What Is a Stream?

A stream is an ordered sequence of bytes of indeterminate length. Input streams move bytes of data into a Java program from some generally external source. Output streams move bytes of data from Java to some generally external target. (In special cases, streams can also move bytes from one part of a Java program to another.)

The word stream is derived from an analogy between a sequence and a stream of water. An input stream is like a siphon that sucks up water; an output stream is like a hose that sprays out water. Siphons can be connected to hoses to move water from one place to another. Sometimes a siphon may run out of water if it's drawing from a finite source like a bucket. On the other hand, if the siphon is drawing water from a river, it may well operate indefinitely. So, too, an input stream may read from a finite source of bytes such as a file or an unlimited source of bytes such as System.in . Similarly, an output stream may have a definite number of bytes to output or an indefinite number of bytes.

Input to a Java program can come from many sources. Output can go to many different kinds of destinations. The power of the stream metaphor is that the differences between these sources and destinations are abstracted away. All input and output operations are simply treated as streams using the same classes and the same methods. You don't need to learn a new API for every different kind of device. The same API that reads files can read network sockets, serial ports, Bluetooth transmissions, and more.

1.1.1. Where Do Streams Come From?

The first source of input most programmers encounter is System.in. This is the same thing as stdin in Cgenerally some sort of console window, probably the one in which the Java program was launched. If input is redirected so the program reads from a file, then System.in is changed as well. For instance, on Unix, the following command redirects stdin so that when the MessageServer program reads from System.in, the actual data comes from the file data.txt instead of from the console:

% java MessageServer < data.txt

The console is also available for output through the static field out in the java.lang.System class, that is, System.out . This is equivalent to stdout in C parlance and may be redirected in a similar fashion. Finally, stderr is available as System.err . This is most commonly used for debugging and printing error messages from inside catch clauses. For example:

try {
 //... do something that might throw an exception
}
catch (Exception ex) {
 System.err.println(ex);
 }

Both System.out and System.err are print streamsthat is, instances of java.io.PrintStream. These will be discussed in detail in Chapter 7.

Files are another common source of input and destination for output. File input streams provide a stream of data that starts with the first byte in a file and finishes with the last byte in that file. File output streams write data into a file, either by erasing the file's contents and starting from the beginning or by appending data to the file. These will be introduced in Chapter 4.

Network connections provide streams too. When you connect to a web server, FTP server, or some other kind of server, you read the data it sends from an input stream connected from that server and write data onto an output stream connected to that server. These streams will be introduced in Chapter 5.

Java programs themselves produce streams. Byte array input streams, byte array output streams, piped input streams, and piped output streams all move data from one part of a Java program to another. Most of these are introduced in Chapter 9.

Perhaps a little surprisingly, GUI components like TextArea and JTextArea do not produce streams. The issue here is ordering. A group of bytes provided as data for a stream must have a fixed order. However, users can change the contents of a text area or a text field at any point, not just at the end. Furthermore, they can delete text from the middle of a stream while a different thread is reading that data. Hence, streams aren't a good metaphor for reading data from GUI components. You can, however, use the strings they do produce to create a byte array input stream or a string reader.

1.1.2. The Stream Classes

Most of the classes that work directly with streams are part of the java.io package. The two main classes are java.io.InputStream and java.io.OutputStream . These are abstract base classes for many different subclasses with more specialized abilities.

The subclasses include:

BufferedInputStream

BufferedOutputStream

ByteArrayInputStream

ByteArrayOutputStream

DataInputStream

DataOutputStream

FileInputStream

FileOutputStream

FilterInputStream

FilterOutputStream

ObjectInputStream

ObjectOutputStream

PipedInputStream

PipedOutputStream

PrintStream

PushbackInputStream

SequenceInputStream

The java.util.zip package contains four input stream classes that read data in compressed format and return it in uncompressed format and four output stream classes that read data in uncompressed format and write in compressed format. These will be discussed in Chapter 10.

CheckedInputStream

CheckedOutputStream

DeflaterOutputStream

GZIPInputStream

GZIPOutputStream

InflaterInputStream

ZipInputStream

ZipOutputStream

The java.util.jar package includes two stream classes for reading files from JAR archives. These will be discussed in Chapter 11.

JarInputStream

JarOutputStream

The java.security package includes a couple of stream classes used for calculating message digests:

DigestInputStream

DigestOutputStream

The Java Cryptography Extension (JCE) adds two classes for encryption and decryption:

CipherInputStream

CipherOutputStream

These four streams will be discussed in Chapter 12.

Finally, a few random stream classes are hiding inside the sun packagesfor example, sun.net.TelnetInputStream and sun.net.TelnetOutputStream. However, these are deliberately hidden from you and are generally presented as instances of java.io.InputStream or java.io.OutputStream only.