Lehekülje Java:Failist lugemine lähteteksti vaatamine

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 <code>BufferedReader</code> või <code>Scanner</code>. Allpool on paar näidet, kuidas faili võib lugeda. Lisaks kahele mainitud andmetöötlus objekti on failide lugemiseks veel täiendav võimalus <code>Files.readAllLines()</code>.

Iga konkreetne andmetöötlus objekt tahab saada argumendiks <code>Path</code> tüüpi objekti. <code>Path</code> 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:
<pre>
1
2
3 4
tere 5
tere
6
</pre>

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

[[Category:Java juhend|Failist lugemine]]