas an unsigned byte can range from 0 to 255.

Certification Practice

The following question is the kind of thing you could expect to be asked on a Java programming certification test. Answer it without looking at today's material or using the Java compiler to test the code.

Given:

import java.io.*;

public class Unknown {
public static void main(String[] arguments) {
String command = "";
BufferedReader br = new BufferedReader(new
InputStreamReader(System.in));
try { 
command = br.readLine(); 
}
catch (IOException e) { }
}
}

Will this program successfully store a line of console input in the String object named command?

1. Yes.

2. No, because a buffered input stream is required to read console input.

3. No, because it won't compile successfully.

4. No, because it reads more than one line of console input.

The answer is available on the book's Web site at http://www.java21days.com. Visit the Day 15 page and click the Certification Practice link.

Exercises

To extend your knowledge of the subjects covered today, try the following exercises:

1. Write a modified version of the HexRead program from Day 7, "Threads, Exceptions, and Assertions," that reads two-digit hexadecimal sequences from a text file and displays their decimal equivalents.

2. Write a program that reads a file to determine the number of bytes it contains and then overwrites all those bytes with zeroes (0). (For obvious reasons, don't test this program on any file you intend to keep; the data in the file will be wiped out.)

Where applicable, exercise solutions are offered on the book's Web site at http://www.java21days.com.