Java:Failist lugemine

Java sisend-väljund näib liiga keeruline. Üks hea omadus, mis selle keerulisusega saavutatakse, on see, et nii klaviatuurilt kui ka failist saab andmeid lugeda samade kõrgema kihi objektidega nagu BufferedReader või Scanner. Allpool on paar näidet, kuidas faili võib lugeda. Lisaks kahele mainitud andmetöötlus objekti on failide lugemiseks veel täiendav võimalus Files.readAllLines().

Iga konkreetne andmetöötlus objekt tahab saada argumendiks Path tüüpi objekti. Path konstrueeritakse kasutades faili asukohta kõvakettal. Allpool on koodinäide, kuidas luua erinevad andmetöötlusobjektid.

Koodinäide:

<source lang="java"> import java.io.BufferedReader; import java.io.IOException; import java.nio.file.Files; import java.nio.file.Path; import java.nio.file.Paths; import java.util.ArrayList; import java.util.List; import java.util.Scanner;

public class ReadFile {

/** * Filename to be read. */ public static final String FILENAME = "test.txt";

public static void main(String[] args) { // Files.readAllLines (Java 1.7+) try { for (String line : readSmallFile(FILENAME)) { System.out.println(line); } } catch (IOException e) { e.printStackTrace(); } // BufferedReader try { System.out.println(readFileBuffered(FILENAME)); } catch (IOException e) { e.printStackTrace(); }

// Scanner try { System.out.println(readFileScanner(FILENAME)); } catch (IOException e) { e.printStackTrace(); }

// read only numbers using Scanner try { for (int number : readOnlyNumbers(FILENAME)) { System.out.println(number); } } catch (IOException e) { e.printStackTrace(); } }

/** * Reads (a small) file and returns list of lines (strings). * @param filename Filename to be read. * @return List of lines. * @throws IOException */ public static List<String> readSmallFile(String filename) throws IOException { Path path = Paths.get(filename); List<String> lines = Files.readAllLines(path); return lines; }

/** * Reads file using Scanner. * @param filename filename to be read. * @return The contents of the file as one string. * @throws IOException */ public static String readFileScanner(String filename) throws IOException { String ret = ""; Path path = Paths.get(filename); Scanner scanner = new Scanner(path); while (scanner.hasNextLine()) { // "\n" -> newline ret += scanner.nextLine() + "\n"; } scanner.close(); return ret; } /** * Read file using BufferedReader. * @param filename Filename to be read. * @return The contents of the file as one string. * @throws IOException */ public static String readFileBuffered(String filename) throws IOException { String ret = ""; Path path = Paths.get(filename); BufferedReader reader = Files.newBufferedReader(path); String line; while ((line = reader.readLine()) != null) { ret += line + "\n"; } reader.close(); return ret; }

/** * Reads only numbers from the input file. Everything else is ignored. * @param filename Filename to be read. * @return A list of numbers from the file. * @throws IOException */ public static List<Integer> readOnlyNumbers(String filename) throws IOException { List<Integer> numbers = new ArrayList<Integer>(); Path path = Paths.get(filename); Scanner scanner = new Scanner(path); while (scanner.hasNext()) { if (scanner.hasNextInt()) { numbers.add(scanner.nextInt()); } else { // we have to read the token scanner.next(); } } return numbers; }

} </source>

Kui arendate Eclipse'is, siis vaikimisi loetakse faili projekti juurkaustast. Seega minul on fail "test.txt" kohe projekti all. Sisu on järgmine:

1
2
3 4
tere 5
tere
6

Koodis on näide, kuidas lugeda failist ainult numbreid. Ehk siis juba tuttav Scanner objekt kasutatav ka failide puhul.