Line Filters in Fortress

Here’s the translation of the Go line filter program to Java, along with explanations in Markdown format suitable for Hugo:

A line filter is a common type of program that reads input on stdin, processes it, and then prints some derived result to stdout. grep and sed are common line filters.

Here’s an example line filter in Java that writes a capitalized version of all input text. You can use this pattern to write your own Java line filters.

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;

public class LineFilter {
    public static void main(String[] args) {
        // Wrapping System.in with a BufferedReader gives us a convenient
        // readLine method that reads the next line of input.
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));

        try {
            String line;
            // readLine returns null when it reaches the end of the input
            while ((line = reader.readLine()) != null) {
                // Convert the line to uppercase
                String ucl = line.toUpperCase();

                // Write out the uppercased line
                System.out.println(ucl);
            }
        } catch (IOException e) {
            // Check for errors during reading
            System.err.println("error: " + e.getMessage());
            System.exit(1);
        }
    }
}

To try out our line filter, first make a file with a few lowercase lines.

$ echo 'hello'   > /tmp/lines
$ echo 'filter' >> /tmp/lines

Then use the line filter to get uppercase lines.

$ cat /tmp/lines | java LineFilter
HELLO
FILTER

In this Java version, we use BufferedReader to read input line by line from System.in. The readLine() method returns null when it reaches the end of the input, which is how we detect when to stop reading.

We use a try-catch block to handle potential IOExceptions that might occur during reading. If an error occurs, we print it to System.err and exit with a non-zero status code.

The toUpperCase() method is used to convert each line to uppercase, and System.out.println() is used to print the result.

Remember to compile the Java file before running it:

$ javac LineFilter.java

Then you can use it in a pipeline as shown in the example above.