Before writing a Java program that reads data from `System.in` and copies each byte read to `System.out` as a byte, a developer should have the following prerequisite Java knowledge:
-
Understanding Java Input/Output (I/O) Streams
- Familiarity with byte streams and how Java handles input and output.
- Knowledge of the
InputStream and OutputStream classes in the java.io package.
System.in is an instance of InputStream, and System.out is an instance of PrintStream, which is a subclass of OutputStream.
-
Working with
System.in and System.out
- Understanding how
System.in.read() reads one byte at a time and returns it as an int (0-255) or -1 when the end of the stream is reached.
- Knowing that
System.out.write(int b) writes a byte to the standard output stream.
-
Handling Exceptions (
IOException)
- Since I/O operations can fail, knowing how to handle
IOException using try-catch blocks or propagating the exception using throws IOException.
-
Using Loops for Continuous Reading
- Understanding while loops to repeatedly read bytes from
System.in until the end of the input stream is reached.
- Recognizing that
read() returns -1 when input is EOF (End of File).
-
Closing Streams Properly (Optional)
- While
System.in and System.out do not need to be closed in most cases, developers should understand the importance of closing streams in general when working with files or sockets.
-
Basic Command-Line Interaction
- Knowing how to run a Java program from the command line.
- Using CTRL+D (on Unix/macOS) or CTRL+Z + Enter (on Windows) to signal EOF when reading from
System.in.
-
Understanding Buffered I/O (Optional for Efficiency)
- Using
BufferedInputStream or BufferedReader for more efficient reading in real-world applications.
Example: Simple Java Program for Byte-by-Byte Copying
import java.io.IOException;
public class CopyBytes {
public static void main(String[] args) throws IOException {
int data;
while ((data = System.in.read()) != -1) { // Read a byte
System.out.write(data); // Write the byte to output
}
System.out.flush(); // Ensure everything is written out
}
}
How it Works
- Reads a byte from
System.in
- Writes the same byte to
System.out
- Runs until EOF (
-1 is returned by System.in.read())
Additional Knowledge (for Advanced Use Cases)
- Reading characters instead of bytes (use InputStreamReader and BufferedReader).
- Handling different encodings (UTF-8 vs. ASCII).
- Using Scanner for formatted input (if the goal changes from byte-wise copying to processing text input).