Searching a file

Many questions about a file are answered by going through its lines once and checking each one. How many lines mention something? Is a certain line there at all? This program counts and prints the lines of a log that contain the word ERROR:

08:00 start
08:15 ERROR disk almost full
09:30 backup done
10:05 ERROR connection lost
10:06 connection back

import java.nio.file.Paths;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        int errors = 0;
        try (Scanner scanner = new Scanner(Paths.get("log.txt"))) {
            while (scanner.hasNextLine()) {
                String line = scanner.nextLine();
                if (line.contains("ERROR")) {
                    System.out.println(line);
                    errors++;
                }
            }
        } catch (Exception e) {
            System.out.println("Error: " + e.getMessage());
        }
        System.out.println("Errors: " + errors);
    }
}

08:15 ERROR disk almost full
10:05 ERROR connection lost
Errors: 2

Any String method can do the checking: contains for a part of the line, startsWith for its beginning, equals for the whole line, equalsIgnoreCase when capitals shouldn't matter.

Is a line there?

To find out whether something is in the file at all, keep a boolean that starts as false and becomes true when a line matches. The answer is known only after the loop, once every line has been checked. Here the user types a discount code, and the program looks for it in the file of valid codes:

WELCOME5
SPRING10
FRIEND20

import java.nio.file.Paths;
import java.util.Scanner;

public class Main {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String code = scanner.nextLine();
        boolean found = false;
        try (Scanner fileScanner = new Scanner(Paths.get("codes.txt"))) {
            while (fileScanner.hasNextLine()) {
                if (fileScanner.nextLine().equals(code)) {
                    found = true;
                }
            }
        } catch (Exception e) {
            System.out.println("Error: " + e.getMessage());
        }
        if (found) {
            System.out.println("Code accepted");
        } else {
            System.out.println("Unknown code");
        }
    }
}

SPRING10

Code accepted

• equals compares whole lines. With contains, the code RING1 would be accepted too, since it's inside SPRING10.
• The answer is printed after the loop. Printed inside it, there would be an answer for every line of the file.