LESSON 371 OF 410 · PROJECTS: COMMAND-LINE TOOLS

CLI Tool: cat v3 — Show Special Chars (-A) in Java

Practice

Now build the -A flag of the cat tool, as a new function. It makes whitespace visible: every tab character (\t) is replaced by the two characters ^I and every line ends with a literal $ marker (so trailing spaces become obvious). Lines are then joined with newlines.

Note: real cat -A also shows other control characters (such as ^M) and non-ASCII bytes; this version only handles tabs and line ends, like cat -T -E. A blank line becomes just $, and an empty list gives an empty string.

Function signature:

public String showAllCat(List<String> lines)

Example:

Input: lines = ["hello\tworld", "foo"]

Output: "hello^Iworld$\nfoo$"

When printed:
hello^Iworld$
foo$

Another example (a blank line, trailing spaces):

Input: lines = ["a\t", "", "x  "]

Output: "a^I$\n$\nx  $"

When printed:
a^I$
$
x  $

Use a StringBuilder and walk the lines by index. For each line, replace tabs with "^I", append the result, then append "$". Don't forget the "\n" separator between lines.

Java's String.replace(CharSequence, CharSequence) handles the literal swap:

sb.append(lines.get(i).replace("\t", "^I"));
sb.append("$");

The full solution:

class Solution {
    public String showAllCat(List<String> lines) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < lines.size(); i++) {
            if (i > 0) sb.append("\n");
            sb.append(lines.get(i).replace("\t", "^I"));
            sb.append("$");
        }
        return sb.toString();
    }
}

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CLI Tool: cat v3 — Show Special Chars (-A) in Java

Cat V3 in Java