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.