Reading and Writing

Using DataInputStream and DataOutputStream

This page shows you how to use the java.io DataInputStream and DataOutputStream classes. It features an example, DataIOTest, that reads and writes tabular data (invoices for Java merchandise). The tabular data is formatted in columns, where each column is separated from the next by tabs. The columns contain the sales price, the number of units ordered, and a description of the item, like this:

19.99   12      Java T-shirt
9.99    8       Java Mug

DataOutputStream, like other filtered output streams, must be attached to some other OutputStream. In this case, it's attached to a FileOutputStream that's set up to write to a file named invoice1.txt.

DataOutputStream dos = new DataOutputStream(
                           new FileOutputStream("invoice1.txt"));

Next, DataIOTest uses DataOutputStream's specialized writeXXX methods to write the invoice data (contained within arrays in the program) according to the type of data being written:

for (int i = 0; i < prices.length; i ++) {
    dos.writeDouble(prices[i]);
    dos.writeChar('\t');
    dos.writeInt(units[i]);
    dos.writeChar('\t');
    dos.writeChars(descs[i]);
    dos.writeChar('\n');
}
dos.close();

Note that this code snippet closes the output stream when it's finished.

Next, DataIOTest opens a DataInputStream on the file just written:

DataInputStream dis = new DataInputStream(
                          new FileInputStream("invoice1.txt"));

DataInputStream also must be attached to some other InputStream; in this case, a FileInputStream set up to read the file just written--invoice1.txt. DataIOTest then just reads the data back in using DataInputStream's specialized readXXX methods.

try {
    while (true) {
        price = dis.readDouble();
        dis.readChar();       // throws out the tab
        unit = dis.readInt();
        dis.readChar();       // throws out the tab
        desc = dis.readLine();
        System.out.println("You've ordered " + unit + " units of " + desc + " at $" + price);
        total = total + unit * price;
    }    
} catch (EOFException e) {
}
System.out.println("For a TOTAL of: $" + total);
dis.close();

When all of the data has been read, DataIOTest displays a statement summarizing the order and the total amount owed, and closes the stream.

Note the loop that DataIOTest uses to read the data from the DataInputStream. Normally, when reading you see loops like this:

while ((input = dis.readLine()) != null) {
    . . .
}

The readLine method returns a value, null, that indicates that the end of the file has been reached. Many of the DataInputStream readXXX methods can't do this because any value that could be returned to indicate end-of-file may also be a legitimate value read from the stream. For example, suppose that you wanted to use -1 to indicate end-of-file? Well, you can't because -1 is a legitimate value that can be read from the input stream using readDouble, readInt, or one of the other read methods that reads numbers. So DataInputStream's readXXX methods throw an EOFException instead. When the EOFException occurs the while (true) terminates.

When you run the DataIOTest program you should see the following output:

You've ordered 12 units of Java T-shirt at $19.99
You've ordered 8 units of Java Mug at $9.99
You've ordered 13 units of Duke Juggling Dolls at $15.99
You've ordered 29 units of Java Pin at $3.99
You've ordered 50 units of Java Key Chain at $4.99
For a TOTAL of: $892.88

Reading and Writing