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Java OutputStream

Jakob Jenkov
Last update: 2020-03-25

The Java OutputStream class, java.io.OutputStream, is the base class of all output streams in the Java IO API. Subclasses of OutputStream include the Java BufferedOutputStream and the Java FileOutputStream among others. To see a full list of output streams, go to the bottom table of the Java IO Overview page.

Java OutputStream Subclasses

Here are some of the well-known subclasses of the Java OutputStream class:

OutputStream's and Destinations

A Java OutputStream is typically connected to some data destination - as mentioned in the Java IO Overview, like a file, network connection, pipe, memory buffer etc. This is also explained in more detail in the Java IO Overview text. The OutputStream's data destination is where all data written to the OutputStream will eventually end.

write(byte)

The write(byte) method is used to write a single byte to the Java OutputStream. The write() method of an OutputStream takes an int which contains the byte value of the byte to write. Only the first byte of the int value is written. The rest is ignored.

Subclasses of OutputStream may have alternative write() methods. For instance, the DataOutputStream allows you to write Java primitives like int, long, float, double, boolean etc. with its corresponding methods writeBoolean(), writeDouble() etc.

Here is an OutputStream write() example:

OutputStream outputStream = new FileOutputStream("c:\\data\\output-text.txt");

while(hasMoreData()) {
  int data = getMoreData();
  outputStream.write(data);
}
outputStream.close();

This OutputStream write() example first creates a FileOutputStream to which the data will be written. Then the example enters a while loop. The condition to exit the while loop is the return value of the method hasMoreData(). The implementation of hasMoreData() is not shown, but imagine that it returns true if there is more data to write, and false if not.

Inside the while loop the example calls the method getMoreData() to get the next data to write to the OutputStream, and then writes that data to the OutputStream. The while loop continues until hasMoreData() returns false.

Note: The proper exception handling has been skipped here for the sake of clarity. To learn more about correct exception handling, go to Java IO Exception Handling.

write(byte[])

The Java OutputStream has to write-methods that enable you to write an array of bytes to the OutputStream at a time. These write-methods are:

  • write(byte[] bytes)
  • write(byte[] bytes, int offset, int length)

The write(byte[] bytes) method writes all the bytes in the byte array to the OutputStream.

The write(byte[] bytes, int offset, int length) method writes length bytes starting from index offset from the byte array to the OutputStream.

Both of these methods return the actual number of bytes written to the OutputStream, if not the whole array, or length bytes could be written as requested.

Here is an example of writing an array of bytes to a Java OutputStream using the second of these two write methods:

OutputStream outputStream = new FileOutputStream("/usr/home/jakobjenkov/output.txt");

byte[] sourceBytes = ... // get source bytes from somewhere.

int bytesWritten = outputStream.write(sourceBytes, 0, sourceBytes.length);

This example will instruct the OutputStream to attempt to write all bytes in the sourceBytes array to the OutputStream. After executing this code the bytesWritten variable will contain the number of bytes actually written to the OutputStream.

Write Performance

It is faster to write an array of bytes to a Java OutputStream than writing one byte at a time. The speedup can be quite significant - up to 10 x higher or more. Therefore it is recommended to use the write(byte[]) methods whenever possible.

The exact speedup you get depends on the underlying OS and hardware of the computer you run the Java code on. The speedup depends on issues like memory speed, hard disk speed and buffer sizes, or network card speed and buffer sizes, depending on which destination the OutputStream sends its data to.

Transparent Buffering via BufferedOutputStream

You can get transparent buffering of bytes written to a Java OutputStream by wrapping it in a Java BufferedOutputStream . All bytes written to the BufferedOutputStream will first get buffered inside an internal byte array in the BufferedOutputStream. When the buffer is full, the buffer is flushed to the underlying OutputStream all at once. Here is an example of wrapping a Java OutputStream in a BufferedOutputStream:

int bufferSize = 8 * 1024;

OutputStream outputStream =
    new BufferedOutputStream(
          new FileOutputStream("c:\\data\\output-file.txt"), bufferSize);

You can read more about the BufferedOutputStream in my BufferedOutputStream tutorial.

flush()

The Java OutputStream's flush() method flushes all data written to the OutputStream to the underlying data destination. For instance, if the OutputStream is a FileOutputStream then bytes written to the FileOutputStream may not have been fully written to disk yet. The data might be buffered in OS memory somewhere, even if your Java code has written it to the FileOutputStream. By calling flush() you can assure that any buffered data will be flushed (written) to disk (or network, or whatever else the destination of your OutputStream has). Here is an example of flushing data written to a Java OutputStream by calling its flush() method:

outputStream.flush();

Close an OutputStream

Once you are done writing data to a Java OutputStream you should close it. You close an OutputStream by calling its close() method. Here is an example of closing a Java OutputStream:

OutputStream outputStream = new FileOutputStream("c:\\data\\output-text.txt");

while(hasMoreData()) {
    int data = getMoreData();
    outputStream.write(data);
}
outputStream.close();

The concrete implementations of hasMoreData() and getMoreData() are left out, but they are not really super important to understand the principle of this example. What matters is, that once the while loop ends, and you are done writing data to the OutputStream, its close() method is called, which closes the OutputStream .

The above example is not fully robust though. In case the write() method throws an exception, the close() method will never get called. The exception will make the program exit whatever method the above code is located in.

Instead, you should use the Java try with resources construct to close the OutputStream. Here is an example that closes a Java OutputStream using the try-with-resources construct:

try( OutputStream outputStream = new FileOutputStream("c:\\data\\output-text.txt")) {
    while(hasMoreData()) {
        int data = getMoreData();
        outputStream.write(data);
    }
}

Once the try block is exited, the close() method of the OutputStream is called automatically, because the OutputStream was declared inside the parentheses of the try block. Even if an exception is thrown from inside the try block, the close() method is still called before the exception is propagated up the call stack.

Convert OutputStream to Writer

The Java OutputStream is a byte based stream. You can convert a OutputStream to a character based Writer using the Java OutputStreamWriter class. Here is an example of converting a Java OutputStream to a Writer using OutputStreamWriter:

OutputStream outputStream       = new FileOutputStream("c:\\data\\output.txt");
Writer       outputStreamWriter = new OutputStreamWriter(outputStream);

outputStreamWriter.write("Hello World");

You can read more about how to use the OutputStreamWriter including how to set the character encoding to use when converting characters to bytes in my OutputStreamWriter tutorial.

Jakob Jenkov

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