How do I find files in a directory using DirectoryStream?

By I Wayan Saryada in Core API, IO 0 Comment

In this example we are going to learn to use the DirectoryStream, which part of java.nio.file package, to find files in a directory. We begin by creating a Path, the directory where the search will be conducted.

After that we create a DirectoryStream using Files.newDirectoryStream(). To create a directory stream we passed two arguments, the starting path and a glob expression. Below we use *.txt glob expression to filter all text files in the F:/Temp.

To get all the entries of the directory stream we can use a foreach loop as can be seen below. We iterate each entry, which is a Path object. And we print the entries file name using the getFileName() method.

package org.kodejava.io;

import java.io.IOException;
import java.nio.file.DirectoryStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class FindFilesInDirectory {
    public static void main(String[] args) {
        Path dir = Paths.get("F:/Temp");

        try (DirectoryStream<Path> stream = Files.newDirectoryStream(dir, "*.txt")) {
            for (Path entry : stream) {
                System.out.println(entry.getFileName());
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

You can also notice that in this example we are using the try-with-resource statement. This mean that the DirectoryStream will be closed appropriately when the process is done. This type of try statement is an improvement introduced in JDK 7.

How do I create and delete a file in JDK 7?

By I Wayan Saryada in Core API, IO 0 Comment

In this example you’ll learn how to create and delete a file. Using the new Files class helper from the JDK 7 you can create a file using the Files.createFile(Path) method. To delete a file you can use the Files.delete(Path) method.

Before create a file and delete a file we can check to see if the file exists or not using the Files.exists(Path) method. In the code snippet below we’ll create a file when the file is not exist. And we’ll delete the file if the file exists.

package org.kodejava.io;

import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class CreateDeleteFile {
    public static void main(String[] args) {
        try {
            // Create a config.cfg file under D:Temp directory.
            Path path = Paths.get("F:/Temp/config.cfg");
            if (!Files.exists(path)) {
                Files.createFile(path);
            }

            // Delete the path.cfg file specified by the Path.
            if (Files.exists(path)) {
                Files.delete(path);
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}

How to recursively list all text files in a directory?

By I Wayan Saryada in Core API, IO 0 Comment

In this example you’ll learn how to use the Files.walkFileTree() to walk through file tree. This method requires two parameters. The first parameter is the starting file, in this example we’ll start from drive F:/Temp. And the second parameter is the file visitor to invoke for each file. Here we’ll create a file visitor call FindTextFilesVisitor which extend the java.nio.file.SimpleFileVisitor.

To get all the text files (files end with .txt) we override the visitFile() defined by the SimpleFileVisitor. In this method we check if the file ends with .txt extension and print the file name when the extension matches. And we continue to walk the file tree by returning FileVisitResult.CONTINUE.

package org.kodejava.io;

import java.io.IOException;
import java.nio.file.*;
import java.nio.file.attribute.BasicFileAttributes;

public class WalkFileTree {
    public static void main(String[] args) {
        try {
            Path startDir = Paths.get("F:/Temp");
            Files.walkFileTree(startDir, new FindTextFilesVisitor());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    /**
     * FindTextFilesVisitor.
     */
    static class FindTextFilesVisitor extends SimpleFileVisitor<Path> {
        @Override
        public FileVisitResult visitFile(Path file, BasicFileAttributes attrs) throws IOException {
            if (file.toString().endsWith(".txt")) {
                System.out.println(file.getFileName());
            }
            return FileVisitResult.CONTINUE;
        }
    }
}

Instead of listing files, you can modify the code snippet above for instance use it to delete all the files that ends with .bak. Simply change the extension and replace the print-out statement with a file delete statement in the visitFile() method.

How to remove non ASCII characters from a string?

By I Wayan Saryada in Regex 0 Comment

The code snippet below remove the characters from a string that is not inside the range of x20 and x7E ASCII code. The regex below strips non-printable and control characters. But it also keeps the linefeed character n (x0A) and the carriage return r (x0D) characters.

package org.kodejava.regex;

public class ReplaceNonAscii {
    public static void main(String[] args) {
        String str = "Thè quïck brøwn føx jumps over the lãzy dôg.";
        System.out.println("str = " + str);

        // Replace all non ascii chars in the string.
        str = str.replaceAll("[^\\x0A\\x0D\\x20-\\x7E]", "");
        System.out.println("str = " + str);
    }
}

Snippet output:

str = Thè quïck brøwn føx jumps over the lãzy dôg.
str = Th quck brwn fx jumps over the lzy dg.

How to verify digital signature of a signed data?

By I Wayan Saryada in Security 2 Comments

In this example we will learn how to verify the digital signature of the previously signed data. To sign the data you can see the previous example on this post How to create a digital signature and sign data?.

Here the code snippet:

package org.kodejava.security;

import java.nio.file.Files;
import java.nio.file.Paths;
import java.security.KeyFactory;
import java.security.PublicKey;
import java.security.Signature;
import java.security.spec.X509EncodedKeySpec;

public class VerifyDigitalSignature {
    public static void main(String[] args) {
        try {
            byte[] publicKeyEncoded = Files.readAllBytes(Paths.get("publickey"));
            byte[] digitalSignature = Files.readAllBytes(Paths.get("signature"));

            X509EncodedKeySpec publicKeySpec = new X509EncodedKeySpec(publicKeyEncoded);
            KeyFactory keyFactory = KeyFactory.getInstance("DSA", "SUN");

            PublicKey publicKey = keyFactory.generatePublic(publicKeySpec);
            Signature signature = Signature.getInstance("SHA1withDSA", "SUN");
            signature.initVerify(publicKey);

            byte[] bytes = Files.readAllBytes(Paths.get("README.md"));
            signature.update(bytes);

            boolean verified = signature.verify(digitalSignature);
            if (verified) {
                System.out.println("Data verified.");
            } else {
                System.out.println("Cannot verify data.");
            }
        } catch (Exception e) {
            e.printStackTrace();
        }
    }
}