LESSON 372 OF 410 · PROJECTS: COMMAND-LINE TOOLS

CLI Tool: cat v4 — Concatenate Multiple Files in Java

Practice

The real cat command can take many files as arguments and prints them one after another. Build that behavior. Given a list of "files" where each file is itself a list of lines, flatten all lines in order and join them with newlines.

A file can be empty (it adds no lines), and the list can hold no files at all; the result is then an empty string.

Function signature:

public String concatCat(List<List<String>> allLines)

Example:

Input: allLines = [["hello", "world"], ["foo", "bar", "baz"]]

Output: "hello\nworld\nfoo\nbar\nbaz"

When printed:
hello
world
foo
bar
baz

Build a flat List<String> with a nested loop, preserving order. Then return String.join("\n", flat).

List<String> flat = new ArrayList<>();
for (List<String> fileLines : allLines) {
    for (String line : fileLines) {
        flat.add(line);
    }
}
return String.join("\n", flat);

The full solution:

class Solution {
    public String concatCat(List<List<String>> allLines) {
        List<String> flat = new ArrayList<>();
        for (List<String> fileLines : allLines) {
            for (String line : fileLines) {
                flat.add(line);
            }
        }
        return String.join("\n", flat);
    }
}

STEP 1 OF 5

Guided

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CLI Tool: cat v4 — Concatenate Multiple Files in Java

Cat V4 in Java