DataOutputStream Class

Package: java.io

The DataOutputStream class is the main class you’ll work with for writing data to binary files. This class builds on the BufferedOutputStream class by adding the ability to write primitive data types. The BufferedOutputStream class builds on the FileOutputStream class by adding buffered output for the sake of efficiency. And the FileOutputStream class provides the basic capabilities of writing characters to an output stream.

The DataOutputStream class is one of many Java I/O classes that use streams. For more information, see Streams (Overview).

Constructor

Constructor	Description
DataOutputStream (OutputStream out)	Creates a data output stream for the specified output stream.

Methods

Method	Description
void close()	Closes the file.
void flush()	Writes the contents of the buffer to the hard drive.
int size()	Returns the number of bytes written to the file.
void writeBoolean (boolean value)	Writes a boolean value to the output stream. It throws IOException.
void writeByte (byte value)	Writes a byte value to the output stream. It throws IOException.
void writeChar(char value)	Writes a char value to the output stream. It throws IOException.
void writeDouble(double value)	Writes a double value to the output stream. It throws IOException.
void writeFloat(float value)	Writes a float value to the output stream. It throws IOException.
void writeInt(int value)	Writes an int value to the output stream. It throws IOException.
void writeLong(long value)	Writes a long value to the output stream. It throws IOException.
void writeUTF(String value)	Writes a string stored in UTF format to the output stream. It throws EOFException, IOException, and UTFDataFormat Exception.

Creating a DataOutputStream

To create a DataOutputStream object, you typically must also create a BufferedOutputStream object, a FileOut putStream object, and a File object. Here is a typical sequence:

File file;

FileOutputStream fstream;

BufferedOutputStream bstream;

DataOutputStream out;

file = new File(“myfile.bin”);

fstream = new FileOutputStream(file);

bstream = new BufferedOutputStream(fos);

out = new DataOutputStream(bos);

Note that the FileOutputStream class has an optional boolean parameter that you can use to indicate that the file should be appended, if it exists. To use this feature, call the FileOutputStream constructor like this:

fstream = new FileOutputStream(file, true);

For more information, see FileOutputStream Class.

Writing to a binary stream

After you successfully connect a DataOutputStream to a file, you can call the various write methods to write different data types to the file. The following code writes a string value followed by an integer and a double:

out.writeUTF(“This is a string”);

out.writeInt(42);

out.writeDouble(99.997);

These methods throw IOException. As a result, you should enclose them in a try/catch block. (For more information, see try Statement in Part 2.)

If you use the BufferedOutputStream class to connect to the output file, the BufferedOutputStream object accumulates data in its buffer until it decides to write the data to the hard drive. If you want, you can force the buffer to be written to the hard drive by calling the flush method, like this:

out.flush();

Also, when you finish writing data to the file, close the file by calling the close method, like this:

out.close();

The flush and close methods also throw IOException, so you need a try/catch block to catch the exception.