Besides System.in and System.out, the other sources of data in a Java program include files, network connections, and other programs.
These other data sources are also provided as input or output streams.

Files, whatever their contents, are read and written through input and output streams.
The java.io.FileInputStream class is used to read a file. The java.io.FileOutputStream class is used to write a file.

Network connections are also a fertile source of streams. When you connect to an Internet server, you read the data the server sends you via an input stream, and you send data back to the server via an output stream.

Java programs themselves produce streams.

1. ByteArrayInputStreams
2. ByteArrayOutputStreams
3. StringBufferInputStreams
4. PipedInputStreams, and
5. PipedOutputStreams

all use the stream metaphor to move data from one part of a Java program to another.