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Thinking in Java
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Input streams

Parts 1 through 4 demonstrate the creation and use of input streams. Part 4 also shows the simple use of an output stream.

1. Buffered input file

To open a file for character input, you use a FileInputReader with a String or a File object as the file name. For speed, you’ll want that file to be buffered so you give the resulting reference to the constructor for a BufferedReader. Since BufferedReader also provides the readLine( ) method, this is your final object and the interface you read from. When you reach the end of the file, readLine( ) returns null so that is used to break out of the while loop.

The String s2 is used to accumulate the entire contents of the file (including newlines that must be added since readLine( ) strips them off). s2 is then used in the later portions of this program. Finally, close( ) is called to close the file. Technically, close( ) will be called when finalize( ) runs, and this is supposed to happen (whether or not garbage collection occurs) as the program exits. However, this has been inconsistently implemented, so the only safe approach is to explicitly call close( ) for files.

Section 1b shows how you can wrap System.in for reading console input. System.in is an InputStream, and BufferedReader needs a Reader argument, so InputStreamReader is brought in to perform the adaptation.

2. Input from memory

This section takes the String s2 that now contains the entire contents of the file and uses it to create a StringReader. Then read( ) is used to read each character one at a time and send it out to the console. Note that read( ) returns the next byte as an int and thus it must be cast to a char to print properly.

3. Formatted memory input

To read “formatted” data, you use a DataInputStream, which is a byte-oriented I/O class (rather than char-oriented). Thus you must use all InputStream classes rather than Reader classes. Of course, you can read anything (such as a file) as bytes using InputStream classes, but here a String is used. To convert the String to an array of bytes, which is what is appropriate for a ByteArrayInputStream, String has a getBytes( ) method to do the job. At that point, you have an appropriate InputStream to hand to DataInputStream.

If you read the characters from a DataInputStream one byte at a time using readByte( ), any byte value is a legitimate result, so the return value cannot be used to detect the end of input. Instead, you can use the available( ) method to find out how many more characters are available. Here’s an example that shows how to read a file one byte at a time:

//: c12:TestEOF.java
// Testing for end of file while reading a byte at a time.
import java.io.*;

public class TestEOF {
  // Throw exceptions to console:
  public static void main(String[] args)
  throws IOException {
    DataInputStream in = new DataInputStream(
      new BufferedInputStream(
        new FileInputStream("TestEOF.java")));
    while(in.available() != 0)
      System.out.print((char)in.readByte());
  }
} ///:~


Note that available( ) works differently depending on what sort of medium you’re reading from; it’s literally “the number of bytes that can be read without blocking.” With a file, this means the whole file, but with a different kind of stream this might not be true, so use it thoughtfully.

You could also detect the end of input in cases like these by catching an exception. However, the use of exceptions for control flow is considered a misuse of that feature.

4. File output

This example also shows how to write data to a file. First, a FileWriter is created to connect to the file. You’ll virtually always want to buffer the output by wrapping it in a BufferedWriter (try removing this wrapping to see the impact on the performance—buffering tends to dramatically increase performance of I/O operations). Then for the formatting it’s turned into a PrintWriter. The data file created this way is readable as an ordinary text file.

As the lines are written to the file, line numbers are added. Note that LineNumberInputStream is not used, because it’s a silly class and you don’t need it. As shown here, it’s trivial to keep track of your own line numbers.

When the input stream is exhausted, readLine( ) returns null. You’ll see an explicit close( ) for out1, because if you don’t call close( ) for all your output files, you might discover that the buffers don’t get flushed, so they’re incomplete.
Thinking in Java
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   Reproduced courtesy of Bruce Eckel, MindView, Inc. Design by Interspire