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How to Use DataInputStream and DataOutputStream

This page shows you how to use the java.io DataInputStream and DataOutputStream classes. It features an example, DataIODemo, that reads and writes tabular data (invoices for merchandise). The tabular data is formatted in columns separated by tabs. The columns contain the sales price, the number of units ordered, and a description of the item. Conceptually, the data looks like this, although it is read and written in binary form and is non-ASCII:

19.99   12      Java T-shirt
9.99    8       Java Mug

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

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

Next, DataIODemo uses DataOutputStream's specialized write 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 ++) {
    out.writeDouble(prices[i]);
    out.writeChar('\t');
    out.writeInt(units[i]);
    out.writeChar('\t');
    out.writeChars(descs[i]);
    out.writeChar('\n');
}
out.close();

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

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

DataInputStream also must be attached to another InputStream; in this case, a FileInputStream set up to read the file just written, invoice1.txt. Then DataIODemo just reads the data back in using DataInputStream's specialized read methods.

try {
    while (true) {
        price = in.readDouble();
        in.readChar();       //throws out the tab
        unit = in.readInt();
        in.readChar();       //throws out the tab
        char chr;
        desc = new StringBuffer(20);
        char lineSep = System.getProperty("line.separator").charAt(0);

        while ((chr = in.readChar() != lineSep) {
            desc.append(chr);
        }

        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);
in.close();

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

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

while ((input = in.read()) != null) {
    . . .
}

The read method returns a value, null, which indicates that the end of the file has been reached. Many of the DataInputStream read methods can't do this, because any value that could be returned to indicate the end of file may also be a legitimate value read from the stream. For example, suppose that you want 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 methods that reads numbers. So DataInputStreams read methods throw an EOFException instead. When the EOFException occurs, the while (true) terminates.

When you run the DataIODemo 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: $892.8800000000001

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