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

## Writing Your Own Filtered Streams

> The following is a list of steps to take when writing your own filtered input and output streams:
> - Create a subclass of `FilterInputStream` and `FilterOutputStream`. Input and output streams often come in pairs, so it's likely that you will need to create both input and output versions of your filter stream.
> - Override the `read` and `write` methods.
> - Override any other methods that you might need.
> - Make sure the input and output streams work together.
> This section shows you how to implement your own filtered streams through an example that implements a matched pair of filtered input and output streams. Many thanks to David Connelly from the Java team for providing us with this example.

> Both the input and the output stream use a checksum class to compute a checksum on the data written to or read from the stream. The checksum is used to determine whether the data read by the input stream matches that written by the output stream.

> Four classes and one interface make up this example program:

> - The filtered input and output stream subclasses--`CheckedOutputStream` and `CheckedInputStream`.
> - The `Checksum` interface and the `Adler32` class compute a checksum for the streams.
> - The `CheckedIOTest` class defines the `main` method for the program.

### The `CheckedOutputStream` Class

> The `CheckedOutputStream` class is a subclass of `FilterOutputStream` that computes a checksum on data as it's being written to the stream. When creating a `CheckedOutputStream`, you must use its only constructor:

> >     public CheckedOutputStream(OutputStream out, Checksum cksum) {
> >         super(out);
> >         this.cksum = cksum;
> >     }

> This constructor takes an `OutputStream` argument and a `Checksum` argument. The `OutputStream` argument is the output stream that this `CheckedOutputStream` should filter. The `Checksum` argument is an object that can compute a checksum. `CheckedOutputStream` initializes itself by calling its superclass constructor and initializing a private variable, `cksum`, with the `Checksum` object. The `CheckedOutputStream` uses `cksum` to update the checksum each time data is written to the stream.

> `CheckedOutputStream` needs to override `FilterOutputStream`'s `write` methods so that each time the `write` method is called, the checksum is updated. `FilterOutputStream` defines three versions of the `write` method:

> 1. `write(int i)`
> 2. `write(byte[] b)`
> 3. `write(byte[] b, int offset, int length)`
> `CheckedOutputStream` overrides all three of these methods:

> >     public void write(int b) throws IOException {
> >         out.write(b);
> >         cksum.update(b);
> >     }

> >     public void write(byte[] b) throws IOException {
> >         out.write(b, 0, b.length);
> >         cksum.update(b, 0, b.length);
> >     }

> >     public void write(byte[] b, int off, int len) throws IOException {
> >         out.write(b, off, len);
> >         cksum.update(b, off, len);
> >     }

> The implementations of these three `write` methods are straightforward: Write the data to the output stream that this filtered stream is attached to, then update the checksum.

### The `CheckedInputStream` Class

> The `CheckedInputStream` class is very similar to the `CheckedOutputStream` class. `CheckedInputStream` is a subclass of `FilterInputStream` that computes a checksum on data as it's being read from the stream. When creating a `CheckedInputStream`, you must use its only constructor:

> >     public CheckedInputStream(InputStream in, Checksum cksum) {
> >         super(in);
> >         this.cksum = cksum;
> >     }

> This constructor is similar to the constructor for `CheckedOutputStream`.

> Just as `CheckedOutputStream` needed to override `FilterOutputStream`'s `write` methods, `CheckedInputStream` must override `FilterInputStream`'s `read` methods so that each time the `read` method is called, the checksum is updated. As with `FilterOutputStream`, `FilterInputStream` defines three versions of the `read` method and `CheckedInputStream` override all of them:

> >     public int read() throws IOException {
> >         int b = in.read();
> >         if (b != -1) {
> >             cksum.update(b);
> >         }
> >         return b;
> >     }

> >     public int read(byte[] b) throws IOException {
> >         int len;
> >         len = in.read(b, 0, b.length);
> >         if (len != -1) {
> >             cksum.update(b, 0, len);
> >         }
> >         return len;
> >     }

> >     public int read(byte[] b, int off, int len) throws IOException {
> >         len = in.read(b, off, len);
> >         if (len != -1) {
> >             cksum.update(b, off, len);
> >         }
> >         return len;
> >     }

> The implementations of these three `read` methods are straightforward: Read the data from the input stream that this filtered stream is attached to, then if any data was actually read, update the checksum.

### The `Checksum` Interface and the `Adler32` Class

> The `Checksum` interface defines four methods for checksum objects to implement; these methods reset, update, and return the checksum value. You could write a `Checksum` class that computes a specific type of checksum such as the CRC-32 checksum. Note that inherent in the checksum is the notion of state. The checksum object doesn't just compute a checksum in one go. Rather, the checksum is updated each time information is read from or written to the stream for which this object computes a checksum. If you want to reuse a checksum object, you must reset it.

> For this example, we implemented the `Adler32` checksum, which is almost as reliable as a CRC-32 checksum but can be computed much faster.

### A Program for Testing

> The last class in the example, `CheckedIOTest`, contains the `main` method for the program.

> >     import java.io.*;

> >     public class CheckedIOTest {
> >         public static void main(String[] args) throws IOException {

> >            Adler32 inChecker = new Adler32();
> >            Adler32 outChecker = new Adler32();
> >            CheckedInputStream in = null;
> >            CheckedOutputStream out = null;

> >            try {
> >                in = new CheckedInputStream(
> >     			   new FileInputStream("farrago.txt"),
> >     			   inChecker);
> >                out = new CheckedOutputStream(
> >     			    new FileOutputStream("outagain.txt"),
> >     			    outChecker);
> >            } catch (FileNotFoundException e) {
> >                System.err.println("CheckedIOTest: " + e);
> >                System.exit(-1);
> >            } catch (IOException e) {
> >                System.err.println("CheckedIOTest: " + e);
> >                System.exit(-1);
> >            }

> >            int c;

> >            while ((c = in.read()) != -1)
> >               out.write(c);

> >            System.out.println("Input stream check sum: " +
> >     			  inChecker.getValue());
> >            System.out.println("Output stream check sum: " +
> >     			  outChecker.getValue());

> >            in.close();
> >            out.close();
> >         }
> >     }

> The `main` method creates two `Adler32` checksum objects, one each for a `CheckedOutputStream` and a `CheckedInputStream`. The example requires two checksum objects because the checksum objects are updated during calls to `read` and `write` and those calls are occurring concurrently.

> Next, `main` opens a `CheckedInputStream` on a small text file, `farrago.txt`, and a `CheckedOutputStream` on an output file named `outagain.txt`, which doesn't exist until you run the program for the first time.

> The `main` method reads the text from the `CheckedInputStream` and simply copies it to the `CheckedOutputStream`. The `read` and `write` methods use the `Adler32` checksum objects to compute a checksum during reading and writing. After the input file has been completely read (and consequently the output file has been completely written), the program prints out the checksum for both the input and output streams (which should match) and then closes them both.

> When you run `CheckedIOTest`, you should see this output:

> >     Input stream check sum: 736868089
> >     Output stream check sum: 736868089

### Filtering Random Access Files

> The filtered streams in `java.io` all inherit from `InputStream` or `OutputStream`, which implement sequential access files. So if you subclass `FilterInputStream` or `FilterOutputStream` your filtered streams will also be sequential access files. Writing Filters for Random Access Files later in this lesson shows you how to re-write this example so that it works on a `RandomAccessFile` as well as on a `DataInputStream` or a `DataOutputStream`.

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