Line Filters in Minitab

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));

        String line;
        try {
            // 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, which is analogous to os.Stdin in Go. The readLine() method serves a similar purpose to the Scan() method in the Go example.

We use a try-catch block to handle potential IOExceptions that might occur during reading, which is equivalent to checking for errors after scanning in the Go version.

The toUpperCase() method in Java is used to convert each line to uppercase, similar to strings.ToUpper() in Go.

Finally, we use System.out.println() to print each uppercased line, which is analogous to fmt.Println() in Go.

Remember to compile the Java file before running it:

$ javac LineFilter.java
$ cat /tmp/lines | java LineFilter

This pattern can be adapted to create various types of line filters in Java.

Markdown provided above is suitable for Hugo and maintains the structure and explanation of the original example, adapted for Java.